Chapter 9: Heredity and Evolution- Exercise- 2

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Chapter 9: Heredity and Evolution

Class: X

Exercise number 2

Question 1

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

Answer 1

Mendel chose pea plants (as they reproduce quickly). He took pure breeding tall (TT) and dwarf (tt) pea plants. These plants are the parental or P, generation. He crossed these two plants. The resulting offspring seeds formed after fertilization were called the first filial or F1 generation. All the F1 plants obtained were tall.

Cross-pollination of tall and short plant

Science Class 10 Chapter 9 NCERT

Next, Mendel self-pollinated the F1 plants and observed that one-fourth of the F2 plants were short. All plants obtained in the F2 generation were not tall. So, the trait for tall plants had not disappeared; it had been masked or hidden.

Science Class 10 Chapter 9 NCERT

Self-pollination of F1 plants

Mendel concluded with the result of this experiment that, the F1 tall plants were not true breeding. They were carrying traits of both short height and tall height. The tall trait is dominant over the dwarf trait, and that’s the only reason they appeared tall.

Question 2

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

Answer 2

Then, Mendel crossed pea plants having round green seeds (RRyy) with pea plants having

wrinkled yellow seeds (rrYY).

Dihybrid crosses

Science Class 10 Chapter 9 NCERT

F1 generation of pea plants will have both the characters of green round seeds and yellow wrinkled seeds in them, because the F1 plants are formed after crossing pea plants having green round seeds and pea plants having yellow wrinkled seeds. But, as we know that yellow seed colour and round seeds are dominant characters, therefore, the F1 plants will have yellow round seeds. Then this F1 offspring was self-pollinated and the F2 offspring 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.

Independent inheritance of two different traits

Science Class 10 Chapter 9 NCERT

More than two factors are involved in the above cross, and these are independently inherited.

Question 3

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?

Answer 3

Since we do not know about the blood group of all the progeny, we do not have sufficient information to determine which blood group is dominant. Blood group A can genotypically AA or AO. Therefore, the information given is incomplete to come to a conclusion.

Question 4

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

Answer 4

Human beings, have two female X chromosomes XX and wo male chromosome, one X and

one Y chromosomes XY. As we know, the gametes receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome, that is: 22+X OR 22+ Y. And the females have XX sex chromosomes, their gametes can only have X sex chromosome, that is: 22+X

Science Class 10 Chapter 9 NCERT