Determining Unknown Genotypes

In the garden pea, the gene that determines red flowers is dominant over that which determines white flowers. A plant with red flowers may be either homozygous or heterozygous for this characteristic. To establish its correct genotype it is crossed with a homozygous recessive plant. If their offspring bear red flowers, then this indicates that the red-flowered plant is homozygous, that is, it is from a pure line.

If on the other hand, their offspring bear a mixture of white and red flowers in equal proportions giving the ratio of 1:1, this indicates that the red-flowered plant was heterozygous

A cross between an individual of unknown genotype with an individual of recessive genotype is known as a test cross.

Unknown genotypes can also be determined by carrying out selfing experiments. For example, a phenotypically tall plant is either genotypically homozygous or heterozygous for this trait. If selfed and all its offspring are phenotypically tall then the parental genotype is TT, that is, homozygous dominant. However, if after selfing both tall and dwarf offspring are produced in the ratio 3:1, then the parental genotype is Tt, that is, heterozygous.