Dihybrid Cross

Unlike a monohybrid cross, a dihybrid cross deals with two traits. Here are a couple problems.

1. In mice, the ability to run normally is a dominant trait. Mice with this trait are called running mice (R). The recessive trait causes mice to run in circles only. Mice with this trait are called waltzing mice (r). Hair color is also inherited in mice. Black hair (B) is dominant over brown hair (b). Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous black mouse. What is the offspring phenotypic ratio?

        First we have to cross a  heterozygous running, heterozygous black mouse (RrBb) with a homozygous running, homozygous black(RRBB). The results would be 4 homozygous running, homozygous black (RRBB), 4 homozygous black, heterozygous running(RrBB), 4 heterozygous black, heterozygous running (RrBb), and 4 heterozygous black and homozygous running(BbRR). So the answer would be all running, black. 

2. Cross a homozygous running, heterozygous black mouse with a waltzing brown mouse. What are the parental genotypes?

          The genotypes would be RRBb x rrbb

3. Cross a heterozygous running, brown mouse with a heterozygous running, homozygous black mouse. What are the genotypes for the offspring at the running allele?

           So you would just focus on the crossing the running gametes so you would cross Rr x Rr. and the running genotypes would be RR, Rr, rr, because when you cross Rr x Rr you get 1 RR: 2 Rr: 1 rr.

4. Cross a heterozygous running, heterozygous black mouse with a heterozygous black mouse. What is the offspring phenotypic ratio?

           So you would cross RrBb x RrBb and you would get 9 running black : 3 running brown: 3 waltzing, black: 1 waltzing, brown.