16.3: Test Cross

Alleles are different forms of the same gene. Humans and other diploid organisms inherit two alleles of every gene, one from each parent.

An allele is recessive if its effects are masked by another allele at the same gene location. For example, pea plants can have purple or white flowers. In this case, white flowers are recessive since a single copy of the allele for purple flowers will result in a plant with purple flowers, even if they also have the allele for white flowers.

If an organism exhibits a known recessive trait, determining the organism’s genotype—its pair of inherited alleles—is simple. Only one genotype, pp (two recessive alleles), produces white flowers.

For an organism with a dominant trait—like a pea plant with purple flowers—determining genotype is not as straightforward. Two genotypes, PP and Pp, produce purple flowers.

Scientists use test crosses to determine the genotypes of organisms exhibiting simple dominant traits. A test cross involves breeding, or crossing, the organism in question with one displaying the recessive counterpart of its dominant trait.

In a test cross for a pea plant with purple flowers, the plant is bred with a pea plant that has white flowers (pp genotype). The flower color of the resulting offspring reveals whether the parent with purple flowers is homozygous (PP) or heterozygous (Pp).

If the plant is homozygous, all of the offspring will inherit one of its dominant P alleles and one of the other parent’s recessive p alleles. Since dominant alleles mask recessive alleles, all offspring will have purple flowers, the dominant trait.

If the plant is heterozygous, however, about 50% of the offspring will inherit its recessive p allele, along with another recessive allele from the other parent. Therefore, about half the offspring from this cross will have white flowers. In this way, test crosses can reveal unknown parental genotypes.

A test cross is a technique that can be used to determine the genotype of an organism expressing a dominant trait, like purple flower color in pea plants.

Because the dominant version, or allele, of a gene will always be expressed over the recessive allele, organisms with the homozygous dominant genotype, two dominant alleles, will appear the same as organisms with the heterozygous genotype, one dominant allele.

A purple-flowered pea plant of unknown genotype can be crossed to a pea plant that has the recessive phenotype, white flowers, and thus is known to have the homozygous-recessive genotype, pp.

In one scenario, if all of the offspring from the test cross display the dominant phenotype, the unknown parent must be homozygous dominant for purple flowers. All of the offspring are heterozygous, receiving one recessive allele from the recessive parent and one dominant allele from the other parent.

However, if the offspring include equal numbers of organisms, displaying the dominant phenotype and organisms displaying the recessive phenotype, the unknown parent must be heterozygous.

All of the offspring still receive their recessive allele from the recessive parent; half of the offspring received a dominant allele from the heterozygous parent and exhibit the dominant phenotype, while the other half received a second copy of the recessive allele from the heterozygous parent and exhibit the recessive phenotype.