What is a monohybrid Cross?

The crossing of two heterozygotes for one trait. ##Aa xx Aa##

It always yields a genotypic ratio of 1:2:1 AA:Aa:aa (homozygous dominant:heterozygous:homozygous recessive.).

In mendelian genetics, It always yields a phenotypic ratio of 3:1 dominant trait:recessive trait. It is not to be confused with a dihybrid cross, which involves crossing two heterozygotes for two traits (##AaBb xx AaBb##)

The genotypic ratio is always the same (1:2:1), but the phenotypes are sometimes different.

If the alleles are incompletely dominant, the phenotypic ratio will be 1:2:1. An example is red and white roses. RR is red, WW is white, but RW is pink, a third phenotype just for heterozygotes.

The same is true for codominant alleles, except the phenotypes are both expressed rather than mixed.