Dec 5

Consider the general form of a dihybrid cross (AaBb x AaBb). If alleles A and B are simple dominants, we observe a 9:3:3:1 Mendelian ratio. What kinds of interactions among the loci will produce:

* 9:7

* 9:3:4

* 15:1

Remember that for two loci, the numbers will always add to 16 and the ratios will usually be some combination of 9,3,3 and 1. So,

9:7 must come from 9 : (3+3+1). If the genotype is recessive at a or at b or both, the individual will have a recessive phenotype. That usually happens when there is a two step pathway and mutations at either step will produce no product.

9:3:4 comes from 9 : 3 : (3+1). In that case, one of the recessives looks the same as the double recessive. If the first step of a pathway is blocked (aa), it doesn't matter what the genotype of the second locus is (bb or B-).

15:1 comes from (9+3+3) : 1. The individual must be recessive at both loci to show the recessive phenotype. In a biochemical pathway, that happens when there are duplicate enzymes. As long as either one is functional the product will be made.