Punnett square

The Punnett square is a visual representation of Mendelian inheritance, a fundamental concept in genetics discovered by Gregor Mendel.

[10] For multiple traits, using the "forked-line method" is typically much easier than the Punnett square.

Zygosity refers to the grade of similarity between the alleles that determine one specific trait in an organism.

Homozygosity, with homo relating to same while zygous pertains to a zygote, is seen when a combination of either two dominant or two recessive alleles code for the same trait.

This can include lethal effects and epistasis (where one allele masks another, regardless of dominant or recessive status).

Since dominant traits mask recessive traits (assuming no epistasis), there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green.

The ratio 9:3:3:1 is the expected outcome when crossing two double-heterozygous parents with unlinked genes.

A problem is converted to a series of monohybrid crosses, and the results are combined in a tree.

As stated above, the phenotypic ratio is expected to be 9:3:3:1 if crossing unlinked genes from two double-heterozygotes.

A Punnett square showing a typical test cross . (green pod color is dominant over yellow for pea pods [ 1 ] in contrast to pea seeds, where yellow cotyledon color is dominant over green [ 2 ] ).
Punnett squares for each combination of parents' colour vision status giving probabilities of their offsprings' status, each cell having 25% probability in theory.
There are also Punnett squares for epistasis . In these cases the genotype epistatic over the other genes hinders their expression in the phenotype.