Think of it this way, each parent has two copies (alleles) of the gene. Each parent has one blue (b) and one brown (B) allele. There is an equal chance of getting one or the other from each parent, so the proportions will be equal of the following cross:

Bb X Bb = BB+Bb+bB+bb

The first three offspring each have at least one dominant B gene, and thus have brown eyes, while the fourth has two copies of the recessive blue allele, and thus has blue eyes. This is the classic 3:1 Mendelian ratio. The reason for dominance in this case is that the B allele produces brown eye pigment, while the b allele is defective. Therefore, just one B allele is enough to give pigment, while two null copies (blue) are needed in order to reveal the complete absence of pigment (blue). So much for Hitler’s theory that blue eyed people are superior – they are just defective in making pigment!

The parents in the above example are called heterozygotes (not to be confused with heterosexuals, although they are probably that also), because they have one of each allele. Two of the offspring are homozygotes: BB throws only brown alleles, and bb throws only blue ones. Thus, if you cross two blue-eyed (bb) parents, you get only blue-eyed children (if this is not the case – oops!). However, brown eyed children probably don’t know if they are Bb or BB, so it is harder to predict offspring than it is with bb X bb.

I hope you ace the test! By the way, this question finds its way into a test in almost any genetics course, so learn it well if you are a student of biology.