NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

NCERT Solutions for Class 10 Science Chapter 9 Intext Questions

Page Number: 143

Question 1.
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier ?
Answer:
Trait B, because it is present in more members of the population. It is likely to have arisen earlier and has now spread to 60% of the population. Trait A is new and has spread to only 10% of the population.

Question 2.
How does the creation of variations in a species promote survival ?
Answer:
The variations provide stability to the population of various species by preventing them from getting wiped out during adverse conditions.
The natural environment also changes, and variations in species which become suited to the environment help it to survive.

Page Number: 147

Question 1.
How do Mendel’s experiments show that traits may be dominant or recessive ? [AICBSE 2015]
Answer:
Mendel took pea plants with contrasting characteristics tall plant and dwarf (or short) plant. On cross pollination, he got all tall plants in first generation (F₁). But by the self¬pollination of F₁ tall plants, the plants of second generation consisted of tall and short pants in the ratio of 3 : 1. On the basis of these experiments, the characteristics appeared in first generation were called dominant (i.e. tall plants) and the characteristics that did not appear were called recessive (dwarf i.e. plants).

Question 2.
How do Mendel’s experiments show that traits are inherited independently ? [AICBSE 2015]
Answer:
Mendel took two pairs of alternate expression of two traits and carried out dihybrid crosses by crossing them. The traits appeared in first generation were termed as dominant. When he used these F₁ progeny to generate F₂ progeny by self-pollination plants of different types were produced. In some plants both the traits were dominant, while in some plants both were recessive and some plants exhibited mixed traits. This indicates that traits are inherited independently.

Question 3.
A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant ? Why or why not ?
Answer:
This information is not enough. This is because each individual is carrying two alleles. The recessive trait can occur only when who alleles are similar. It blood group A is dominant and O is recessive, then daughter can have blood group O only when both recessive alleles occur together in mother, and father has one allele of O and other of A.

Question 4.
How is the sex of the child determined in human beings ?
OR
“The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it.” Justify this statement with the help of a flow chart showing determination of sex of a newborn. [CBSE (Delhi) 2013]
Answer:
Half of the male gametes (sperms) carry X chromosome and other half carry Y chromosomes. All the female gametes carry only X chromosomes. When a sperm fertilizes an egg, the following situations become possible.

1. (i) When a sperm carrying X chromosome fertilises an egg that contains only X chromosome), the resulting zygote develops into a female (XX condition).
2. (ii) When a sperm carrying Y chromosome fertilises an egg (that contains only X chromosome), the resulting zygote develops into a male (XY condition).

Thus there are 50 – 50 chances of a male or female child and none of the parents may Sex determination in humans be considered responsible for it.
The sex-determination mechanism is shown alongside.

Page Number: 150

Question 1.
What are the different ways in which individuals with a particular trait may increase in a population ?
Answer:
Different ways in which individuals with a particular trait may increase in a population are as follow :

1. If it gives the benefit of survival through natural selection.
2. Due to a sudden increase in a particular trait in a population, i.e., by genetic drift.

Question 2.
Why are traits acquired during the life-time of an individual not inherited ?
Answer:
The traits acquired during the life-time are changes in the non-reproductive cells of the organisms and are not capable of being passed on to the next generation.

Question 3.
Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics ?
Answer:
The small numbers of surviving tigers are a cause of worry from the point of view of genetics because in tigers there are negligible genetic variations. Due to this they are not well adapted. The rapid environmental changes cannot be favouable for them. If these changes are not controlled, tigers would be wiped out.

Page Number: 151

Question 1.
What factors would lead to the rise of a new species ?
Answer:
The factors that would lead to the rise of a new species are the following :

1. Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of pupulation.
2. Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
3. Variations caused in individuals due to a natural selection.

Question 2.
Will geographical isolation be a major factor in the speciation of a self- pollinating plant species ? Why or why not ?
Answer:
The geographical isolation cannot be major factor in the speciation of a self-pollinating plant species because it does not have to look the plants for its process of reproduction to be carried out.

Question 3.
Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ?
Answer:
Geographical isolation cannot be a major factor in the speciation of an asexually reproducing organism because it does not require any other organism to carry out reproduction.

Page Number: 156

Question 1.
Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Answer:
If similar characteristics are shown in different organisms, then these are considered to be inherited from the common ancestry. It also shows the closeness of the species.
For example, bats and birds have some similarity in their wings, so they are closely related, while lizard and squirrel do not have wings so these are not closely related to the birds and bats.

Question 2.
Can the wing of a butterfly and the wing of a bat be considered homologous organs ? Why or why not ?
Answer:
The wings of a butterfly and the wings of a bat cannot be considered to be homologous organs because they have different basic designs though they are used for the same purpose of flying. They are analogous organs.

Question 3.
What are fossils ? What do they tell us about the process of evolution ?
Answer:
Fossils : Fossils are the remains or traces of a dead organism. These are formed through the formation of sedimentary rocks. They provide following information on the process of evolution.

1. They tell about the changes that occured on the earth’s surface and the corresponding organisms.
2. They tell about the gradual development of complex structured organisms from simple structured organisms.
3. It is known through them that birds are evolved from reptiles.
4. They state that angiosperms are developed from pteriodophytes and gymnosperms.
5. They exhibit the process of humana evolution.

Page Number: 158

Question 1.
Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species ?
Answer:
This is because although genetic make up of humans may be slightly different in different races of people, there is no reproductive isolation. Reproductive isolation differentiates one species from the other. Human beings different in size, colour and looks can marry among themselves and produce fertile offspring.

Question 2.
In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design ? Why or why not ?
Answer:
Bacteria is a primitive organism as they came into being very early in evolution. But these organisms are still surviving in the present conditions after millions of years. This is because they have adapted well to the changing environment over these years. Same is the case for all other organisms like spiders, fishes and chimpanzees which have adapted to their environment and have survived. Therefore, all the organisms which exist have a body design which is better as it is suited to their environment.

NCERT Solutions for Class 10 Science Chapter 9 Textbook Chapter End Questions

Question 1.
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short.
This suggests that the genetic make-up of the tall parent can be depicted as:
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer:
(c) TtWW

Question 2.
An example of homologous organs is :
(a) our arm and a dog’s fore-leg
(b) our teeth and an elephant’s tusks
(c) potato and runners of grass
(d) all of the above
Answer:
(d) All of the above

Question 3.
In evolutionary terms, we have more in common with :
(a) a Chinese school-boy
(b) a chimpanzee
(c) a spider
(d) a bacterium
Answer:
(a) A Chinese school-boy

Question 4.
A study found that children with light coloured eyes are likely to have parents with light coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive ? Why or why not ?
Answer:
This information is not complete. On the basis of this, it cannot be decided light colour trait is dominant or recessive. So it cannot be said until one does not know the nature of this trait in the parents.

Question 5.
How are the areas of study-evolution and classification interlinked ?
OR
‘Two areas of study namely ‘evolution’ and ‘classification’ are interlinked”. Justify this statement. [AICBSE 2016]
Answer:
Classification of organisms is based on relative similarities and differences among organisms. Resemblances in organisms are because they have arisen from a common ancestor and differences in them are due to adaptations to different types of environment. Since the organisms can be graded in order of increasing complexity it indicates at the concept of evolution.

Question 6.
Explain the terms analogous and homologous organs with examples. [CBSE 2011,2013, 2014]
Answer:
Analogous organs : Those organs which have different basic structure (or different basic design) but have similar appearance and perform similar functions are called analogous organs.
For example, The wings of an insect and a bird are analogous organs.

Homologous organs :  Those organs which have the same basic structure (or same basic design) but different functions are called homologous organs.
For example, The wing of a bat, flipper of a seal, front leg of a horse and arm of a man are homologous organs.

Question 7.
Outline a project which aims to find the dominant coat colour in dogs.
Answer:
Suppose a black homozygous male is mated with a white homozygous female. If the progeny has all black dogs then the dominant coat colour is black.

