Question

A pea plant with blue flowers denoted by BB is cross- bred with a pea plant with white flowers denoted by ww.
A. What is the expected colour of the flowers in F1 generation?
B. What will be the percentage of plants bearing white flowers in F2 generation, when the flowers were selfed?

Answer 1

Hint: B allele is dominant over b allele. B allele encodes blue trait and blue colour is dominant over white colour. BB and Bb genotypes encode blue colour. Whereas bb encodes white colour.

Complete answer:
A. It is given that there are two parents having blue and white flowers. Genotype of the blue flower is BB. Whereas white flower plants bear bb genotypes. Because bb encodes white flowers.

Cross-bred:
Parents: BB (Blue) $\times$ bb (white)
BB parent produces B allele bearing gametes. Bb parent produces b allele bearing gametes.

Punnett square:

gametes	b
B	Bb ( Blue)

The phenotype of Bb genotype is blue. Because B allele is dominant over b allele.

B. It is given that F1 plants are self-crossed. The genotype of F1 is Bb.

Cross-bred:
Parents: Bb (blue) $\times$ Bb (Blue)
Bb parents produce B and b gametes by the process of meiosis.

Punnett square:

Gametes	B	b
B	BB (Blue)	Bb(Blue)
B	Bb (Blue)	bb (White)

Diagrammatic representation:

One out of four types are white colour bearing progenies.
Percentage of white colour’s flower bearing progenies is,
$\left ( \dfrac{\text{Favorable outcomes}}{\text {total outcomes}} \right )\times 100$
Favorable outcomes=1
Total outcomes= 4
Percentage of plants bearing white flower in F2 generation is,
$ \left ( \dfrac{1}{4} \right ) \times 100 = 25 \times 100= 25\% $

Note: Genotype of heterozygous plant is Bb. It produces blue flowers. The presence of either a single or two allele in the genotype gives Blue color. The genotype of white flower bearing plant is ww.