Question

In corn, green photosynthetic tissue requires a dominant allele C. Corn plants that are homozygous cc for a recessive allele are pure white. A cross between green plants produced 936 green and 282 white plants. The cross was

Multiple Choice

A. CC X CC

B. Cc X cc

C. Cc X Cc

D. CC X Cc

E. CC X cc

Answer 1

We are looking at the phenotype in corn plants determined by a gene that has two alleles namely, C and c. C is dominant while c is recessive. CC and Cc produce green tissue while cc produces white tissue.

Since this involves a single trait, we can analyze the crosses using Mendelian genetics. Let us analyze each of the crosses mentioned. However, the outcome was obtained from a cross between green plants. Hence the only possible parental genotypes are CC and Cc. Thus, options B (Cc crossed with cc) and E (CC crossed with cc) are rejected.

1. CC vs CC

The Punnett square for this cross is:

	C	C
C	CC	CC
C	CC	CC

Since the cross involves parents without any recessive alleles, the offsprings produced will not have any white tissue. As can be seen from the Punnett square, all the offsprings are CC. Thus the phenotypic ratio is green: white = 4:0.

2. Cc vs Cc

The Punnett square for this cross is:

	C	c
C	CC	Cc
c	Cc	cc

Thus the phenotypic ratio is Green: White = 3: 1.

3. CC vs Cc

The Punnett square for this cross is:

	C	C
C	CC	CC
c	Cc	Cc

Thus the genotypic ratio is Green: White = 4: 0. Thus, no white plants are produced by this cross.

Now, in the given study, let the phenotypic ratio be P. Therefore,

This is the phenotypic ratio as would have been expected from a cross between Cc and Cc.

Thus, the correct option is C. Cc vs Cc