Question 8.
Explain the importance of fossils in deciding evolutionary relationships.
Answer:
Fossils play important role in providing evolutionary evidences because by knowning the age of fossils we can know about the evolution process of an organism.
For example, a fossil bird called archaeopteryx that looked like a bird had many other features of reptiles. It had feathered wings like those of birds, but teeth and tail like those of reptiles. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles.

Question 9.
What evidence do we have for the origin of life from inanimate matter ? [CBSE 2011, 2014]
Answer:
A British scientist J.B.S. Haldane at first in 1929 suggested that life is originated from inanimate matter. According to him life must have developed from the simple inorganic molecules which were present at that time. Later, Miller and Urey in 1953 presented its evidences. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like methane, ammonia and hydrogen sulphide, etc. over water. This was maintained at a temperature just below 100°C and electric sparks were then passed through the mixture of gases to stimulate lightning for about one week. At the end of one week, it was found that about 15 per cent of carbon (from methane) had been converted into simple compounds and amino acids which make up protein molecules formed in living organisms. This experiment provides the evidence that the life originated from inanimate matter (or lifeless matter) like inorganic molecules.

Question 10.
Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually ? [CBSE 2011,2014]
Answer:
During sexual reproduction there is ‘crossing over’ of chromosomes, that gives rise to variations. These variations are inherited and increase the chances of survival of an organism.

1. In sexual reproduction variations may occur due to errors in DNA copying.
2. There may be variations due to interchange of homologous chromosomes during crossing over of male and female.
3. In sexual reproduction, it is not predetermined that which gamete would fuse with another gamete. It depends only on chance. It is also a reason of variation.
   These variations enable the organisms to adapt themselves to the changing conditions and also help to give rise to new species.

Question 11.
How is the equal genetic contribution of male and female parents ensured in the progeny ? [CBSE 2011, 2013]
Answer:
Genetic material in most organisms is present in pairs of chromosomes. Gametes in the sexually reproducing organisms are formed by the process of meiosis during which half of the genetic material goes into each gamete. When the gametes from male and female parents fuse with each other during sexual reproduction, the normal complement is restored. Half of the genetic material comes from the female and half from the male.

Question 12.
Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement ? Why or why not?
Answer:
Yes, variations that confer an advantage to an individual organism are inherited. The organism can survive longer in an environment and maintain its existence in the population.

NCERT Solutions for Class 10 Science Chapter 9 Heredity and Evolution

Heredity and Evolution: Heredity; Mendel’s contribution- Laws for inheritance of traits, Sex determination : brief introduction; Basic concepts of evolution.

Page 143

Question 1.
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% Of the same population, which trait is likely to have arisen earlier ?
Answer: As species are asexually reproducing, there would be only very minor differences generated due to small inaccuracies in DNA copying, so trait B, which exists in 60% of the same population may get inherited earlier while trait A, which exists in 10% of the population may be originated late due to variations. Thus, trait B have arisen earlier since it is present in 60% of the same population.

Question 2.
How does the creation of variations in a species promote survival ?
Answer: Natural selection selects the individuals having useful variations which ensure their survival in the prevailing conditions of environment. Variant individuals that can withstand or cope with prevailing environment will survive better and will increase in number through differential reproduction.

Page 147

Question 1.
How do Mendel’s experiments show that traits may be dominant or recessive ?
Answer:
Mendel took pea plants with contrasting characteristics – tall plant and dwarf (short) plant. On cross pollination, he got all tall plants in F1 generation. Then by self pollination of F1 tall plants, he produced second generation (F2) consisting of tall and short plants in the ratio of 3 : 1. Then he concluded that, ‘T’ (tall) trait is dominant while ‘t’ trait for shortness is recessive.

Question 2.
How do Mendel’s experiments show that traits are inherited independently ?
Answer:
In a dihybrid cross made by Mendel, it was observed that when two pairs of traits or characters were considered; each trait expressed independent of the other. Thus, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits.

Question 3.
A man with blood group A marries a woman with blood O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O is dominant ? Why or why not ?

Answer:
No. This information is not sufficient to determine which of the traits − blood group A or O − is dominant. This is because we do not know about the blood group of all the progeny.Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.

Question 4.
How is the sex of the child determined in human beings?

Answer:
The females carry two X-chromosomes. Females produce one type of gametes (eggs) with same type of chromosomes (22 + X). Males have one X and one Y- chromosome. Among the male gametes, half of the sperms carry X-chromosome (22 X) and half
carry Y-chromosome (22 + Y). Thus, female is homogametic and male is heterogametic. When a sperm carrying X- chromosome fertilises an egg, the zygote develops into female (XX condition). When sperm carrying Y-chromosome fertilises an egg, the zygote develops into a male (XY condition). Thus, sex is determined at the time of fertilisation.

Page 150

Question 1.
What are the different ways in which individuals with a particular trait may increase in a population ?
Answer:
Different ways are : variation, natural selection and genetic drift (isolation).

Question 2.
Why are traits acquired during the lifetime of an individual not inherited ?
Answer:
Because acquired characters bring changes only in non-reproductive tissues and cannot change the genes of the germ cells. Thus, acquired traits cannot be passed to next generation.

Question 3.
Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics ?
Answer:
(i) If any natural calamity occurs and kills these small number of surviving tigers, they can become extinct resulting in the loss of some genes forever.
(ii) Small number will lead to little recombination and, therefore, lesser variations. These both are very important for giving better survival chances to the species.
(iii) Less number of species means lesser extent of diversity and lesser number Of traits which reduces the chances of adaptability with respect to the change in the environment.

Page 151

Question 1.
What factors could lead to the rise of a new species ?
Answer:
Genetic variations, natural selection and reproductive isolation could lead to the rise of a new species.

Question 2.
Will geographical isolation be a major factor in the speciation of a self-pollinating plant species ? Why or why not ?
Answer:
No, because pollination occurs on the same plant in self-pollinating plant species.

Question 3.
Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ?
Answer:
No, because asexual reproduction involves single parent or organism.

Page 156

Question 1.
Give an example of characteristics being used to determine how close two species am in evolutionary terms ?
Answer:
Homologous organs, analogous organs and vestigial organs help to identify evolutionary relationships amongst the species.

Question 2.
Can the wing of butterfly and the wing of a bat be considered homologous organs ? Why or why not ?
Answer:
No, wing of a bat and wing of a bird cannot be considered as homologous organs because they have different basic structure.

Question 3.
What are fossils ? What do they tell us about the process of evolution ?
Answer:
Fossils are the impression or remains of ancient life found preserved in the sedimentary rocks. Fossils are direct evidences of evolution. Fossils also help to identify evolutionary relationship between apparently different species. They also tell about the extent of evolution that has taken place.

Page 158
Question 1.
Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species ?
Answer:
They look different because of interaction of genes with environment which results in change in their appearance. But they belong to the same species as they have same number of chromosomes and can breed among themselves.

Question 2.
In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better body design’ why or why not ?
Answer:
No, because different designs are the product of evolution and different species have different body design to suit or adapt to their environment.

Page 159

Question 1.
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing whfte flowers. The progeny all bore violet flowers, but almost half of them are short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer:
(c) Genetic make-up of tall plant can be depicted by TtWW.

Question 2.
An example of homologous organs is
(a) our arm and a dogs fore-leg.
(b) our teeth and an elephants tusks.
(c) potato and runners of grass.
(d) All of the above.
Answer:
(d) Both organs in all options have same basic structural design but have different functions and appearance.

Question 3.
In evolutionary terms, we have more in common with
(a) a Chinese school-boy.
(b) a chimpanzee.
(c) a spider.
(d) a bacterium.
Answer:
(a) A Chinese school-bpy is also a human being.

Question 4.
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer:
We can say that light eye colour trait is dominant because only dominant traits are expressed in the first generation.

Question 5.
How are the areas of study – evolution and classification— inteilinked?
Answer:
Evolution and classification are interlinked with each other in many ways. Classification is the most important term to explain evolution. It is based on the similarities and differences between two species or among two organisms. More closer the characteristics, the moe doser is the evolution and chances to be in the same group of classification. Thus, the classification of species is a reflection of their evolutionary relationship.

