Class Ten Science NCERT Chapter 9 Solutions

 Chapter 9: Heredity and Evolution

 Section A – Solution to Textbook Exercises

Q.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 species, which trait is likely to have arisen earlier?

Ans. In asexual reproduction, offspring are produced from single parents. There may be small inaccuracies in DNA copying which can develop new traits. They will be in a smaller proportion than the traits already present. Therefore, trait B which exists in 60% of the population must have arisen earlier than trait A which occurs in 10% of the population.

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

Ans. The variations generated in offspring do not have equal chances to survive and get inherited in the next generation. The inheritance of such characteristics or variations depends on a number of environmental factors as well as on the nature of variation. Same variations are preadaptations which can be beneficial under certain environmental condition. For example, in a heatwave, most of the bacteria will die but a few having pre-adaptation or variation to tolerate heat wave, will survive and multiply.

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


Illustrate Mendel’s experiment to show that traits may be dominant or recessive by taking tall/short plants as a character in garden pea.

Ans. Mendel took pea plants with contrasting characters some with short and some with the tall stem. He produced the progeny of first-generation (F1) from them. All the plants were tall in the F1 generation, there was no intermediate characteristic. Mendel used the progeny of F1 as parent plants and produced the progeny of F2 generation to test whether the tallness of F1 progeny was the same as their parents. He noticed that the progeny of F2 generation were not all tall. One-fourth of progeny were short. This characteristic of shortness proves that both the characteristics (tallness and shortness) were inherited from the parents to F1 progeny. In F1 progeny only tallness character was expressed. However, the second-generation progeny (F2 progeny) expressed both the characters in a particular ratio i.e. 3: 1 phenotypically and 1: 2: 1 genotypically. In a cross, trait (T) which expresses itself in the hybrid (Tt) is called dominant. In such a hybrid, only T is sufficient to express its trait i.e. tallness. While the trait which does not express itself in the hybrid is called recessive trait. In such a trait both the copies should be tt.

Q.4. How do Mendel’s experiments show that traits are inherited independently?

 Mendel crossed a variety of pea plant with round, yellow seeds with another variety having wrinkled green seeds, the F1 progeny showed only round yellow seeds. After self-fertilisation of F1 plants, the F2 progeny obtained, showed four different types of plants having seeds in a ratio 9 (round, yellow) : 3 (round, green) : 3 (wrinkled, yellow): 1 (wrinkled, green). This is called a dihybrid ratio (9 : 3 : 3: 1). This shows that each trait is inherited independently of others.