A cross is carried out between two guinea pigs that each carry one B allele and one b allele. Grouping the offspring by the pair of alleles they carry rather than by appearance, what ratio is expected?

Explication

Sorting by allele pairs gives four equally likely results, and the mixed pair can be assembled in two ways while each matching pair can be assembled in only one, so the middle group is twice the size of either outside group. Counting three animals with at least one B allele against one with two b alleles merges the mixed group into the group carrying two B alleles, which sorts the litter by appearance and answers a different question about the same cross.