Question 6.
Explain the terms analogous and homologous organs with examples.
Answer:
Analogous organs are those organs which have different basic structural designs and developmental origins but have similar appearance and perform similar functions.
Examples:
Wings of an insect and wings of a bat.
Homologous organs are those organs which have the same basic structural design and developmenta’ origin but have different functions and appearance.
Examples: Forelimbs of frog and forelimbs of human.

Question 7.
Outline a project which alms to find the dominant coat colour in dogs.
Answer:
A homozygous black (RB) male dog and a homozygous white (bb) female dog is taken and given to mate and produce offspring in F1 generation. If black colour is dominant out of every 4 dogs, 3 will be black and if white colour is dominant 3 out of 4 dogs will be white.

Question 8.
Explain the importance of fossils in deciding evolutionary relationships.
Answer:
Fossils and their study is useful to know about the species which are no longer alive. They provide evidence and missing links between two classes. They are helpful in forming a sequence of organisms in the pathway of evolution. Thus, fossils have importance in deciding evolutionary relationships.

Question 9.
What evidence do we have for the origin of life from inanimate matter?
Answer:
Stanley L. Miller and Harold C. Urey provided evidence regarding origin of life from inanimate matter. They assembled an atmosphere similar to that existed on early earth. The atmosphere had molecules like ammonia, methane, hydrogen sulphide and water, but no oxygen. The mixture was maintained at a temperature just below 100◦C and sparks were passed through the mixture of gases. At the end of a week, 15% carbon from methane had been converted to simple compounds of carbon like aminoacids which make up protein molecules. So, life arose afresh on earth.

Question 10.
Explain how sexual reproduction gives rise to more viable variations that asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually ?
Answer:
Variations occurring during sexual reproduction may be due to:

1. Separation of homologous chromosomes (by chance only) during gamete formation.
2. Crossing over (recombination) of homologous chromosomes.
3. Fertilisation of gametes to form zygote.
4. Errors during DNA copying or mutations.

In asexually reproducing organisms only errors during DNA copying or mutations cause variations.
Since the extent of variations is much larger in sexually reproducing organisms, therefore, the chances of evolution is also much in sexually reproducing These variations enable the organisms to adapt themselves to the changing conditions and also help to face the struggle for Over the time, they and rise to new species.

Question 11.
How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer:
Genetically organisms are of types

(i) Haploid : They have single set of chromosomes, where each chromosome is represented singly. As the chromosomes are the bearer of genes so haploids have single set of genes. A single gene determines the expression of character.
(ii) Diploid : ‘They have two sets Of homologous chromosomes, where the chromosome occur in pair, one maternal contributed by the mother through her ovum and the second Of the pair is contributed by the male parent through his sperm. The resultant cell zygote produces by the fusion of male and female gametes have two sets of chromosomes –  each set contributed’ by each parent. In diploids a character is controlled by two genes/factors. Both the father and mother contribute practically equal amount of genetic material to the child. It means that each trait can be influenced by both paternal and maternal DNA.

Question 12.
Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement ? Why or why not ?
Answer:
No, many of the times the variations are not advantageous to an individual organism but still survive in a population, e.g., take the case of free ear lobe and attached ear lobe. Most of the other variations not only give survival advantage to an individual but also contribute to genetic drift. Thus, we can say that most of the variations lead to better adaptation of an organism to the changing environment. In this way, it gives survival advantage to that organism and will also survive in the coming population.

Multiple Choice Questions (MCQs) [1 Mark each]

Question 1.
An example of homologous organs is [NCERT]
(a) our arm and a dog’s foreleg
(b) our teeth and an elephant’s tusks
(c) potato and runners of grass
(d) All of the above
Answer:
(a) Our arm and a dog’s foreleg is the example of homologous organs.

Question 2.
The science, which deals with study of heredity and variations is called
(a) phylogeny
(b) embryology
(c) genetics
(d) palaeontology
Answer:
(c) The genetics is the study of heredity and variations and includes their occurrence, causes, benefits, disadvantages, significance, etc.

Question 3.
Archaeopteryx is a connecting link between
(a) reptiles and aquatic animals
(b) birds and insects
(c) reptiles and birds
(d) birds and dinosaurs
Answer:
(c) Archaeopteryx is a connecting link- between the reptiles and birds. It appears like a bird, but has other features which are present in reptiles, e.g. it has wings like bird, but teeth and tail like the reptilians.

Question 4.
For palaeontological studies a scientist will gather the evidences from
(a) study of homology
(b) study of analogy
(c) fossils
(d) All of these
Answer:
(d) Study of homologous and analogous organs indicates the origin and modification in organisms and study of fossils indicates the age and features of an organism.

Question 5.
In evolutionary terms, we have more in common with [NCERT]
(a) a Chinese school boy
(b) a chimpanzee
(c) a spider
(d) a bacterium
Answer:
(a) Chinese school boy because both of us belong to the same species, i.e. Homo sapiens.

Question 6.
Aditya was observing some organisms in lab and tried to compare them. The presence of which organs will confirm to him that they share evolutionary history?
(a) Analogous organs
(b) Paralogous organs
(c) Homologous organs
(d) None of these
Answer:
(c) Homologous organs are present in organisms who share evolutionary history. However, these organs perform different functions in different organisms.

Question 7.
New species may be formed if
I. DNA undergoes significant changes in germ cells. .
II. chromosome number changes in the gamete.
III. there is no change in the genetic material.
IV. mating does not take place.
(a) I and II
(b) I and III
(c) II, III and IV
(d) I, II and III
Answer:
(a) New species may be formed if the DNA changes are severe enough, such as a change in the number of chromosome. This leads to new variations.

Question 8.
Which of the following statements is not true with respect to variation?
(a) All variations in a species have equal chance of survival.
(b) Change in genetic composition results in variation.
(c) Selection of variants by environmental factors forms the basis of evolutionary processes.
(d) Variation is minimum in asexual reproduction.
Answer:
(a) All variations in a species do not have equal chances of survival. Some of the variations may be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such, a newborn cell dies soon.

Question 9.
Select the statement that describes characteristics of genes. .
(a) Genes are specific sequence of bases in a DNA molecule.
(b) A gene does not code for proteins.
(c) In individuals of a given species, a specific gene is located on a particular chromosome.
(d) Each chromosome has only one gene.
Answer:
(b) Genes are stretches of DNA found on chromosomes of a cell. A gene contains information for making proteins in a cell. A specific gene is located on a particular chromosome in individuals of a given species.

Question 10.
If a round, green seeded pea plant (RRyy) is crossed with wrinkled, yellow seeded pea plant (rrYY), the seeds produced in F1 – generation will be [NCERT Exemplar]
(a) round and yellow
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow
Answer:
(a) The cross between RRyy and rrYY seeds will produce RrYy (round and yellow) seeds in F1-generation, because round and yellow are the dominant traits.

Question 11.
From the list given below, select the character, which can be acquired but not inherited. [NCERT Exemplar]
(a) colour of eye
(b) colour of skin
(c) size of body
(d) nature of hair
Answer:
(c) Acquired traits develop in response to the environment. The size of the body is an acquired trait because it can vary based on the availability of less or more food. The other three colour of eye and skin and nature of hair are characters inherited from the parents.

Question 12.
According to the evolutionary theory, formation of a new species is generally due to [NCERT Exemplar]
(a) sudden creation by nature.
(b) accumulation of variations over several generations.
(c) clones formed during asexual reproduction.
(d) movement of individuals from one habitat to another
Answer:
(b) Accumulation of variations over several generations forms new species. Genetic drift accumulates different changes in sub-populations of a species. Also, natural selection may also operate differendy in the different geographic locations. Eventually, different groups of new species will be formed.

