Test Crosses

Whenever we have a dominant allele, we express the dominant phenotype. This means we cannot be certain of our genotype. We may be heterozygous, or homozygous dominant. If the allele is represented by the letter A (or a), we could be Aa or AA.

How do we find out the genotype of an individual expressing the dominant phenotype? In humans, it is a bit unethical, so we would struggle to do so without mapping their genes - time consuming and costly at the moment!! In plants and some animals, we do a test cross, breeding it with an individual expressing the recessive phenotype.