NCERT Solutions for class 10th Science Chapter 9 Heredity and Evolution


Question 1. If a trait Aexists 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?

Sol. In asexual reproduction, cell division results in the creation of similar cells with the identical copies of DNA. Therefore, in such species, a trait may come into existence due to random mutation induced by various environmental factors. It can be observed that the induced trait occurs in few species which is then inherited by future generations, on replication over a period of time. Thus, if 60% ofa population contains this trait, it means that there members have been replicating themselves for a longer period of time than those in the population where the trait is seen in only 10% of the population. Thus, trait B is likely to have arisen earlier.

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

Sol. A variation is the deviation in characters of an individual from the group to which it belongs or deviation in characters of the offspring from those of the parents. The survival of the variation depends upon its nature. Depending on the nature of variations, different individuals would have different kinds of advantage. These variations may lead to the advantages of the individuals due to positive adaptation of the trait. This is how the creation of variations in a species promotes survival of the species.

Question 3. How do Mendel’s experiments show that traits may be dominant or recessive?

Sol. Mendel conducted a monohybrid cross from tall (TT) pea plant with dwarf(tt) pea plant as shown in the figure. He obtained tall pea plants in first generation (F1) and when first generation was self crossed, both tall and dwarf plants were obtained in a ratio of3:1 i.e., 3 tall plants and 1 dwarf plant.

Thus, as three-fourths of the plants in second (F2) generation were tall and
one-fourth was dwarf. So he concluded that, tall trait was dominant while dwarf trait was recessive.

Question 4. How do Mendel’s experiments show that traits are inherited independently?
Sol. Gregor Mendel stated that during a dihybrid cross (crossing of two pairs of traits), when two or more characteristics are inherited then, an assortment of each pair of traits is independent of the other which means that during gamete formation, one pair of trait segregates from another pair of traits independently. Mendel crossed pea plants with two pairs of contrasting characters i.e., tall plants with round seeds and short plants with wrinkled seeds as shown in the given figure:

When these FI plants were self pollinated to generate F2 generation, he found that some F2 progeny were tall plants with round seeds and some were short plants with wrinkled seeds. At the same time there are tall plants but had wrinkled seed, while others were short, but had round seeds. Thus, Mendel showed that tall plant/short plant trait and the round seed/wrinkled seed trait are independently inherited.

Question 5. A man with blood group A marries a woman with blood group O and their daughter has blood group 0. Is this information enough to tell you which of the traits – blood group A or O – is dominant ? Why or why not ?

Sol. No, the given information is not sufficient to tell that which trait – blood group i.e., A or O is dominant. This is because we cannot predict that whether father or mother is homozygous or heterozygous. According to Mendel, in blood heredity, blood group A is always dominant and blood group O is always recessive. Here, father’s blood group can be AA (homozygous) or AO (heterozygous) genotypically, whereas mother can be OA or 00. For daughter to be born with blood group 0, she must receive O type gene one each from father as well from mother, for this father must have heterozygous AO blood group and mother must have homozygous blood group 00.

Question 6. How is the sex of the child determined in human beings ?

Sol. In human beings, out of23-pairs of chromosomes 22-pairs of chromosomes are called autosomes and only one pair is called sex-chromosome. These sex-chromosomes are also of two types i.e. ‘X’ and ‘Y’ sex-chromosomes.

The sex of a child depends on what happens at fertilisation like:
(i) If a sperm carrying X chromosome fertilises an ovum (or egg), which carries X chromosome, then the child will be a girl (or female). This is because the child will be having XX combination of sex chromosome.
(ii) If a sperm carrying Y chromosome fertilises an ovum (or egg), which carries X chromosome, then the child will be a boy (or male). This is because the child will be having XY combination of sex chromosome.

Note: Sex determination, is a system which decides the sexual characteristics of an organism or offspring. It helps to determine whether the organism will be male or a female, which are the two most common sexes.

Question 7. What are the different ways in which individuals with a particular trait may increase in a population ?

Sol. There are two ways in which individuals with a particular trait may increase in a population. These are-
(i) Natural selection– Organism with a particular trait selected naturally because it increases the survival rate of the given species, which leads to the increase in a population.
(ii) Genetic drift-An accident in small population may result in surviving of the organism with a particular variant only. This may increase the frequency of some gene in that population which have no survival advantage.
(iii) Poor nourishment may reduce the population of individuals but when nourishment conditions improve the population starts increasing again.