Question 13.
Select the incorrect statement. [NCERT Exemplar]
(a) Frequency of certain genes in a population changes over several generations resulting in evolution.
(b) Reduction in weight of the organism due to starvation is genetically controlled.
(c) Low weight parents can have heavy weight progeny.
(d) Traits which are not inherited over generations do not cause evolution.
Answer:
(b) The weight reduction due to starvation will not change the DNA of the germ cells, because low weight is not a trait that is genetically controlled or inherited. Also, low weight parents may have heavy weight progeny.

Question 14.
In human males all the chromosomes are paired perfectly except one. This/these unpaired chromosome is/are
I. large chromosome
II. small chromosome
III. Y-chromosome IV X-chromosome
(a) I and II
(b) Only III
(c) III and IV
(d) II and IV
Answer:
(c) In human males, one pair called the sex chromosomes are unpaired. Here, one is a normal-sized X-chromosome while other is a short Y-chromosome. Women have a perfect pair of sex chromosomes, both called X.

Question 15.
Rajneesh was studying the fossils of two different types, fossil A was found in upper layer of Earth and B in deeper layers. What can be predicted regarding the age of these fossils?
(a) A has recently become extinct
(b) B has become extinct recently
(c) The time of extinction cannot be determined
(d) None of the above
Answer:
(a) Since, fossil A was found in upper layer of earth, it suggests that the organism has become extinct recently. Fossil B found in deeper layer must have become extinct long time ago and deposition of other layers occurred over it during this period.

Question 16.
A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic makeup of the tall parents can be depicted as [NCERT]
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer:
(c) Parent with genotype TtWW produce two types of gametes TW and tW, while the other with genotype ttww produce only one type of gamete W.

CBSE Class 10 Science Notes Chapter 9 Heredity and Evolution

Heredity and Inherited Traits: Mendel’s Experiment; Sex determination.
Heredity refers to the transmission of characters from parents to offsprings. An inherited trait is a particular genetically determined feature that distinguishes a person from the others for example; attached or free ear lobes in human beings.

Rules for the inheritance of traits:
Mendel’s contribution: The rules for inheritance of traits in human beings are related to the fact that both mother and father contribute an equal amount of genetic material i.e. DNA to their offspring. So an offspring will get two versions of that trait from the two parents. Mendel worked out rules for inheritance of these traits. Gregor Johann Mendel regarded as the ‘Father of Genetics’ performed his experiments with garden peas (Pisum sativum) in the garden behind his monastery. He observed a number of contrasting characters in garden peas and observed their inheritance.

Some important terms
1. Chromosomes are long thread-like structures present in the nucleus of a cell which contain hereditary information of the cell in the form of genes.

2. DNA is a chemical in the chromosome which carries the traits in a coded form.

3. Gene is the part of a chromosome which controls a specific biological function.

4. Contrasting characters: A pair of visible charactes such as tall and dwarf, white and violet flowers, round and wrinkled seeds, green and yellow seeds etc.

5. Dominant trait: The character which expresses itself in a (Ft) generation is dominant trait. Example : Tallness is a dominant character in pea plant.

6. Recessive trait: The character which does not express itself but is present in a generation is recessive trait. Ex. dwarfism in the pea plant.

7. Homozygous: A condition in which both the genes of same type are present for example; an organism has both the genes for tallness it is expressed as TT and genes for dwarfness are written as tt.

8. Heterozygous: A condition in which both the genes are of different types for example; an organism has genes Tt it means it has a gene for tallness and the other for dwarfness only tall character is expressed.

9. Genotype: It is genetic make up of an individual for example; A pure tall plant is expressed as TT and hybrid tall as Tt.

10. Phenotype: It is external appearance of the organism for example; a plant having Tt composition will appear tall although it has gene for dwarfness.

11. Homologous pair of characters are those in which one member is contributed by the father and the other member by the mother and both have genes for the same character at the same position.

Mendel’s Experiment: Mendel started his experiment on the pea plants. He conducted first monohybrid and then dihybrid crosses.

Monohybrid Cross: The cross in which Mendel showed inheritance of dominant and recessive characters is monohybrid cross. To observe inheritance of single pair of contrasting characters

he took pure tall (genotype TT) and pure dwarf (genotype tt) pea plants and cross pollinated them to obtain first generation or first filial generation. In this figuration (F1 generation) he obtained only tall plants. This meant that only one of the parental traits was seen, not the mixture of the two. The plants of F generation or progeny are then self pollinated to obtain F2 generation or progeny. Now all plants were not tall. He obtained 75% tall plants and 25% dwarf plants i.e. the phenotypic ratio was 3:1. This indicates that in the F, generation both tall and dwarf traits were inherited but tallness expressed it self. Tallness is a dominant trait and dwarfness is a recessive trait. F2 generation has a genotypic ratio of 1 : 2 : 1 of three types of plants represented by TT, Tt and tt as shown in the cross.

Conclusion: Phenotypic ratio—Tall : Dwarf 3 : 1
Genotype ratio—Pure Tall : Hybrid Tall : Pure Dwarf 1 : 2 : 1

Law of Dominance: When parents having pure contrasting characters are crossed then only one character expresses itself in the Ft generation. This character is the dominant character and the character/factor which cannot express itself is called the recessive character.

Dihybrid Cross: Mendel also carried out experiments to observe inheritance of two pairs of contrasting characters, which is called dihybrid cross. He cross breed pea plants bearing round green seed with plants bearing wrinkled and yellow seeds. In the Fx generation he obtained all round and yellow seeds it means round and yellow traits of seeds are dominant features while wrinkled and green are recessive. He self-pollinated the plants of F: generation to obtain F2 generation, he obtained four different types of seeds round yellow, round green, wrinkled yellow and wrinkled green in the ratio of 9 : 3 : 3 : 1. He concluded that traits are independently inherited

Conclusion

• Round and yellow seeds-9.
• Round and green seeds-3.
• Wrinkled and yellow seeds-3.
• Wrinkled and green seeds-1.

How do traits get expressed?
Cellular DNA is the information source for making proteins in the cell.
A part of DNA that provides information for one particular protein is called a gene for that protein for example; the height of a plant depends upon the growth hormone which is in turn controlled by the gene. If the gene is efficient and more growth hormone is secreted the plant will grow tall. If the gene for that particular protein gets altered and less of it is secreted when the plant will remain short. Both the parents contribute equally to the DNA of next generation during sexual reproduction. They actually contribute a copy of the same gene for example; when tall plant is crossed with short plant the gametes will have single gene either for tallness or for shortness. F1 generation will get one gene for tallness and other for shortness also.

How do germ cells i.e. gametes get single set of genes from parents who have two copies in them ?
Each gene set is present, not as a single long thread of DNA, but as separate independent pieces each called a chromosome. Each cell gets two copies of the chromosome, one from each parent. Each germ cell or gamete has one copy of it because there is reductional division in the sex organs at the time of formation of gametes. When fertilization takes place normal number of chromosomes is restored in the progeny ensuring the stability of DNA of the species.

How is the sex of a newborn individual determined?
It is the process by which sex of a newborn can be determined.

Different species use different strategies for this :

• In some animals the temperature at which fertilized eggs are kept determines whether the developing animals will be males or females.
• Some animals like snails can change sex indicating that sex is not genetically determined.
• In human beings sex of the individual is determined genetically; means genes inherited from the parents decide the sex of the offspring.

Sex determination in human beings: In human beings, all chromosomes are not paired. 22 chromosomes are paired but one pair called sex chromosome is odd in not having a perfect pair in males. Females have a perfect pair both represented by XX. On the other hand males have a normal sized X but the other is short called Y so it is shown as XY. All gametes or ova formed by the homogenetic female are similar i.e. have X chromosome. Males heterogenetic form two types of sperms i.e. half with X chromosome and the other half with Y chromosome. Sex of the baby will depend on fertilization. There are two possibilities :

Autosomes: Those chromosomes which do not play any role in sex determination.

Sex chromosomes: Those chromosomes which play a role in determining sex of the newborn.

• If the sperm having X chromosome fertilizes with ovum with X chromosome then the baby will have XX chromosome and it will be female.
• If the sperm having Y chromosome fertilizes with ovum with X chromosome then the baby will have XY chromosomes and it will be male.

