测试十字架

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.