Question 8. Why are traits acquired during the life-time of an individual not inherited?

Sol. The traits acquired during the life-time of an individual not inherited because certain changes occur in the structure or function of the body parts due to environmental factors which cannot be passed on to the DNA of the germ cells. The traits are only inherited if changes take place in the genetic material. Therefore, the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution.

Question 9. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

Sol. When a population is small, the scope and number of variations is limited which leads to the reduction in their diversity and traits. The small numbers of surviving tigers a cause of worry from the point of view of genetics because it would cause loss in genetic variability. Small number of tigers means fewer chances of variations which is necessary for the maintaining the stability of a species by natural selection. It can also cause sudden extinction of the tigers and loss of their genes forever, adversely affecting the diversity of nature.

Question 10. What factors could lead to the rise of a new species?

Sol. The factors which are mainly responsible for giving rise of a new species are:
(i) Genetic drift
(ii) Natural selection
(iii) Genetic variation
(iv) Geographical isolation

Question 11. Will geographical isolation be a major factor in the speciation of a self­ pollinating plant species ? Why or why not?

Sol. No, geographical isolation will not be a major factor in the speciation of a self­ pollinating plant species because pollination occurs within the same plant. Geographical isolation signifies the separation of two populations. But here self- pollinating plants exists together evolves homozygous lines. Hence, there will be no variations and no speciation will take place.
Note:Speciation is a process within evolution that leads to the formation of new, distinct species that are reproductively isolated from one another.

Question 12. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Sol. No, geographical isolation will not be a major factor in the speciation of an organism that reproduces asexually because asexual reproduction involve only single parent. Variations can occur only when the copying of DNA is not accurate. Therefore, geographical isolation cannot prevent the formation of new species in an asexually reproducing organism.

Question 13. Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Sol. The more characteristics two species will have in common, the more closely they are related. And the more closely they are related, the more recently they will have had a common ancestor. Forelimbs of humans and wings of birds are closely related in an evolutionary terms. Since, the forelimb of human and wings of birds have similar structure but they both performs different functions. Thus, presence of homologous organs provides evidence for evolution that they have evolved from the same ancestor.

Note: Homologous organs, analogous organs or vestigial organs help to identify evolutionary relationships.

Question 14. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Sol. The wings of butterfly or bat have similar function but dissimilar in the origin. Such organs are called analogous organs. So they are not homologous organs.

Question 15. What are fossils? What do they tell us about the process of evolution?

Sol. Fossils are the remnants or impressions of the extinct organisms which were obtained from the earth after digging. These impressions and the remnant are concerned to those organisms which were millions of years ago in the living conditions but not yet.
Fossil provides us the information about the life forms which existed earlier. It indicates the time when these organisms existed on the earth. It also indicates about the extent of evolution that has taken place when they are compared with the present life forms. They provide evidences of evolution by revealing the characteristics of the past organism and the changes that have occurred in these organisms to give rise to the present organisms.

Question 16. 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 ?

Sol. Human beings look very different from each other due to the change in phenotypic characters. Their genotypic characters are same so their organs are same but due to change in geographical conditions they become different in size, colour and look to the progeny of some species.

Question 17. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

Sol. Evolution cannot always be equated with progress or better body designs. Evolution simply creates more complex body designs. However, this does not mean that the simple body designs are inefficient. It is a process of progressive change whereby complex body forms of living organisms emerged over a period of time but previous ones also continued to exist, though many life forms failed to exist due to environmental change. Thus, bacteria, spiders, fish and chimpanzees have evolved from common ancestors forming branches at different stages.


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) TIWW (b) Timv
(c) TtWW (d) TtWw

Sol. (c) Since, all the progeny bore violet flowers, it implies that the tall plant with violet flowers has genotype ‘WW’ for violet flower colour. Since, the progeny obtained is both tall and short, the parent plant was not a pure tall plant and bears genes that determine short height of the plant. Therefore, the genotype of the plant with respect to height would be ‘Tt’. So, ifa cross is carried out between tall parent with violet flowers (TtWW) and short parent with white flowers (ttww), the progeny obtained is TtWw (8):ttWw (4):ttWw(4). All the progeny bear violet flowers but half of them are tall and half are short.

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.

Sol. (d)Homologous organ have same origin but different functions like each of the organ above. Our teeth and elephant’s tusks, our arm and a dog’s fore-leg and potato and runners of grass are the examples of 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

Sol. (a) In evolutionary terms, we have more in common with a Chinese school boy. A Chinese boy is a member of same species i.e., Homo sapiens.