Evolution: Acquired and inherited traits, Speciation, Evolution and classification, Evolution by stages, Human evolution.

Evolution: It is the sequence, of gradual, irreversible changes which took place in the primitive organisms over millions of years to form new present-day species. Variations that resulted in formation of new species occurred basically due to errors in DNA copying as well as due to sexual reproduction.

An Illustration to show variations in a population: A group of twelve red beetles live in green bushes and reproduce sexually so are likely to develop variations. There are the following possibilities

First situation: Crows eat these beetles as they can easily pick up red ones in the green bushes There is a colour variation during sexual reproduction and green beetles appears, it reproduces and its population increases. Crows are not able to see green beetles so their population continues to increase but that of red beetles decreases. This type of variation gives a survival advantage.

Second situation: Due to a colour variation few blue beetle appear forming blue population. Crows can see both red and blue and eat them. Initially there are more of red beetles and less of blue. There is sudden calamity, an elephant kills red beetles by stamping on bush, blue beetles survive reproduce and increase in number. In this case there is no survival advantage but provides diversity without any adaptation.

Third situation: As the population of beetles increases, the bushes suffer from a disease and the availability of food for beetles decreases. The size of beetles decrease but after a few years as the plant disease is eliminated and enough food is available for the beetles they come back to their normal size. This type of change is not inherited.

Acquired Traits: Acquired traits are those which are not inherited over generations as they are caused due to change in the non-reproductive tissue and are not passed on the DNA of the germ cells for example; the size of the beetles in the population decreased due to scarcity of food.

Inherited Trait: Inherited traits are caused due to changes in the DNA of germ cells which are inherited from generation to generation, for example; formation of green beetles in the population of red beetles.

Acquired Traits and Inherited Traits

Acquired Traits	Inherited Traits
(i) These are the traits which are developed in an individual due to special conditions.	(i) These are the traits which are passed from one generation to the next.
(ii) They cannot be transferred to the progeny.	(ii) They get transferred to the progeny.
(iii) They cannot direct evolution, e.g. low weight of starving beetles.	(iii) They cannot direct evolution, e.g. low weight of starving beetles.

Charles Darwin’s Idea of Evolution: His concept of evolution was based on the idea that new species were formed due to variations that occurred in the organisms Nature played an important role in selecting the organisms having suitable variations.

Speciation: It means the development of one or more species from an existing species The factors that could lead to rise of a new species are :

Gene flow: It means the exchange of genetic material by interbreeding between populations of the same species or between individuals within a population. It increases the variation in the genetic composition of a population.

Genetic drift: It is random change in the frequency of alleles in a populate over successive generation due to errors in the gametes. The process is rapid in smaller population. Genetic drift can lead to accumulation of changes in the generations.

Natural selection: According to Darwin, natural selection also plays an important role in bringing about evolution of new species of plants and animals. According to him variations existed between the individuals of a population and some natural phenomena eliminated those individuals which were less adapted. The surviving population would pass the hereditary advantageous features to their offsprings. With time this process would give rise to organisms different from the original population and new species are formed.

Isolation: When a population of a species splits into two, it cannot reproduce with each other and forms a new species, for example; when a population of beetles feed on bushes on a mountain range, some may start feeding on nearby bushes finding entry into a new subpopulation. They reproduce with them so genes enter in a new population. Ultimately the two groups will be incapable of reproducing with each other and new species will be formed.

Evolution and Classification: The organisms show certain features, like appearance and behaviour which are called characteristics for example; Plants can perform photosynthesis. The basic characteristics are shared by a large number of organisms. More characteristics which two species have in common more closely are related, if they are more closely related then they have common ancestors (explain the example of brother sister and cousins). Evolutionary relationships can be traced with the help of the following :

Homologous organs: Those organs which have the same basic structural design and developmental origin but perform different functions and appearance, for example; Forelimbs of frog, lizard, bird, bat and human beings. They have same design of bones but they perform different functions.

Analogous organs: Those organs which have different basic design and developmental origin but have similar appearance and perform a similar function, for example; wings of bat and bird. Wings of bat are folds of skin attached between fingers. But wing of birds are modified forelimbs.

Study of Fossils: Fossils are preserved remains of living organisms that lived in the past. When living organisms die their bodies decompose but some parts of their body may be in such an environment that they do not decompose for example; if a dead insect gets caught in hot mud it will not decompose quickly but the mud will harden and retain impressions of the body parts of the insects. These impressions are also called fossils: The age of fossil can be estimated in two ways :
The fossils that occur closer to earth surface are more recent to those found in deeper layers.
The second method is isotope dating i.e. detecting the ratio of different isotopes of the same element in the fossil material.

Significance of fossils: Fossils are formed layer by layer in the earths crust. The animals and plants which existed earlier are buried in the deeper layer which ones found in the upper layer. It is found that, deeper fossils have simpler structure than found than upper layer. Complete fossil record of animals like horse, camel, man has helped us to study the stages of evolution.

Evolution by stages: Evolution is a continuous and gradual process, complicated organs did not evolve by a single DNA change but were formed by bit by bit change over generations for example; complex organs like eyes were created by bit by bit changes, in between the rudimentary eye in some insects also provided a fitness advantage. The structure of eye in all organisms is different enough to have evolutionary origins. Some organs even developed for one particular function but later become useful for quite a different function, e.g Feathers developed to provide warmth to the animal but later helped in flight.

Some dinosaurs had feathers although they could not fly, this shows that birds are closely related to reptiles, since dinosaurs were reptiles Some dissimilar looking structures also evolved from common ancestors. The current example of such a process is wild cabbage plant from which different vegetables are generated by artificial selection rather than natural selection

• Selection of short distance between the leaves has led to formation of cabbage that, we eat.
• Selection for arrested flower development had led to broccoli,
• Selection for sterile flowers had made cauliflower,
• Selection for swollen-stem had formed kohlrabi.
• Selection for large leaves had formed leafy vegetable kale,
• Selection for colored leaves formed red cabbage.

To sum up we can say that evolutionary relationships can be established by

• Study of Homologous organs
• Study of Analogous organs
• Study of fossils
• Changes in DNA during reproduction

Evolution versus Progress: Evolution can not be called progress from lower forms to higher forms. It is basically forming more complex designs while the simpler once also keep growing. Evolution is generation of diversity with the help of environmental selection. Bacteria which were formed first have the capacity to live in diverse conditions and are still flourishing; on the other hand human beings which are highly evolved species can not be called the pinnacle of evolution but yet another species in the evolving life forms.

Human Evolution: Human evolution has been studied with the help of excavation; time dating and fossil study All human beings belong to single species i.e. Homo sapiens. Human species have come from Africa. Some of our ancestors left Africa while others stayed on. These migrants slowly spread across the planet i.e. West Asia, Central Asia, Eurasia, South Asia and East Asia They traveled to Indonesia, the Philippines, Australia and America They traveled forward and backward sometimes separating and sometimes coming back to mix with each other. They had come into being as an accident of evolution.

Although there is a great diversity of human forms all over the world get all humans are single species.

• They didn’t go in a single line.
• They went forward and backward.
• Moved in and out of Africa.
• Sometimes come back to mix with each other.

Genetics: Branch of science that deals with heredity and variation.

Heredity: It means the transmission of features/characters/traits from one generation to the next generation.

Variation: The differences among the individuals of a species/population are called variations.

Mendel and his work on Inheritance.
Gregor Johann Mendel started his experiments on plant breeding and hybridisation. Mendel was known as Father of Genetics.
The plant selected by Mendel was Pisutn sativum (garden pea). Mendel used a number of contrasting characters for garden pea.

Sex Determination: Phenomenon of decision or determination of sex of an offspring.

Factors Responsible for Sex Determination:

• Environmental: In some animals, the temperature at which the fertilised eggs are kept decides the gender. Example, in turtle.
• Genetic: In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosomes (XX – female; XY – male).

