Class 10 Science · Chapter 8

8. Heredity

Complete solutions to the intext questions and exercises. CBSE Class 10 Science (NCERT).

Intext Questions (Page 129)

1If 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

In asexual reproduction, a trait passes from one generation to the next. The older a trait is, the more individuals will have it. Trait B is present in more individuals (60%). So it is likely to have arisen earlier.

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

Variations make some individuals different from others. When the environment changes, some of these individuals may be better suited to survive. For example, bacteria that can bear heat will survive a heat wave. These survivors reproduce and keep the species alive.

Intext Questions (Page 133)

1How do Mendel’s experiments show that traits may be dominant or recessive?

Mendel crossed tall pea plants (TT) with short pea plants (tt). All the plants of the F1 generation were tall. The short trait did not appear.

When F1 plants were self-pollinated, the F2 generation had tall and short plants in the ratio 3 : 1. So the short trait was present in F1 but was hidden.

The trait that is expressed (tall) is dominant. The trait that is hidden (short) is recessive.

Monohybrid cross between a tall and a short pea plant.

2How do Mendel’s experiments show that traits are inherited independently?

Mendel crossed pea plants with round yellow seeds (RRYY) with plants having wrinkled green seeds (rryy). All F1 plants had round yellow seeds.

In F2, he got four types of plants: round yellow, round green, wrinkled yellow and wrinkled green, in the ratio 9 : 3 : 3 : 1.

New combinations (round green and wrinkled yellow) appeared. This shows that seed shape and seed colour are inherited independently of each other.

3A 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?

No, this information is not enough.

The daughter could get blood group O in more than one way. We need data from a large number of such families. Only then can we know which trait is dominant.

In fact, blood group A is dominant over O. If the father has genes A and O (AO), he can pass on O. Then the daughter gets O from both parents and has blood group O.

4How is the sex of the child determined in human beings?

Women have a perfect pair of sex chromosomes, XX. Men have a mismatched pair, XY.

All eggs from the mother carry an X chromosome. A sperm from the father may carry X or Y.

If a sperm with X fertilises the egg, the child will be a girl (XX).
If a sperm with Y fertilises the egg, the child will be a boy (XY).

So the sex of the child depends on which chromosome it gets from the father.

Sex determination in human beings.

Exercises

1A 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

Answer: (c) TtWW

All progeny have violet flowers, so the tall parent is WW. Half the progeny are short, so the tall parent is Tt.

2A 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?

No, we cannot say.

Both parents and children have the same light eye colour. We do not know the genes the parents carry. To know which trait is dominant, we need crosses between parents with light eyes and parents with dark eyes. We also need to study many generations.

3Outline a project which aims to find the dominant coat colour in dogs.

Select a pure black dog and a pure white dog (say) of the same breed.
Cross them and study the coat colour of the puppies (F1 generation).
The colour shown by all the F1 puppies is the dominant colour.
Then cross two F1 dogs. In the F2 generation, the dominant and recessive colours will appear in the ratio of about 3 : 1.
Repeat with many pairs of dogs to get reliable results.

4How is the equal genetic contribution of male and female parents ensured in the progeny?

Each cell of the body has two sets of chromosomes. But germ cells (sperm and egg) have only one set. When a sperm fuses with an egg, the zygote gets one set from the father and one set from the mother. So both parents contribute equally to the genes of the progeny.

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