CBSE Class 10 Science 2022 · Paper 31/6/2 · Question 15

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)Write any two contrasting visible characters other than the ones shown above, taken by Mendel for his experiment.
(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

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

F_{1}

seeds are round and yellow. (b) Any two other pairs that Mendel used, for example tall and short (dwarf) plants, and violet and white flowers. (Others: axial and terminal flower position, inflated and constricted pods, green and yellow pods.) (c)(i) Every

F_{1}

plant (

R r Y y

) carries the factors for wrinkled (

r

) and green (

y

) seeds, but they are masked by the dominant

R

and

Y

. The factors are not lost or blended: the two copies of each separate when gametes form, and the shape and colour factors are sorted into the gametes independently. On self-pollination some zygotes receive

r

from both gametes (

r r

) or

y

from both (

y y

), and these seeds show the recessive traits again.

F_{2}

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

= 9 : 3 : 3 : 1

. (c)(ii) OR : (I) A dominant trait is one that is expressed even when only one copy of its factor is present, as in the hybrid

F_{1}

R R

and

R r

seeds are both round. (II) A recessive trait is one that is masked by the dominant factor and is expressed only when both copies are recessive: only

r r

seeds are wrinkled. The recessive trait stays hidden in

F_{1}

and reappears in about one quarter of the

F_{2}

seeds.

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From the board paper

CBSE Class 10 Science 2022 · Paper 31/6/2, Question 15 — all 15 questions with answers →

CBSE Class 10 Science 2022 · Paper 31/6/2 · Question 15

Answer

From the board paper

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