Sex Chromosomes: In human beings, there are 23 pairs of chromosomes. Out of these 22 chromosome pairs are called autosomes and the last pair of chromosomes that help in deciding the gender of that individual are called sex chromosome.
XX – female; XY – male
The cross done shows that half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus sex of children will be determined by what they inherit from their father, and not from their mother.

Acquired Traits:

• These are the traits which are developed in an individual due to special conditions.
• They cannot be transferred to the progeny.
• They cannot direct evolution, for example, the low weight of starving beetles.

Inherited Traits:

• These are the traits which are passed from one generation to the next.
• They get transferred to the progeny.
• They are helpful in evolution, for example, the colour of eyes and hair.

Microevolution: It is the evolution which takes place on a small scale. Example, change in body colour of beetles.

Speciation: It is the process of formation of new species. A species is a group of similar individuals that belong to a population that can interbreed and produce fertile offspring. Speciation takes place when the variation is combined with geographical isolation.

Gene flow: It is the exchange of genetic material by interbreeding between populations of the same species or individuals. Gene flow occurs between populations that are partly but not completely separated.

Genetic Drift: It is the random change in the frequency of alleles (gene pair) in a population over successive generations.
Genetic drift takes place due to:

• severe changes in the DNA.
• change in the number of chromosomes.

Natural Selection: The process by which nature selects and consolidates those organisms which are more suitably adapted and possesses favourable variations.

Evolution and classification. Both evolution and classification are interlinked.

• Classification of species is a reflection of their evolutionary relationship.
• The more characteristics two species have in common the more closely they are related.
• The more closely they are related, the more recently they have a common ancestor.
• Similarities among organisms allow us to group them together and to study their characteristics.

Tracing Evolutionary Relationships:

• Homologous Organs: Morphological and anatomical evidences. These are the organs that have same basic structural plan and origin but different functions.
  Example, forelimb of a horse (running), wings of bat (flying), paw of a cat (walk/ scratch/ attack) — same basic structure but different functions.
• Analogous Organs: These are the organs that have different origin and structural plan but same functions.
  Example, wings of a bat (elongated fingers with skin folds), wings of bird (feathery covering along the arm) — different structures but same functions.
• Fossils: The remains and relics of dead organisms that lived in the remote past. Fossils provide evidence of evolution. Example, a fossil called Archaeopteryx has feathered wings like birds but teeth and tail like reptiles hence suggesting that birds and reptiles had a common ancestor.

Artificial Selection: Humans have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artificial selection. Example, wheat (many varieties obtained due to artificial selection).

1. Heredity : It refers to the transmission of characters or traits from the parents to their offspring. Heredity is the continuity of features from one generation to another which are present in fertilised egg or zygote. The zygote develops into an organism of a particular type only.

2. Genetics : It is the branch of biology which deals with heredity and variation. Genetics is to help our understanding of heredity by knowing how offspring inherit characteristics from their parents.

3. Variation : It means the differences in the characters or traits among the individuals of a species. Variations occur during reproduction both because of error in DNA copying and as a result of sexual reproduction. Variations contribute to evolution.
Causes of variations:

• Different combinations of genetic material.
• Some positive gene mutations.
• Interaction of genes with environmental changes (adaptations).

Importance of variations:

• It forms, the. basis of heredity.
• It causes adaptations due to which organism can easily adjust to its changing environment.
• Accumulation of variations forms the basis of evolution.

Remember!
Variations are produced both in sexual and asexual reproduction but amount of variations produced in asexual reproduction are subtle (so little) that they are hardly noticeable as compared to variations caused due to sexual reproduction.

4. Genotype : The genetic constitution of an organism e.g., Genotype of human male is 44 + XY and
genotype of human female is 44 + XX

5. Phenotype : The appearance of the organism, i.e., the way in which genotype is expressed. Phenotype is the result of interaction of genes with the environment.
e.g., Red colour may be controlled by a pair of genes RR. Now if genotype is RR phenotype will be red only but if genotype is Rr then also phenotype will be red since R is a dominant gene.

6. Gene : It is the basic unit of inheritance by which characters are transferred from parents to their offspring. Gene consists of a specific length of DNA on a chromosome. A specific Segment of DNA that provides the information for one protein is called gene for that protein.
According to Mendel, both parents must contribute equally to the DNA of the progeny during sexual reproduction. As both parents determine the trait in the progeny, so both parents must be contributing a copy of the same gene.

7. Chromosomes : These are the long threads present in the nucleus of every cell. Chromosomes are made- up of DNA and protein. Each chromosome contains very long molecule of DNA.
Remember!
Each gene set is present as separate independent pieces each called a chromosome. Each cell have two copies of each chromosome, one each from male and female parents. Every germ cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species. Such mechanism of inheritance is used by all sexually and asexually reproducing organisms.

8. Allele: It is an alternative form of a gene occupying the same position on a chromosome and affecting the same characters but in two alternative ways, e.g., the free and attached ear lobe are the alleles of ear lobe character.
Expressing allele of a gene :

• Homozygous dominant in capital letters, e.g., tallness(TT)
• Homozygous recessive in smalMetters, e.g., shortness or dwarfness (tt)
• Heterozygous (Tt)-lt will be called hybrid tall.

9. Dominant allele: An allele that affects the phenotype of an organism both in heterozygous and homozygous condition. It is denoted by a capital letter, e.g., tallness in pea plant is denoted by ‘T.

10. Recessive allele: An allele that affects the phenotype of the organism in absence of a dominant allele, i.e., in homozygous recessive individuals. It is denoted by a small alphabet, e.g., dwarfness in pea plant is denoted by’t’.

11. Homozygous: When both alleles of a particular gene are the same, e.g., TT

12. Heterozygous : When both alleles of a particular gene are different, e.g., Tt

13. Diploid : Cells or organism containing two sets of genes, e.g., human body cells. Diploid cells have genetic constitution of 2n.

14. Haploid : Cells or organism containing one set of genes, e.g., human reproductive cells (sperms and ova). Haploid cells have genetic constitution of n.

15. Monohybrid cross : A cross between two parents taking the alternative traits of one single character, e.g., A cross between tall and dwarf pea plants.
Monohybrid Ratio :

• In F₁ generation : 100% hybrid
• In F₂ generation : phenotypic ratio is 3 : 1 and genotypic ratio is 1 : 2 : 1

16. Dihybrid cross: A cross between two parents taking into consideration alternative traits of two different characters, e.g., A cross between two pea plants one having round, green seeds and the other having wrinkled, yellow seeds.
Dihybrid Ratio :

• F₁ ratio is 100% Hybrid type.
• F₂ ratio : Phenotypic is 9 : 3 : 3 : 1 and Genotypic . ratio is very complex.

17. Human Blood Groups: There are four types of blood groups A, B, AB or O. These are controlled by a gene which is denoted by symbols I^A, I^B and I^O (sometimes also denoted as i). The genes I^A and I^B show no dominance over each other (they are codominant, i.e., both expresses themselves independently). But these both genes are dominant over the gene I^O. Therefore, blood group of a person depends on the type of genes present, e.g., (i) Blood group A has the following gene types :

18. Determining sex of a newborn individual genetically:

• In human beings the sex of the individual is ” determined genetically.
• There are 23 pairs of chromosomes of which 22 are similar in male and female and are known as autosomes.
• The remaining one is sex chromosome which is XY in males and XX in females.
• Males produce two types of sperms X and Y, while female produces one type of egg X.
• If a X type of sperm fertilizers the egg then the sex of baby will be female (XX).
• If Y type of sperm fertilizers the egg then the sex of the baby will be male (XY).
  

19. Mendel’s experiment to show that traits may be dominant or recessive:

• Mendel conducted breeding experiments in garden pea.
• selected pure plant of a tall/short plant.
• produced first generation plants by crossing them.
• found that all plants were tall.
• produced the second generation by self-fertilization of hybrids.
• found that three-quarter of the plants was tall and one quarter was short.
  

