CBSE Class 10 Science 2022 · Paper 31/1/3 · Question 11

Heredity 3 marks Medium

A red-eyed individual is crossed with a white-eyed individual to produce

F_{1}

progeny with red eyes. When

F_{1}

individuals are intercrossed,

F_{2}

progeny is formed with both red as well as white-eyed individuals.

(a)How is the dominant trait identified ?
(b)What are recessive traits ?
(c)If 12 individuals are produced in $F_{2}$ generation, then how many white-eyed individuals would be obtained ? Calculate the ratio of red-eyed individuals to white-eyed individuals.

Answer

Red is dominant; recessive traits stay hidden in $F_{1}$ ; 3 of the 12 are white-eyed, and red : white is $3 : 1$ .

(a) When two pure-breeding individuals with contrasting traits are crossed, the trait that alone appears in the

F_{1}

progeny is the dominant trait. Here all the

F_{1}

individuals have red eyes, so red eye colour is dominant. (b) Recessive traits are traits that remain hidden in the

F_{1}

generation in the presence of the dominant factor and reappear in the

F_{2}

generation; in Mendel's account they are expressed only when both copies of the factor are recessive, as in

r r

. White eye colour is the recessive trait here. (c) The

F_{1}

individuals are

R r

, and

R r \times R r

gives

1 R R : 2 R r : 1 r r

, so one quarter of the

F_{2}

is expected to be white-eyed:

\frac{1}{4} \times 12 = 3

white-eyed individuals (and

9

red-eyed). The ratio of red-eyed to white-eyed individuals is

9 : 3 = 3 : 1

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