4. Summer squash fruits can be shaped like a disc, a sphere, or a football. The

Question

4. Summer squash fruits can be shaped like a disc, a sphere, or a football. The spherical shape occurs when either of two genes are homozygous recessive. When both genes are homozygous recessive, the fruit is football shaped. A. Using A and B to represent the dominant alleles, and a and b for the recessive alleles, what are the possible genotypes for: disc-shaped fruits spherical fruits football fruits: B. If a cross is made between homozygous disc and football squash plants, what fruit types would be seen in the F2 generation? In what ratios?

Explanation / Answer

a) If A and B are dominant to a and b, the possible genotypes of the fruits are as follows:

b) Crossing disc and football fruits: AABB x aabb = AaBb in F1 generation. In F2 generation:

Genotype ratio - 1 AABB : 2 AABb : 2 AaBB : 4 AaBb : 1 AAbb : 2 Aabb : 1 aaBB : 2 aaBb : 1 aabb - 1:2:2:4:1:2:1:2:1.

Phenotype ratio - 9 disc shaped fruits : 6 spherical fruits : 1 football fruit - 9:6:1

So, the observed phenotype ratio will be 9:6:1

c) The genotype of the F1 plant is AaBb. Here the test cross involves breeding of the F1 AaBb with a phenotypically recessive individual aabb.

Genotype ratio - 4 AaBb : 4 Aabb : 4 aaBb : 4 aabb - 1:1:1:1

Phenotype ratio - 4 disc shaped fruits : 8 spherical fruits : 4 football fruits - 1:2:1

The observed phenotype ratio will be 1:2:1

AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb

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