Dominant and recessive epistasis

*Reading across, each row gives the phenotypic ratios of progeny from a dihybrid cross (AaBb x AaBb).

*Reading across, each row gives the phenotypic ratios of progeny from a dihybrid cross (AaBb x AaBb).

Crosses rarely produce exactly 16 progeny; therefore, modifications of a dihybrid ratio are not always obvious. Modified dihybrid ratios are more easily seen if the number of individuals of each phenotype is expressed in sixteenths: x number of progeny with a phenotype 16 total number of progeny where x/16 equals the proportion of progeny with a particular phenotype. If we solve for x (the proportion of the particular phenotype in sixteenths), we have:

number of progeny with a phenotype X 16

total number of progeny

For example, suppose we cross two homozygous individuals, interbreed the F1 and obtain 63 red, 21 brown, and 28 white F2 individuals. Using the preceding formula, the phenotypic ratio in the F2 is: red = (63 X 16)/112 = 9; brown = (21 X 16)/112 = 3; and white = (28 X 16)/112 = 4. The phenotypic ratio is 9:3:4

A final point to consider is how to assign genotypes to the phenotypes in modified ratios owing to gene interaction. Don't try to memorize the genotypes associated with all the modified ratios in Table 5.2. Instead, practice relating modified ratios to known ratios, such as the 9:3:3:1 dihybrid ratio. Suppose we obtain 15/16 green progeny and 1/16 white progeny in a cross between two plants. If we compare this 15:1 ratio with the standard 9:3:3:1 dihybrid ratio, we see that 9/16 + 3/16 + 3/16 equals 15/16. All the genotypes associated with these proportions in the dihybrid cross (A_B_, Abb, and aaB_) must give the same phenotype, the green progeny. Genotype aabb makes up 1/16 of the progeny in a dihybrid cross, the white progeny in this cross.

In assigning genotypes to phenotypes in modified ratios, students sometimes become confused about which letters to assign to which phenotype. Suppose we obtain the following phenotypic ratio: 9/16 black : 3/16 brown : 4/16 white. Which genotype do we assign to the brown progeny, A bb or aaB_? Either answer is correct, because the letters are just arbitrary symbols for the genetic information. The important thing to realize about this ratio is that the brown phenotype arises when two recessive alleles are present at one locus.

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