20. Homologous chromosomes: A pair of corresponding chromosomes of the same shape and size, one from each parent.

21. Autosomes and Sex chromosomes : The identical » chromosome pairs are called autosomes. The
chromosome pair which is different are called sex chromosomes. Humans have 23 pairs of chromosomes. 1-22 pairs are autosomes while 23rd pair (XX in females and XY in males) which are designated as X and Y are sex chromosomes.

22. Molecular Phylogeny: It is the study of evolutionary relationships by comparing DNA of different species.

23. Natural selection : Natural selection is one of the basic mechanisms of evolution, along with mutation, migration and genetic drift. Natural selection means the environmental conditions prevailing around an organism against which organism adapts itself, grows – and reproduces further. This leads to a change in the composition of genes within a population further causing evolution. Thus, it can be said that,
Natural selection results in adaptation in population to fit their environment better. Thus, natural selection direct evolution in the population of a particular species.

24. Fossils of the information which they provide regarding evolution: Fossils are the remains of ancient life forms, which got preserved somehow in the layers of earth, snow or oil.
Information given by fossils:

• They reveal that the life forms which existed earlier do not exist today which indicate that the living forms are ever changing (evolving).
• They are used to guess the time when a particular organism existed on earth. It is done through carbon dating.

25. Genetic drift: The change in the frequency of some genes in a population which provides diversity without any survival advantage is called genetic drift.

26. The various ways in which individuals with a particular trait may increase in a population : Differences in population are responsible for the diversity such as, colour of eyes, hair, shape of ear lobes. This occurs due to : (i) Sexual reproduction (ii) Inaccuracies during DNA replication (iii) Due to environmental changes. This diversity will increase with time as these variations can be passed on only through DNA/genes during reproduction through reproductive tissue (germ cells or gametes).

• If these variations give survival advantage, then such traits are selected in nature and such traits increase in a population.
• Due to genetic drift. This occurs due to geographical or reproductive isolation. It results in the change in gene frequency in a particular: population.
• Migration which leads to gene flow in and out of the population.
• The mutation caused due to particular type of environment. ,
• Acquired traits due to particular type of environment.

27. Evidence of evolution: Errors in DNA copying (mutation) and sexual reproduction lead to variations which form the basis of evolution. Characteristics that
are common in different kinds of living organisms provide evidence in favour of evolution.

28.Evolution : Evolution can be defined as a naturally occurring slow, continuous and irreversible process of change. The gradual change of living organisms from pre-existing organisms since the beginning of life is called organic evolution. Whereas, gradual change in elements from one form to another with time is termed as inorganic evolution, i.

29.Inherited traits : are those traits which are passed from one generation to another through specific genes. Any change in DNA of the germ cells will be passed.
30. Acquired traits : are those traits which are acquired by the organism in its lifetime, e.g., removal of tail cannot change the genes of the germ cells of the mice thus cannot be passed to next generation.

31. Speciation : It means the origin of new species from the existing ones. It happens when different populations of the same species evolve along different lines.
How speciation occurs ?

• It occurs when two populations are isolated (both geographically and reproductively) leading to almost no gene flow between the two populations.
• Over generations, genetic drift will accumulate different changes in each sub-population.
• Natural selection may also operate differently in these different locations.
• Together natural selection and genetic drift will cause such changes (severe changes in the DNA) that these two groups will not be able to reproduce with each other even if they happen to meet.
• When DNA changes occur to larger extent, it may lead to change in the number of chromosomes or gene expression, eventually the germ cells of the two groups cannot fuse with each other. This leads to emergence of new species.

32. Estimating Age of Fossil: There are 2 methods :

• Relative method : On digging, the fossils which are closer to the surface are more recent than the fossils found in deeper layers.
• Dating fossils (carbon dating method): It is done by detecting the ratios of different isotopes of the same element (i.e., isotope of C-14 which is radioactive) in the fossil material.

33. Evolution by stages :

Complex organs like eye has evolved from rudimentary organs, (e.g., rudimentary eye in flatworm might be useful enough to give only a fitness advantage and the structure of eye in different organisms is different indicating them to have different evolutionary origins) not by a single DNA change but created bit-by-bit over generations.

A change that is useful for one property to start with can become useful later for quite a different
function (e.g., Feathers might start as providing insulation in cold weather. But later, they might be useful for flight. Some heavy birds and reptiles also have feathers but they do not fly.

Some very dissimilar looking structures evolve from a common ancestral design, e.g., wild cabbage was cultivated as a food plant and many different vegetables were generated by selection over last two thousand years, (a) Selection of very small distances between the leaves gave rise to cabbage we eat. (b) Selection for arrested flower development gave rise to broccoli, (c) Selection for sterile flowers gave rise to cauliflower (d) Selection for swollen parts gave rise to kohlrabi.(e) Selection for larger leaves gave rise to leafy vegetable kale. It suggests that, if these selections were not done then there would have been only wild cabbage.

34. Homologous organs are organs having same origin and basic structure but they appear different and perform different functions in various organisms,
e.g.,

• Forelimbs of horse and arms of man.
• Wings of birds and flippers of whale.

Similarities in basic structure of (homologous) organs in different organisms, indifferent groups indicate common ancestry.

35. Analogous organs are organs, which look similar because they perform same function, but they do not have same origin and basic structure.
e.g.,

• Wings of birds and wings of insects.
• Fins of fish and flippers of the whale.

NCERT  Exemplar Class 10 Science Chapter 9 Heredity and Evolution

Short Answer Questions

Question 1. Do genetic combination of mothers play a significant role in determining the sex of new born?
Answer. No, mothers have no role in determining the sex of the new born. Mothers have a pair of X chromosome. And all children will inherit an ‘X’ chromosome from their mother regardless of whether they are boys or girls.

Question 2. Mention three important features of fossils which help in the study of evolution.
Answer. The three important features of fossils that help in the study of evolution are as follows:

1.  It helps in establishing the time period in which the organisms lived.
2.  It helps in establishing evolutionary traits among organisms and their ancestors.
3.  It is the mode of preservation of the remains of ancient species.

Question 3. In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation.
Answer. The sex of a child is determined by the type of sex chromosome contributed by male gamete. The statistical probability of getting either a male or female child is 50: 50, because the ratio of male gametes containing X chromosome and those containing Y chromosome is 50 : 50.

Question 4. What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?
Answer. Homologous organs are those organs which have the same basic structural design and developmental origin but have different functions and appearance. Example: The forelimb of a frog, a lizard, a bird and a man seem to be built from the same basic design of bones, but they perform different functions.
Yes, it is necessary that homologous structures always have a common ancestor in order to carry out the different activities. Otherwise there cannot be any similarity in basic plan, internal structure, development or origin.

Question 5. Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution.
Answer. Diversity of animals does not mean that they have diverse ancestry because common ancestry greatly limit the extent of diversity. If the animals are inhabiting in the same habitat their evolution by speciation and geographical isolation is also not likely. Animals having a common ancestor have developed new traits forming various groups of animals.

Question 6. A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.
Answer. The women produces ova with ‘X’ chromosome and man produces sperms with X and Y chromosome.
If the husband of the woman transfer X chromosome, then child will be a girl. On the other hand, if the husband transfer Y chromosome, the child will be a boy. In the case, the husband is always transferring X chromosome and hence, all the children are girl.

Long Answer Questions

Question 7. Give reasons why acquired characters are not inherited.
Answer. Acquired characters are not inherited because they do not produce change in the DNA of germ cells, only those characters which have a gene for them can be inherited.

Question 8. Does geographical isolation of individual of a species lead to formation of a new species? Provide a suitable explanation.
Answer. Yes, geographical isolation of individual of a species lead to the formation of a new species.
Geographical isolation of a population leads to genetic drift and there will be no gene flow between it and the parent species. Inbreeding in small population will reproduce among themselves and generate new variations. Accumulation of those variations over several generations will lead to formation of new species.

Question 9. Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria? Provide a suitable explanation.
Answer. Both bacteria and human beings perform all activities of life to live in their environment. Human beings have more complex organisation and differentiation which are absent in bacteria. Since, complexity and differentiation develop only through evolution, humans are more evolved than bacteria.

