You cross two purebreeding whiteflowered plants together and

You cross two pure-breeding white-flowered plants together and in the F1 you get a

purple-flowered plant. You allow that purple-flowered F1 plant to self-fertilize and

observe in the F2 a ratio of 9 purple-flowered plants to 7 white-flowered plants.

a. How many genes control flower color in this example? (Circle one)

one gene two genes

b. List all the genotype(s) that produce white-flowered plants.

Solution

In the given example there should be two genes which are interfering with the flower colour because otherwise two purebred white color flowered plants can never give purple flowered plants. Let us suppose B denotes purple flowers which is the dominant allele over b which denotes white flowers. Another gene A eill interfere with the flower colour allowing the deposition of pigments in the flowers. This allele A is dominant over a, which suggests that when the genotype is aa, the flower will be white colored irrespective of whether B/b is present because aa will not allow any deposition of pigments on tge flowers. Thus the possible genotypes of purple flowered plants will be, AABB, AaBB, AABb, AaBb. The possible genotypes of white flowered plants will be aaBB, aaBb, AAbb, Aabb, aabb. Now in the given question two purebred white flowered plants gave a purple flowered plant. The cross can thus be between aaBB(white) & AAbb(white) to produce purple flowered plant AaBb. This cross cannot be between aaBB & aabb although both being pure bred because then in F1, the offspring would be aaBb which will be white and not purple flowered plant. Thus the cross was made between aaBB© AAbb to produce AaBb in F1.

If self crossed AaBb©AaBb