How To Determine An Unknown Genotype Using A Test Cross

Long before the discovery that deoxyribonucleic acid was the molecule responsible for passing down traits from parents to their offspring, the Central European monk Gregor Mendel conducted experiments in pea plants to figure out the workings of the process of heredity. By establishing the principles of genetic dominance and recessiveness, Mendel determined how to find the genotype of an individual by observing its offspring from a test cross.

In Mendelian genetics, each measurable trait, phenotype, of an individual, such as its flower color, stem length, or seed shape, is controlled by a pair of genes. Differences in these traits are caused by different individuals possessing alternate forms of the same genes, known as alleles. For example, the pea plants Mendel studied possessed either rounded seeds or wrinkled seeds. Many of these plants, when left to self-pollinate, were true-breeding, yielding offspring of the same phenotype: round seed parents produced all round seed offspring and vice versa.

However, Mendel noticed that some of the round seed plants, when self-pollinated, produced a mixture of round and wrinkled offspring. Moreover, the self-pollinated wrinkled seed plants never produced round seed progeny. Mendel concluded that the round seed parents in this case must have possessed a wrinkled allele, but that the expression of this gene was masked by the presence of a round allele. Likewise, the true-breeding wrinkled plants must have possessed two copies of the wrinkled allele. Because of this behavior, he designated round seeds as "dominant" and wrinkled seeds as "recessive," and he found that many other traits followed similar patterns.

This discovery meant that an unknown round seed plant could be either homozygous, carrying two dominant alleles, or heterozygous, carrying one dominant and one recessive allele. To distinguish between these possible genotypes, Mendel developed the procedure known as the test cross. He took a wrinkled seed plant, which he knew to be homozygous for the recessive allele, and cross-pollinated it with the mystery plant. He then looked at the phenotypes of the progeny from the cross.

Mendel knew that each progeny received one copy of the gene for seed shape from each parent. Therefore, all were guaranteed to have one recessive allele from the wrinkled parent. If the round seed parent was homozygous, then the offspring would all receive a dominant allele as well, resulting in uniform heterozygosity and round seeds. Conversely, if that parent was heterozygous, half of the offspring would receive a recessive allele, resulting in a one to one mixture of round and wrinkled seed progeny. To Mendel, these visible results revealed the then-invisible workings of heredity.