CBSE Class 10 Science 2022 · Paper 31/6/1 · Question 14

Heredity 4 marks Medium

A student crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds. He found that only one type of seeds were produced in the

F_{1}

generation. When these

F_{1}

generation pea plants were self-pollinated with each other, then in addition to the seed type of

F_{1}

generation, some new types of seed combinations were also obtained in the

F_{2}

generation.

(a)Mention the dominant traits observed in $F_{1}$ generation.
(b)What are the new possible combinations of seeds likely to be observed in $F_{2}$ generation ?
(c)(i)Give reason why the traits which were not visible in the seeds of $F_{1}$ generation reappeared in the seeds of $F_{2}$ generation. Write the ratio of different types of seeds obtained in $F_{2}$ generation in this case.
(c)(ii)OR : What is meant by the terms (I) dominant, and (II) recessive traits ? Explain.

Answer

Round yellow is dominant; the new combinations are round green and wrinkled yellow; masked recessive factors reappear in $F_{2}$ in the ratio $9 : 3 : 3 : 1$ .

(a) Round seed shape and yellow seed colour are the dominant traits: every

F_{1}

seed was round and yellow. (b) The new combinations are round and green seeds, and wrinkled and yellow seeds, which neither parent had. (Wrinkled green seeds, absent from

F_{1}

, also reappear in

F_{2}

, but that is the combination of one of the parents.) (c)(i) The factors for wrinkled shape and green colour were present in every

F_{1}

plant (

R r Y y

) but were masked by the dominant factors; they were not lost. When gametes form, the two factors of each pair separate, and the factors for shape and for colour are passed on independently, so some

F_{2}

seeds receive two recessive factors (

r r

y y

) and show the recessive traits again.

F_{2}

ratio: round yellow : round green : wrinkled yellow : wrinkled green

= 9 : 3 : 3 : 1

. OR (c)(ii) (I) A dominant trait is one that is expressed in

F_{1}

and shows even when only one factor for it is present (

R R

R r

gives round seeds). (II) A recessive trait is one that is hidden in

F_{1}

and shows only when both factors are recessive (

r r

gives wrinkled seeds). For example,

R R \times r r

gives an

F_{1}

of all round seeds (

R r

); selfing

F_{1}

gives

3

round

: 1

wrinkled in

F_{2}

, the recessive trait reappearing in one quarter.

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CBSE Class 10 Science 2022 · Paper 31/6/1 · Question 14

Answer

From the board papers

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