Reginald Punnett created Punnett squares to predict the proportions of possible genotypes in offspring. A genotype refers to the genetic makeup of an organism, while an organism’s phenotype refers to its observable physical traits.  A Punnett square is a visual tool that shows the possible combinations of gametes and the resulting offspring in a cross, often modeled using the F2 generation. It organizes the alleles from each parent to clearly display the four possible outcomes in a monohybrid cross.

Organisms with identical alleles for a particular gene (TT or tt) are described as homozygous, whereas organisms with different alleles for the same gene (Tt) are heterozygous. Punnett squares can also be used for dihybrid crosses, in which two traits are studied simultaneously. In this case, the square expands to four boxes across and four boxes down, showing sixteen possible combinations.

A Punnett square can also be created for dihybrid crosses.  It would be four boxes wide and four boxes tall.

Review:

1. What is the purpose of a Punnett square?
2. Compare homozygous and heterozygous.