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NCERT Solution for Class 10 Science Chapter 9 : Heredity and Evolution

Heredity and Evolution
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The Chapter 9 of NCERT Class 10 Science is Heredity and Evolution. This chapter will take you through the passing of genes from one generation to another and the process of sex determination. With a total of 12 questions in the chapter, the complete solution is provided by Swiflearn in pdf format to have a better understanding of Heredity and Evolution.


NCERT Solutions for Class 10 Science Chapter 9 by Swiflearn are by far the best and most reliable NCERT Solutions that you can find on the internet. These NCERT Solutions for Class 10 Science Chapter 9 are designed as per the CBSE Class 10th Science Syllabus. These NCERT Solutions will surely make your learning convenient & fun. Students can also Download FREE PDF of NCERT Solutions Class 10 Science Chapter 9.

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NCERT Solution for Class 10 Science Chapter 9 Heredity and Evolution PDF


NCERT 10th Science Chapter9

Exercise 9.1

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?


In asexual reproduction, the resulting offspring are identical to the parents (have a near-exact copy of their parents’ DNA). However, sometimes this copying of DNA is not accurate, so the newly formed DNA has some variations giving rise to a new trait. This trait is then inherited by future generations and successive generations keep accumulating variations. Therefore, if trait A exists in 10% of the population and a trait B exists in 60% of the same population we can conclude that trait B has arisen earlier because the trait has been replicating and existing in a higher percentage of the population.


Question 2:
How does the creation of variations in a species promote survival?


Species are adapted to survive in a particular habitat. Drastic changes in the environment can disturb the habitat and prove fatal for certain species. For example, many microorganisms inhabit freshwater ponds. If there is a sudden increase in the temperature of the water, it will kill the bacteria living in it. However, some species of bacteria that are resistant to heat shall survive in the heat. These species will survive and reproduce in the changing environment. If
there were no heat-resistant variants, the entire species of bacteria would be extinct. Thus, the creation of variations in a species promotes its survival.


Exercise: 9.2

Question 1:
How do Mendel’s experiments show that traits may be dominant or


Solution 1:
Mendel conducted experiments on pea plants with a single character ((with two alternative traits) called monohybrid cross. He crossed true-breeding tall plants (TT) with true-breeding dwarf plants. In the first filial generation, he received only tall plants. The dwarf trait did not appear in the F1 generation. Self-pollination of the F1 progeny resulted in both tall and dwarf plants. Based on the monohybrid crosses, Mendel concluded that out of the two traits of a character, one is dominant and the other is recessive. The dominant trait always expresses
itself while the expression of the recessive trait is masked in the presence of the dominant trait, recessive trait. The recessive trait is expressed only in homozygous condition. Also, the tall plants in F1 were not true-breeding. They were heterozygous (Tt) tall plants. The dominant allele (T) suppressed the expression of the recessive allele (t).


Question 2:
How do Mendel’s experiments show that traits are inherited


Solution 2:
Mendel crossed two pea plants differing in contrasting traits of two characters i.e. a dihybrid cross. He crossed a pea plant having yellow coloured and rounded seeds with another pea plant having green coloured and wrinkled seed.
The F1generation has all round and yellow seeds. The F2generation, all the characters inherited independently. (Round yellow, round green,
wrinkled yellow, wrinkled green.


Question 3:
A man with blood group ‘A’ married to any woman with blood group ‘O’
and they had a daughter with blood group ‘O’. Is this content enough to
tell you that which of the traits − blood group A or O − is dominant? Why
or why not?


Solution 3:
A man is having blood group A. Hence, he must be having genotype IAIO or IAIO. He will produce two types of gametes IA and IO. A woman is having blood group O. Hence, she is having  genotype IOIO. She will produce one type of gamete IO. Their daughter will get IO allele from the mother and IO allele from the father. Hence, she is having blood group O. Thus, the given information is enough to tell that blood group A is dominant over blood group O as two alleles of IO are required to have blood group O and only one allele of IA is enough to have blood group A.


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


Solution 4:
The sex of the child is determined by the chromosomal combination in parents. Women have a perfect pair of sex chromosomes i.e. XX whereas men have a mismatched pair i.e. XY. In females, all eggs produced will carry the X chromosome. In males, half of the sperms will carry the Y chromosome and the other half will carry the X chromosome. During fertilization, when the egg fuses with a sperm carrying the Y chromosome, the child will be a boy. However, when the egg fuses with a sperm carrying X chromosome, the offspring will
be a girl.