Question 10. Evolution has exhibited a greater stability of molecular structure when compared with morphological structures. Comment on the statement and justify your opinion.
Answer. There is immense diversity in size, form, structure and morphological features in the living world. At the molecular level, these diverse types of organisms exhibit similarity of the basic bio molecules like DNA, RNA, carbohydrates, proteins, etc.

Question 11. Study the following cross and showing self-pollination in Flf fill in the blank and answer the question that follows (Q12, Q13 and Q14):

Answer. Rr Yy Round, yellow.

Question 12. In previous question, what are the combinations of character in the F2 progeny? What are their ratios?
Answer. Round yellow – 9, Round green – 3, Wrinkled yellow – 3 Wrinkled green – 1, Le. 9 : 3 : 3 : 1.

Question 13. Give the basic features of the mechanism of inheritance.
Answer. Basic features for the mechanism of Inheritance are as follows:

1. Characters are controlled by genes,
2. Each gene controls one character.
3. Genes are located oh chromosomes.
4. There may be two or more forms of gene.
5. An individual possess two forms of genes whether similar or dissimilar.
6.  One form may be dominant over the other.
7. The two alleles separate at the time of gamete formation.
8. The two forms are brought together in the zygote.
9. Alleles of different genes located on separate chromosomes behave independent of one another.

Question 14. Give reasons for the appearance of new combinations of characters in the F2 progeny.
Answer. An organism can inherit each character independently. So, in the F2 progeny new combination of character appears. Tall/ Short and Round/Wrinkled seed trait are independently inherited.

Extra Questions – Heredity and Evolution – CBSE Class 10 Science

Question-1
What is heredity?
Solution:
The continuity of features from one generation to another is known as heredity. It is also defined as the transmission of traits from parents to the offsprings.
Question-2
Name the plant on which Mendel performed his experiments.
Solution:
Mendel performed his experiments on the plant, Pisum sativum – the garden pea plant.

Question-3
Define variation.
Solution:
Children do not resemble their parents completely. They possess characters obtained from both the parents. These changes in the phenotypic and genotypic characters are known as variations. Thus a given population of a species has indefinite variants.
Question-4
Define a gene.
Solution:
A gene is a small portion of the DNA, with codes for a particular polypeptide or a protein. In other words, it is the functional unit of the DNA. It is also responsible for the transmission of hereditary characters from the parents to the offspring.
Question-5
Write the expanded form of DNA.
Solution:
The expansion for DNA is Deoxyribonucleic Acid.
Question-6
What are the components of the chromosome?
Solution:
A chromosome consists of two Chromatids joined together at the centromere. The chromaid consists of the DNA material wound over small protein molecules called histones.
Question-7
What is a retrovirus?
Solution:
A retrovirus is a virus that has RNA as its genetic material instead of DNA. HIV virus is a retrovirus.
Question-8
What is a sex chromosome?
Solution:
Chromosomes that are responsible for the determination of sex in an individual are known as sex chromosomes. There are two types sex chromosomes – the X chromosome and the Y chromosome. Males possess one X and one Y chromosome and females possess two X chromosomes.
Question-9
How is sex determined in human beings?
Solution:
The male sex gametes have X and Y chromosomes, whereas the female sex gametes have two X chromosomes. When a sperm containing a Y chromosome fuses with the ovum containing X chromosome, the zygote develops into a male. When a sperm containing the X chromosome fuses with an ovum containing X chromosome, the zygote develops into a female. Thus the sex of an individual is determined by the sex chromosomes X and Y, which is present in the male chromosomes.
Question-10
Define homologous organs.
Solution:
Homologous organs are those organs, which have similar origin and basic plan of development, but may or may not differ in their functions. The forelimbs of a human, a bird and a horse are homologous organs.
Question-11
Explain Darwin’s theory of evolution.
Solution:
Darwin’s theory of evolution is also known as the Theory of Natural Selection or Darwinism. Darwin explained that despite having enormous potential of fertility, the population size of any kind of organism remains within a limit. It is due to the struggle between members of the same species and different species for food, space and mate. This struggle eliminates the unfit individuals. In other words, the fit organisms possess some variations, which are favorable and can leave the progeny to continue the variations. This is called Natural Selection.
Question-12
Define Genetics. What is the contribution of Mendel in the field of genetics?
Solution:
The branch of biology that deals with the study of heredity and variations is known as Genetics. Gregor Johann Mendel was the first person to carry out experiments regarding the heredity of certain characters from one generation to another in a scientific manner. He worked mainly on the garden pea plant. His observations regarding the occurrence of contrasting characters in various generations of garden pea led him to interpret that these are controlled by units which he called, factors. These factors are today known as genes. He is also known as the Father of Genetics.
Question-13
Where are the genes located? What is the chemical nature of genes?
Solution:
Genes are segments of the DNA, which is wound compactly into chromosomes. A gene is composed of a specific sequence of nucleotides. A nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphate group.
Question-14
During which stage can the chromosomes be seen clearly? Write the features of the eukaryotic and prokaryotic chromosomes.
Solution:
Chromosomes are distinctly visible during the Metaphase stage of mitosis of a cell.
The features of eukaryotic and prokaryotic chromosomes are as follows
Eukaryotic chromosomes
1. They are present in large numbers.
2. In higher organisms it occurs in paired condition.
3. They are visible distinctly only during the metaphase stage of mitosis.
4. Each chromosome consists of two chromatics attached together by a centromere.
Prokaryotic chromosomes
1. They are simple in composition when compared to the eukaryotic chromosomes.
2. They are generally circular in shape.
3. There is only one chromosome in a cell.
Question-15
Who provided the evidence of DNA as the genetic material? Write the names of the components of the DNA molecule.
Solution:
Frederick Griffith, Avery, Mc Leod and Mc Carty established that DNA was the carrier of the genetic information.
The DNA molecule is a polynucleotide. A nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphate group.
Question-16
What do you understand by the double helical structure of DNA? Who proposed this structure?
Solution:
James Watson and Francis Crick proposed the double helical structure of the DNA. According to this structure,
• DNA molecule consists of two polynucleotide strands forming a double helix. Each helical turn has a length of 3.4nm in which ten nucleotides are present.
• Each polynucleotide strand has a backbone of sugar and phosphate. The nitrogenous base is attached to the sugar.
• The nitrogenous bases of the two strands of a double helix form a pair with the help of hydrogen bonds. Adenine pairs with thymine by two hydrogen bonds, whereas guanine pairs with cytosine by three hydrogen bonds.
• The hydrogen bonds hold the two strands of the helix together.
Question-17
Describe the different types of chromosomes.
Solution:
The different types of chromosomes are,
Metacentric
It is a chromosome with the centromere near the middle and the two chromatics are of equal lengths.
Sub-metacentric
Here the centromere is situated slightly closer to one end than the other. Thus one chromatic is slightly longer than the other.

Acrocentric
Here the chromosome is situated near one end of the chromosome. Thus one chromatic is very long while the other is very small.

Telocentric
Here the centromere is situated at the tip of the chromosome.

Question-18
How do embryological studies provide evidence for evolution?
Solution:
The embryology of different vertebrates provide very strong evidence favoring organic evolution. The early embryos of different vertebrates show striking similarities. This indicates common origin and ancestry of different vertebrates. Thus embryological studies provide direct evidence for evolution.
Question-19
Define evolution. Describe the contribution of Lamarck.
Solution:
Evolution is referred to as the changes acquired by a species or a certain population of a species gradually over a long period of time. These changes should be heritable.
Contribution of Lanark
According to the Theory of inheritance of acquired characters or Lamarckism, put forward by Lanark, the use and disuse of an organ leads to acquiring change in the features of that organ. These changes are also inherited by the offspring’s. The favorable variations caused due to use and disuse after a considerably long period of time, results in evolution of a new species.
Question-20
How do homologous organs provide evidence in support of evolution?
Solution:
The presence of homologous organs indicates that all vertebrates have a common ancestry. Similarly, all organs and systems of the vertebrates show fundamental similarities, which point towards common ancestry.

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