Heredity and Evolution

Class 10 NCERT Science


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?

Solution :

In asexual reproduction, the reproducing cells produce a copy of their DNA through some chemical reactions. However, this copying of DNA is not accurate and therefore, the newly formed DNA has some variations.$\\$ It can be easily observed in the above figure that in asexual reproduction, very few variations are allowed. Therefore, if a trait is present in only $10$% of the population, it is more likely that the trait has arisen recently. Hence, it can be concluded that trait B that exists in $60$% of the same population has arisen earlier than trait A.

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

Solution :

Sometimes for a species, the environmental conditions change so drastically that their survival becomes difficult. For example, if the temperature of water increases suddenly, most of the bacteria living in that water would die. Only few variants resistant to heat would be able to survive. If these variants were not there, then the entire species of bacteria would have been destroyed. Thus, these variants help in the survival of the species. However, not all variations are useful. Therefore, these are not necessarily beneficial for the individual organisms.

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

Solution :

Mendel showed that when two true-breeding plants of contrasting traits for the same character are crossed the progeny exhibits only one of the traits. He demonstrated this using the monohybrid cross. Monohybrid cross is a cross between two plants in order to study the inheritance of a single character. For example,$\\$ Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these two plants. The seeds formed after fertilization were grown and these plants that were formed represent the first filial or F1 generation. All the F1 plants obtained were tall.

$\text{Cross-pollination of tall and short plant}$$\\$ Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short.$\\$ $\text{Self-pollination of F1 plants}$$\\$ From this experiment, Mendel concluded that the F1 tall plants were not true breeding. They were carrying traits of both short height and tall height. They appeared tall only because the tall trait is dominant over the dwarf trait.

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

Solution :

Mendel’s dihybrid cross demonstrate independent assortment of genes. A dihybrid cross is a cross between two plants with alternate traits for two characters. For example, Mendel crossed pea plants having round green seeds (RRyy) with pea plants having wrinkled yellow seeds (rrYY).$\\$ $\text{ An example of dihybrid crosses}$$\\$ Since the F1 plants are formed after crossing pea plants having green round seeds and pea plants having yellow wrinkled seeds, F1 generation will have the dominant trait for both these characters in them (Law of Dominance). In pea, yellow seed colour and round seeds are dominant characters, Therefore, the F1 plants will have yellow round seeds. Then this F1 progeny was self-pollinated and the F2 progeny was found to have yellow round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds in the ratio of $9:3:3:1.$$\\$ $\text{Independent inheritance of two different traits}$$\\$ The appearance of two new variants (different from the parental types) in the F2 generation showed that the genes for the two characters are assorted independently. In the above cross, more than two factors are involved, and these are independently inherited.

5   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 traits - blood group A or O - is dominant? Why or why not?

Solution :

No. This information is not sufficient to determine which of the traits - blood group A or O - is dominant. This is because we do not know about the blood group of all the progeny. Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.