Exercise: 9.3

Question 1:
What are some different ways in which any individual with a particular
trait may result in an increase in population?


Solution 1:
The different ways in which individuals with a particular trait may increase in a population are: I. Natural selection – Certain variations give a survival advantage to individuals in a population in a changing environment resulting in an increase in their population. II. Genetic drift: Accidents in small populations, even if they give no survival advantage also leads to an increase of certain individuals in a population. III. Some traits are not inherited but acquired by an individual during its lifetime. The acquired characters help the individual to survive
better and increase their population.


Question 2:
Why are traits acquired during the lifetime of an individual not inherited?


Solution 2:
In reproduction, parents pass on the germ cells to the progeny. So, any changes in the germ cells (leading to variation or new traits) are inherited by the progeny. Some traits are acquired by an individual during their lifetime due to environmental influences or other external factors. An acquired trait involves a change in no reproductive tissues (somatic cells). Since acquired traits do not involve any change in the germ cells, therefore, these traits cannot be


Question 3:
Why the fewer numbers of surviving tigers are causing worry, for genetics?


Solution 3:
A small population of any species will accumulate very fewer variations. When such a species reproduces, there are fewer chances of having progeny with some useful variations (that will give better survival advantage). So, in the case of a disease, they will not be able to survive. They will become extinct and the genes will be lost forever.

Exercise: 9.4

Question 1:
Mention a few factors that are leading to the rise of a new species?


Solution 1:
Speciation is an evolutionary process by which reproductively isolated biological populations evolve to become distinct species.
There are some factors which could lead to the formation of new species, they are:
(i) Genetic drift
(ii) Natural selection
(iii) Severe DNA change.
(iv)A variation may occur which does not allow sexual act between two groups
(Reproductive isolation).


Question 2:
Will geographical isolation be a major factor in the speciation of a selfpollinating plant species? Why or why not?


Solution 2:
Geographical isolation prevents the transfer of genes between two species. In a selfpollinating plant, the transfer of pollen occurs from the anther of one flower to the stigma of the same flower or another flower from the same plant. It does not require any external agent for pollination, neither is it dependent on another plant for pollination. So, geographicalisolation cannot affect the speciation of a self-pollinating plant.


Question 3:
Will geographical isolation be a major factor in the speciation of an
organism that reproduces asexually? Why or why not?


Solution 3:
Geographical isolation prevents the transfer of genes (gene flow) between populations of aspecies. In asexual reproduction, a single organism is capable of giving rise to a new individual. In these reproducing organisms, variations can occur only when there is an error  during the copying of DNA. Therefore, geographical isolation cannot affect the formation of
new species in an asexually reproducing organism.


Exercise: 9.5

Question 1:
Give an example of characteristics being used to determine how close two
species are in evolutionary terms.


Solution 1:
Similarities in characteristics help us to trace evolutionary relationships between organisms. Some characteristics in different organisms are similar because they are inherited from a common ancestor. And these provide a link between different stages of evolution of species. Feathers that we see on birds today were also present on dinosaurs. However, dinosaurs did not use these feathers to fly but as insulation. Birds seem to have later adapted the feathers for flight. This indicates that birds are very closely related to reptiles since dinosaurs were reptiles. This also proves that birds and reptiles are closely related and that the evolution of wings started in reptiles.


Question 2:
Can the wing of a butterfly and the wing of a bat be considered as
homologous organs?


Solution 2:
The organs are similar in fundamental structure but are different in functions, they are called homologous organs. The wings of a butterfly and the wings of a bat are not homologous organs as they do not have the same origin or the same basic structure. The wings of a butterfly and wings of a bat are analogous organs as they perform the same function of flight even though their origin and structure are different.


Question 3:
What are fossils? What do they explain to us about the process of


Solution 3:
Fossils are remains or imprints of hard parts of the animals such as skeletal structures that once inhabited earth. They provide us with one of the most acceptable evidence in support of evolution because the study evolutionary past of organisms can be done only in the form of their fossils. By studying the fossils we can identify the changes that have occurred in these organisms to give rise to the present-day forms. They act as a connecting link between the organisms of the past and the organisms of the present.


Exercise: 9.6

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?


Solution 1:
A species is a group of interbreeding individuals. Skin colour, looks, and size are all the observable features of human beings which are generally environmentally controlled. Based on these features, various human races are formed. However, all human beings irrespective of skin colour, height, language, race etc. are capable of reproducing with each other and producing offspring. Therefore, all human beings belong to a single species of Homo sapiens.


