Unit 11: How to Complete Diyhybrid Crosses

Dihybrid Crosses

A dihybrid cross is a way to study two traits at a time. When studying two traits at a time, Mendel's Law of Independent Assortment comes into play. Most traits are inherited independently of each  other. For instance, being right handed does not mean you will have freckles. Example: Being right-handed (R) is dominant to being left-handed (r). Having freckles (F) is dominant to not having freckles (f). Cross a mother who is heterozygous for right-handedness (Rr) and  heterozygous for freckles (Ff) with a father who is also right-handedness (Rr)  and heterozygous for freckles (Ff).  In order to complete a dihybrid cross,  -the first step is to identify the genotypes of the two traits in each organism:  RrFf x RrFf. -The next step is to FOIL (mathmatically) each organisms alleles.  To do so, Multiple the First term of each pair, the Outside term of each pair, the Inside term of each pair, and the Last term of each pair. Next, place each pair of alleles from the FOIL on a different square of the  Punnett diagram. Then place each allele with its partner from the side of the  square. To determine the genotypic ratio, count how many of each kind of genotypes  there are, beginning with the dominant genotypes: RRFF, RRFf, RrFF, RrFf, RRff, Rrff, rrFF, rrFf, rrff =1, 2, 2, 4, 1, 2, 1, 2, 1 The genotypic ration for the above problem is 1:2:2:4:1:2:1:2:1 To determine the phenotypic ratio, count how many of each kind of phenotypes there are, beginning with the dominant phenotype and proceeding in this order: Dominant Dominant = Right-handed with freckles RRFF, RRFf, RrFF, RrFf = 9 Dominant Recessive = Right-handed no freckles RRff, Rrff = 3 Recessive Dominant = Left-handed with freckles rrFF, rrFf = 3 Recessive Recessive = Left-handed no freckles rrff = 1 The phenotypic ratio for the above problem is 9:3:3:1. In fact, anytime both  parents are heterozygous for both traits then the phenotypic ratio will always  be 9:3:3:1! For more information, see the Bozeman Science video "A Beginner's Guide to Punnet Squares".