Monohybrid Crosses

Using the pea plant, Gregor Mendel originally performed monohybrid crosses, breeding experiments that involve a single trait, such as pod color.

For example, in the original parental generation, or P zero, one parent has only green pods.

Its genotype is homozygous for the green pod color allele, denoted by an uppercase G.

The other parent has only yellow pods and is homozygous for the yellow pod allele, written as lowercase g.

After crossing, the first-generation offspring, known as the filial or F1 generation, appear identical and show the same green pod phenotype.

This result shows the Principle of Uniformity, where all F1 offspring show the same phenotype.

It also indicates that green is the dominant trait over yellow. These F1 plants are heterozygous Gg.

When these F1 plants are allowed to self-fertilize, the yellow trait re-emerges in the F2 generation. The F2 offspring inherit homozygous dominant, heterozygous, and homozygous recessive genotypes in a 1:2:1 ratio, resulting in a 3:1 phenotypic ratio of green to yellow pods.

This result shows that yellow is the recessive trait, which comes from the original parent with yellow pods.

It stays hidden in the F1 generation and appears in the F2 generation only when both alleles are recessive.