Section 3: Punnett Squares

Reginald Punnett created Punnett squares to predict the proportions of possible genotypes in offspring.  Genotype refers to the genetic makeup of an organism.  An organism’s phenotype refers to its physical traits.

A Punnett square is a way to visually highlight the four possible combinations of gametes and their offspring in an F2 generation.  Organisms with identical alleles for a particular gene (TT, tt) are homozygous, whereas organisms with different alleles for the same gene (Tt) are heterozygous.

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


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

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