In a cross between two guinea pigs that each carry one B allele and one b allele, what is the chance that an offspring carries one allele of each kind?

Explanation

The mixed pair can be assembled in two ways, taking the dominant allele from one parent or from the other, and all four outcomes of the cross are equally likely. Choosing one in four counts only one of those two routes and leaves out the second way of arriving at the very same pair.