Class 10 Science

Heredity and Evolution

In Text Solution

Question 1: How is the sex of a child determined in human beings?

Answer: 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.

Question 2: 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 is likely to have arisen earlier because it is present in a larger portion of the population.

Question 3: How does the creation of variations in a species promote survival?

Answer: 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.

Question 4: How do Mendel’s experiments show that the traits may be dominant or recessive?

Answer: 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.

Question 5: How do Mendel’s experiments show that traits are inherited independently?

Answer: 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.

Question 6: 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?

Answer: 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.

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

Answer: 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.

Question 8: Why are traits acquired during the lifetime of an individual not inherited?

Answer: Acquired traits do not bring any change in the genotype of an individual. Hence, acquired traits do not get inherited.

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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?

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

Answer: 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.