7.7: A test-cross enables us to figure out which alleles an individual carries.

How can you see something that is invisible to the naked eye? Genes are too small to be seen, and so determining an individual’s genotype requires indirect methods. Suppose you are in charge of the alligators at a zoo. Some of your alligators come from a population in which white, albino alligators have occasionally occurred, although none of your alligators is white. Because white alligators—those having two recessive pigmentation alleles, mm (“m” for melanin)—are popular with zoo visitors, you would like to produce some at your zoo through a mating program.

The problem is that you cannot be certain of the genotype of your alligators. They might be homozygous dominant, MM, or they might be heterozygous, Mm. In either case, their phenotype is normal coloration. How can you figure out the genotype of a particular alligator? This is a challenge to animal breeders, but not an insurmountable one. Genes may be invisible, but their identity can be revealed by a simple tool called the test-cross.

In the test-cross, you cross (i.e., mate) an individual exhibiting a dominant trait but whose genotype is unknown with an individual that is homozygous recessive. Then you examine the phenotypes of their offspring. In the case of your breeding program for albino alligators, you could borrow an albino alligator from another zoo and breed your unknown-genotype alligator (genotype: M_) with that albino alligator (genotype: mm). There are two possible outcomes, and they will reveal the genotype of your unknown-genotype alligator (FIGURE 7-15). If your alligator is homozygous dominant (MM), it will contribute a dominant allele, M, to every offspring. Even though the albino alligator will contribute the recessive allele, m, to all its offspring, all the offspring of this cross will be heterozygous, Mm, and none of them will be albino.

If, on the other hand, your unknown-genotype alligator is heterozygous, Mm, half of the time it will contribute a recessive allele, m, to the offspring. In every one of those cases, the offspring will be homozygous recessive and thus albino.

So the cleverness of the test-cross is that when you cross your unknown-genotype organism with an individual showing the recessive trait (and so having the known genotype of mm), the offspring will reveal the previously unknown makeup of the parent. To be confident in concluding that the unknown-genotype alligator has the MM genotype, though, you’d have to observe many, many offspring. After all, even if its genotype is Mm, quite a few offspring in a row might be normally pigmented, with the genotype Mm. Eventually, however, a heterozygous individual is likely to produce an offspring with the homozygous recessive genotype, mm, and the albino phenotype.