Heredity and Evolution NCERT Solutions For Class 10th SCIENCE





Page No. 143

Q1. 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?
Ans. Trait B is likely to have arisen earlier because it is present in a larger portion of the population.

Q2. How does the creation of variations in a species promote survival?
Ans. Variations take place in response to the changes in the environment. Such variations enable a species to cope with the new changes. Thus, variations help a species in survival.

Page No. 147

Q1. How do Mendel’s experiments show that the traits may be dominant or recessive?
Ans. During monohybrid cross by Mendel, one of the pair of traits did not appear in the F1 generation. But that trait appeared in the F2 generation. Based on this observation, Mendel concluded that a trait could be dominant or recessive.

Q2. How do Mendel’s experiments show that traits are inherited independently?
Ans. During dihybrid cross by Mendel, it was observed that when two pairs of traits 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.

Q3. 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 trait – blood group A or O – is dominant? Why or why not?
Ans. This information is not sufficient. For considering a trait as dominant or recessive, we need data of at least three generations. This data is about only two generations.

Q4. How is the sex of a child determined in human beings?
Ans. Somatic cells in human beings contain 23 pairs of chromosomes. Out of them the 23rd pair is composed of different types of chromosomes which are named as X and Y chromosomes. The 23rd pair contains one X and one Y chromosome in a male. On the other hand, the 23rd pair in a female contains X chromosomes. This means that all the eggs would have X chromosome as the 23rd chromosome, while a sperm may have either X or Y chromosome as the 23rd chromosome. When a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a female child. When a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.

Page No. 150

Q1. What are the different ways in which individuals with a particular trait may increase in a population?
Ans. When a beneficial trait appears, it can increase in a population. The example of blue beetles in the chapter shows this. Since blue beetles could not be spotted by the crows hence more blue beetles could survive. Sometimes, an accident can also lead to proliferation of a new trait in a population, as happened in the example of trampling of bushes by elephants. It can be said that sudden or gradual changes in the environment or some mutation in a species can lead to a particular trait being passed on through generations.

Q2. Why are traits acquired during the lifetime of an individual not inherited?
Ans. Acquired traits do not bring any change in the genotype of an individual. Hence, acquired traits do not get inherited.

Q3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Ans. Small numbers of surviving tigers means that a small gene pool of tigers is left. A smaller population reduces the chances of variations. A time may come when lack of useful variations may result in extinction of tigers. Hence, a small number of surviving tigers is a cause of worry from the point of view of genetics.

Page No. 151

Q1. What factors could lead to the rise of a new species?
Ans. Speciation can happen if two groups of the same species are somehow prevented from interbreeding for several generations. This can happen because of geographical segregation or because of some genetic changes.

Q2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
Ans. In a self-pollinating plant species, geographical segregation cannot be a major factor in speciation, because no new trait can become a part of the genotype in self-pollination plant species. However, there are some chances of some environmental changes which can lead to some variations.

Q3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not ?
Ans. In case of an asexually reproducing organism, geographical isolation cannot be a major factory in speciation because meiosis does not take place during asexual reproduction.

Page No. 156

Q1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.
Ans. Let us take the example of humans and apes. Both of them have similar body design. Body hair and mammary glands are present in both the animals. Hence, these two animals are closely related in evolutionary term. Now take some common characters between a fish and a man. Vertebral column, brain box and jaws are present in both of them. But fish and man look entirely different from each other. Hence, they are not very closely related in evolutionary term; rather are like distant relatives.

Q2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?
Ans. Wings of a butterfly are composed of chitinous membrane, while wings of a bat are composed of bony skeleton. Hence, these are not homologous organs rather analogous organs.

Q3. What are fossils? What do they tell us about the process of evolution?
Ans. The preserved remains of animals or plants or other organisms from the distant past are called fossils. These fossils tell us about many extinct animals and also give insights into how the evolution could have taken place.

Page No. 158

Q1. 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?
Ans. In spite of wide differences in size, colour and looks, human beings can interbreed. Hence, all of them are kept under one species.

Q2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?
Ans. It depends on our perception of ‘better’ design. If complexity of body design is the criterion, then chimpanzee is obviously better than bacteria. But if ability of survival in almost all kinds of habitat is a criterion then bacteria are far ahead than any other group of organisms.



Q1. 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
Ans. (c) TtWW

Q2. 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.
Ans. (d) All of the above

Q3. In evolutionary terms, we have more in common with
(a) A Chinese school-boy.
(b) A chimpanzee
(c) A spider
(d) A bacterium
Ans. (a) a Chinese schoolboy.

Q4. 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?
Ans. No, since two copies of traits are inherited from parents, one from the mother and the other from the 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 are that both parents are contributing recessive allele.

Q5. How are the areas of study- evolution and classification interlinked?
Ans. When we classify organism we look for similarities among organism which allows us to group them. Based on these principles we can work out the evolutionary relationship to the species.

Q6. Explain the terms analogous and homologous organs with examples.
Analogous organs: Such organs which perform a similar function but are different in structure and origin. Example- Wings of birds and wings of insects.
Homologous organs: Such organs which may have different functions but similar structure and origin. Example- forearm of frog, lizard and bird.

Q7. Outline a project which aims to find the dominant coat colour in dogs.

Q8. Explain the importance of fossils in deciding evolutionary relationship.
(i) Study of fossils allows us to make estimates of how far back evolutionary relationship go between organisms.
(ii) Study of the age of fossils allows us to know which organisms evolved earlier and which later.

Q9. What evidence do we have for the origin of life from inanimate matter?
Ans. The evidence was given by Stanley L. Miller and Harold C. Urey in 1953. They assembled an atmosphere similar to that thought to exist on early earth over water. This was maintained by them at a temperature just below 100 degrees Celcius and sparks were passed through the mixture of gases to stimulus lightening. At the end of the week, they found that 15% of the carbon had been converted to simple compounds of carbon including amino acids which make up protein molecules.

Q10. 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?
Ans. Variations arise either because of errors in DNA copying or as a result of sexual reproduction. Due to sexual reproduction genetic variability increases in the population from one generation to another. This happens due to the fact that sexually reproducing organism inherits half the genes from each parent. These variations are very important for the process of evolution.

Q11. 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?
Ans. No, depending on the nature of variations different individuals have been different kinds of advantages. However, when a drastic change occurs in the environment only those organisms in the population will survive which have an advantageous variation in that population to survive in the changed environment.

Q12. How is the equal genetic contribution of male and female parents ensured in the progeny?
Ans. The equal contribution of male and female parents is ensured in progeny during sexual reproduction. Each trait of progeny is determined by a pair of alleles and gametes of male and female contain one allele. Each allele pairs during fertilisation combine together to determine traits. Thus, the traits of progeny are determined by equal genes from male and female.


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