Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous bl
ouse. What is the probable phenotype ratio?

When crossing a heterozygous running, heterozygous black mouse (RrBb) with a homozygous running, homozygous black mouse (RRBB), we can set up a Punnett square. The potential genotypes of the offspring will be RrBB and RrBb. Therefore, the phenotype ratios for the traits will yield all running mice (as R is dominant over r) and all black mice (as B is dominant over b), leading to a phenotype of 100% running and 100% black.

Now, let’s break it down a bit. You end up with offspring that are 50% RrBB and 50% RrBb. This means all are “running” and “black” in phenotype, reflecting the dominance of both traits without variation in appearance. It’s neat how some simple genetics can yield such straightforward results!