In rabbits the allele B is dominant to the allele b. Two rabbits that each carry one B allele and one b allele are bred. What is the chance that an offspring carries two b alleles?

Explanation

Four combinations of parental alleles are equally likely, and only one of them brings a b allele from each parent together. Counting one in three comes from listing the outcomes that are not two B alleles and forgetting that those remaining outcomes are not all the same as each other.