CBSE Class 10 Science 2022 · Paper 31/4/1 · Question 3(b)

Heredity 2 marks Medium

In a cross between red coloured and white coloured flowers, when plants with red coloured flowers of F1 generation were self pollinated, plants of F2 generation were obtained in which 75% of plants were with red flowers and 25% plants were with white flowers. Explain the inheritance of traits in the above cross with the help of a flow chart only along with the ratio of plants obtained.

Answer

Red RR × white rr gives all red Rr in $F_{1}$ ; selfing gives RR, Rr, Rr, rr, so $F_{2}$ has 3 red : 1 white.

Let

R

stand for the form of the gene for red flowers (dominant) and

r

for the form for white flowers (recessive). Parents:

R R

(red)

\times

r r

(white) Gametes:

R

and

r

F_{1}

R r

- all plants have red flowers

F_{1}

self-pollinated:

R r \times R r

Gametes:

R

and

r

from each parent

F_{2}

R R

R r

R r

r r

Ratio of plants in

F_{2}

: 3 red : 1 white (75% red, 25% white); genotype ratio

1 R R : 2 R r : 1 r r

. Red is dominant over white, so only red shows in

F_{1}

; white reappears in

F_{2}

, in the

r r

plants only.

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CBSE Class 10 Science 2022 · Paper 31/4/1 · Question 3(b)

Answer

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