Question 2:
In evolutionary terms, can we say which among bacteria, spiders, fish and
chimpanzees have a ‘better’ body design? Why or why not?


Solution 2:
In evolutionary terms, we cannot say which among spiders, bacteria, chimpanzees and fish have a ‘better’ body design. The evolutionary process takes into account the development of most efficient and suitable features in the design of the body for survival and adaptation favouring a particular niche. For example, organisms with a complex body design may not survive in a particular situation where as, bacteria with the simplest body design can survive in extreme habitats. Evolution only leads to the development of complex body designs and cannot be equated with progress.


Exercise Chapter 9

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


Solution 1:
(c) There are two flower colours violet and white but F1 showed only violet flowers. This means violet flower colour (V) is the dominant trait that masks the recessive trait (v) (white flower colour). The F1 had half tall and half short plants. This indicates both dominant and recessive traits expressed in the F1 progeny. So, we can infer that the tall plants were not true-breeding and must be genetically heterozygous for the character height (Tt), hence, the
genetic make-up of the tall parent can be depicted as TtVV. Therefore, the cross involved in the given question is TtVV × ttvv ↓ TtVv − ttVv Therefore, half the progeny is tall, but all of them have violet flowers.


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.


Solution 2:
(d) All of the Above.
As in all cases, the basic internal structure and origin of the organs are the same though they differ functionally. For example, an elephant’s tusk is a modified incisor. So, the origin is the same but the function is different. Tubers of potato and grass runners are both stem modifications but functions are different, hence these are homologous organs.


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.


Solution 3:
(a) In evolutionary terms, we have more in common with a Chinese schoolboy. Because both belong to the same species Homo sapiens. Chimpanzees and humans both have a common ancestor but are two different species.


Question 4:
A study found that children who have light-coloured eyes are most
probably have parents with light-coloured eyes, on which can we say
anything about whether the light eye colour trait is dominant or recessive?
Why or why not?


Solution 4:
No. From the above statement, we cannot say with certainty whether light eye colour is recessive or dominant. However, since both parents and their children have light eye colour, the possibility is that light eye colour is a recessive trait. Had the light eye colour been a dominant trait, the homozygous light-eyed parents would have only light-eyed children but the heterozygous light-eyed parent might have some recessive dark-eyed children (3: 1 ratio).


Question 5:
How the areas of study are “evolution and classification” are interlinked?


Solution 5:
Classification is the process by which scientists group living organisms based on their similarities. If two species have more of their characteristics in common, then they can be said that they are more closely related, and if two species are more closely related, then we can say that they have a more recent ancestor. For example, in a family, a brother and sister are closely related than their cousins. A brother and sister have a recent common ancestor i.e., their parents. A brother-sister and their cousin are also related but less than the sister and her
brother. The grandparents are the common ancestor for a brother and sister and their cousin. Due to sexual reproduction, there is an accumulation of variation over generations which  leads to the creation of different phenotypes and genotypes as we move down a family (evolution) tree.


Question 6:
Explain the terms homologous and analogous organs with some examples.


Solution 6:
Homologous organs are those that similar in origin but perform some different functions. Forelimbs of frog, lizard, pigeon, bats, whales, horses and humans have the same basic structural plan but different functions. For example, the wings help in flight whereas human forearm helps in various activities.
Analogous organs are those that have similar functions but are different in their structural details and origin. For instance, the wings of a bat and a bird are similar in functionalities, but this similarity does not mean that these animals are more closely related. The basic structure of the wings of insects is different from that of a bird and a bat but similar in function. Also, on careful observation, we will find that the wings of a bat are just the folds of skin that are stretched between its fingers whereas the wings of birds are present all along the arm. Similarly, fins of fishes and flippers of whales are also examples of analogous organs.
Therefore, these organs are analogous.


Question 7:
Outline a project which aims to find the dominant coat colour in dogs


Solution 7:
Coat colour in dogs is governed by a variety of genes. At least 11 genes have been identified that influence the colour of the coat in a dog. A dominant gene is a gene that is expressed in both homozygous as well as heterozygous conditions, whereas the recessive gene is only expressed in homozygous condition.
Let us consider that one parent is homozygous black (BB) and another parent is homozygous brown (bb). Cross between both the parents produces offspring with the genetic make-up: Bb, but we can’t say whether brown is dominant or black is dominant. By observing the off-springs produced, we can say which one among given is a dominant trait. If the F1generation comprises of black we can say black is dominant and if it is brown then we can say brown is dominant.


