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The Punnett square

Recall that in diploid organisms, each parent generates 2 gametes; the progeny inherits one from each parent.  There are 4 possible combinations of the parental gametes that can occur in the progeny. A common way to visualize this is the Punnett square. Below is an example of a monohybrid cross, e.g. one trait or locus, with 2 diploid heterozygous parents.

One quarter of the progeny of this cross would be AA for this locus, one quarter would be aa and one-half would be Aa, giving a genotypic ratio of 1:2:1 (1 AA: 2 Aa: 1 Aa).

Remember that this is a genotypic ratio, not necessarily phenotypic. If allele A were fully dominant, the genotypes AA and Aa would have an identical phenotype, giving a 3:1 phenotypic ratio.