You have 2 true breeding strains of a plant. Both strains have white flowers. Wi

Question

You have 2 true breeding strains of a plant. Both strains have white flowers. Wild-type

flower color in this species is red. When strain #1 was crossed to wild-type plants, the F1 plants

had red flowers. When strain #2 was crossed to wild-type plants, the F1 plants had red flowers.

When the two white flowered mutants were crossed together, the F1 plants had red flowers.

When the F1 plants from crossing the two mutants are self-fertilized, the following F2 progeny were obtained:

F2 phenotypes #

Red flowers 15

White flowers 185

Propose a hypothesis to explain the inheritance of flower color in these plants. Clearly describe how

many genes are involved, the dominance relationships of alleles, and any interactions that may be

occurring between genes. Diagram out the crosses, assigning genotypes and phenotypes to the parental,

F1, and F2 plants.

Explanation / Answer

There is some kind of epistasis which is present. 185:15 is a 12.3:1 ratio (200/16 = 12.5). Any cross that comes out in “16ths” must involve 2 genes (a variation of 9:3:3:1), so this looks like there are two genes involved in flower color and there is some kind of epistasis.

So, if we just call the genes A and B, then the pure-breeding parents must have been AABB (red) and aabb (white)

The F1 is all red, so we know that the white alleles of both genes must be recessive.

Now, the F2 must be AaBb x AaBb, and (as with any F2) cross, we get genotype ratios of 9/16 A-B-, 3/16 Abb, 3/16 aaB- and 1/16 aabb. How can this come out 12.5:1? Well, if only aabb is white (that is, if you have to be homozygous recessive for both genes to be white), then 1/16 of the offspring are white and all the rest are red.

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