Question

In sweet peas, genes C and P are necessary for color in the flower. The flowers are white in the absence of either or both the genes. What will be the percentage of colored flowers in the offsprings of the cross Ccpp × ccPP?
A. 25%
B. 50%
C. 75%
D. 100%

Answer 1

Hint: The dihybrid cross involves the cross of two observable characters. The presence of both dominant alleles C and P is necessary for the coloration of the flower. Absence of any one of them or even both will result in white flowers.

Complete answer:
When both the genes are necessary to express a particular phenotype those genes are called complementary genes. Both C and P genes complement each other for expression of flower color.
This occurs due to the interaction of genes i.e. the expression of an allele of one gene will affect the expression of one allele of a second gene. C and P affect the expression of flower color.
According to the question, the flowers will be coloured when (C) and (P) are present together.
When Ccpp × ccPP –
Ccpp will segregate into gametes- Cp and cp.
ccPP will segregate into gametes cP and cP.
The representation of the cross is as follows-

	Cp	cp
cP	CcpP(coloured)	cpPp(white)

The ratio of the coloured to white is 1:1. So, on crossing Ccpp with ccPP 50% offsprings will be coloured.

Hence, the correct answer is option B, i.e. 50%.

Note: The interaction of complementary genes was studied in Lathyrus odoratus flowers for the pigmentation or coloration of flowers. As both the genes complement each other it does not follow the conventional Mendelian dihybrid ratio 9:3:3:1.