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?

Sol. No, since two copies of traits are inherited from parents, one from mother and the other from father. Unless we know the nature of these two variants of traits we cannot tell which is dominant and which is recessive. Recessive traits appear when both the parents contribute recessive allele. From this statement we can only presume that both parents are contributing recessive allele.

Question 5. How are the areas of study – evolution and classification – interlinked?

Sol. Every species or organisms has inbuilt tendency for genetic variation which plays an important role in the origin of new species and forms the basis for evolution. The more characteristics two species have in common, the more closely they are related. Classification of organisms necessarily involves, organising them in different groups, based on the similarities and differences of characteristics. Classifying organisms helps us in recognising the basic arrangement of a hierarchical structure among diverse species. In fact, classification of species is a reflection of their evolutionary relationship. Thus, we can say evolution and classification are related to each other.

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

  • Homologous organs: The organs of similar structure and origin but different in function are known as homologous organs, e.g., fore limbs of frog, wings of birds, wings of bat, flappers of whale, forelimbs of horse and fore limbs of man are similar in their origin but different in their functions.
  • Analogous organs: Organs similar in functions but different in structure and origin are known as analogous organs, e.g., wings of butterfly and that of bird have similar functions but their origins are different.

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

Sol. Dogs have a variety of genes that govern coat colour. Let us select black and white colour for the study of inheritance of coat colour in dogs by performing the following crosses:
(i)Cross the pure black bred male dog with pure bred white female bitch.
(ii)Cross the pure white bred male dog with pure bred black bitch. Observe the coat colour of the progeny produced.
If progeny is all black then black colour may be dominant and if progeny is all white then white may be dominant.

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

Sol. The remains or impressions of dead animals or plants that lived in the remote past are known as fossils.
Fossils help us in studying evolution in number of ways, some of them are listed below:

  1. The fossils provide evidences for evolution. For example a fossil bird called Archaeopteryx looks like a bird but it has many other features which are found in the reptiles. This is because Archaeopteryx has feathered wings like those ofbirds but teeth and tail like those ofreptiles. Archaeopteryx is therefore, a connecting link between the reptiles and the birds and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide the evidence that the present animals and plants have originated from the previously existing ones through the process of continuous evolution.
  2. It provides us clues to the past and thus trace path of the evolution.
  3. It helps in understanding the previous climate and environment, thus help in catogorising geological time.
  4. It can also help to find out the types of organisms (plants, animals and microbes) that existed at a particular geological time and reconstruct the evolutionary process.

Note: Fossil evidence informs most studies of the origin of life. The age of the earth is about
3.5 billion years ago.

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

Sol. Evidence for the origin of life from the inanimate matter was provided by the
Miller and Urey experiment in which they created an atmosphere similar to the early earth atmosphere where free oxygen is absent. They mixed different gases like ammonia, methane, hydrogen and water vapour and produce a spark in them. At the end ofa week, 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules found in living organisms. This experiment provides the evidence that the life originated from inanimate matter (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?

Sol. Since sexual reproduction involves two parents, large number of variations is produced. These two parents will already have variations accumulated from the previous generations and hence each generation formed would be novel since it has accumulated variations from two parents.
Whereas in asexual reproduction, the organisms have very little variations due to errors in DNA copying while cell division process occurs. The inbuilt tendency for variations in organisms is the basis for evolution. Since sexually reproducing organisms have more variations, they speed up the process of evolution. While the less number of variations mean, asexually reproducing organisms will take long years for evolving into a new species. This is because the generation of variation plays a very important role in evolution process.

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

Sol. The equal genetic contribution of male and female parents in a progeny is through gametes, which have half the amount of DNA as compared to other body cells (non-reproductive cells). So when the fertilisation takes place, half of the genetic material comes from female and half from the female. For example, in human beings, out of 46 chromosomes, half of the chromosome i.e., 23 is contributed by male gamete and half of the chromosomes are contributed by female gamete. In this way, an equal contribution of male and female parents ensured in the progeny.

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?

Sol. This statement is mostly true, but not always because those variations that confer an advantage to an organism will definitely survive in a population because natural selection prefers these variations, e.g., the accumulation of heat resistant variation in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave. But there are several other variations which though do not provide an advantage to the organism in present condition and are inherited to the next generations. Such non-advantageous variations may become advantageous in future when environmental conditions changes.

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