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?

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.

A trait that is visible is eye color