Question 8:
Explain the importance of fossils in deciding evolutionary relationships.


Solution 8:
Fossils are the remains or impression of the organism that once existed on earth and they also give the description of the ancestors of the plants and animals that are alive today. Fossils are buried under layers of earth, mud and silt and are preserved for millions of years. Fossils are formed when an animal dies and is buried under the earth. After which soft tissues of the body quickly decompose leaving the shells or hard bones behind. Over the time sediment, it builds over the top and hardens to form rock. Deeper within the earth, older is the fossil. If
we dig into the earth and start finding fossils, the fossils we get/find closer to the surface are more recent than the fossils obtained deeper.
Let us start 100 million years ago when some invertebrates on the sea-bed dead and buried in the sand. More sand accumulates, and then sandstone forms under pressure. Millions of years later, dinosaurs who were living in that area, died and their bodies are also buried in the mud these then also compressed above the invertebrate fossils. Then, millions of years later, some creatures similar to horse died in that area and got fossilized above the fossils of the dinosaur.
Thus, after digging that area, scientists can easily predict that horse-like animals evolved later than those of the dinosaurs and the invertebrates. Thus, this example suggests that the fossils found closer to the Earth’s surface are more recent ones than the fossils present in deeper layers.


Question 9:
What evidence do we have for the origin of life from inanimate matter?


Solution 9:
Miller-Urey experiment provided evidence for the origin of life from inanimate matter. In the experiment, they created an atmosphere containing molecules like ammonia, methane and hydrogen sulphide, but no oxygen. This atmosphere was similar to the atmosphere present during the primitive Earth. The temperature was maintained constant at 100oC and sparks were passed through the mixture to simulate lightning. After a particular time, a certain amount of the carbon
from methane had been converted to simple compounds of carbon like amino acids. Amino acids are the precursors of the proteins and proteins are the molecules that support life in a basic form.


Question 10:
Explain how sexual reproduction gives rise to more viable variations than
asexual reproduction, and how does this affect the evolution of those of
sexually reproducing organisms?


Solution 10:
In sexual reproduction, two parents are involved. Both parents contribute their genes to the offspring. The child obtains a half number of chromosomes from the father and half from the mother i.e. he inherits one copy of genes from father and another copy from the mother. So, two individuals having different variations combine their DNA to give rise to a new individual. Therefore, sexual reproduction easily leads to variations but on the other side, in asexual reproduction, chance variations can only occur when the copying of DNA is not accurate. Also, during asexual reproduction, the offspring inherits the entire cellular apparatus from the parent. If there are major variations then the resultant DNA will not be
able to survive inside the inherited cellular apparatus. However, in sexual reproduction, more variations are allowed and the resultant DNA is also able to survive, thus making the variations viable. Sexual reproduction helps the organism to adapt better to the environment. Variations help the species to survive in all the conditions. Environmental conditions such as a disease, pests, and food availability can change suddenly affect a place and in such kind of
situations, only those variants that are resistant to these conditions would be able to survive. This will slowly lead to the evolution of a better-adapted species. By this, variation helps in the evolution of sexually reproducing organisms. However, in case of a sudden change in the environment of asexually reproducing species can cause their extinction.


Question 11:
How is the equal genetic contribution of female and male parents ensured
in the progeny?


Solution 11:
In human beings, each somatic cell of the body contains 23 pairs of chromosomes. Out of the 23 pairs of chromosomes, 22 pairs are known as autosomes and the remaining one pair is called sex chromosomes represented as X and Y.
Females have two X chromosomes while males have one X and one Y chromosome. During gametogenesis (gamete formation), meiosis occurs and the gametes receive half the number of chromosomes.
Therefore, the male gametes have 22 autosomes and either X or Y chromosome. On the other hand, the female has 22 autosomes and the X chromosome. During sexual reproduction, the male and female gametes fuse and the number of chromosomes is again restored in the zygote. The progeny receive 22 autosomes and one X or Y chromosome from the male parent and 22 autosomes and one X chromosome from the female parent.


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


Solution 12:
Natural selection is one of the basic mechanisms of evolution and it favours variations that offer a survival advantage. Individuals with survival advantage reproduce within the population and pass on the favourable variation to the progeny and as a result evolution of organisms occurs. However, there can be some other variations that occur by chance (arise only accidentally) and do not offer any survival advantage. If such variations occur in small populations, they can change the frequency of some genes in the population even if they are
not important for survival. This accidental change in the frequency of genes in small populations is referred to as genetic drift. So, both types of variations can survive in a population.

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