Question

B = black

b = white

Two rats, both heterozygous for black fur, mate and have offspring. What is the probability that these two rats will have offspring with white fur?

Answer 1

If they are both heterozygous, then they both have alleles Bb.
Using a Punnett square, we draw the following:

B b
B BB Bb
b bB bb

Since B is dominant, so only bb will give white fur, i.e. 1 out of four is expected to give white fur.
Probability is therefore 1/4.