Genotype and Phenotype One of Mendel's most revolutionary insights followed directly from his observations of F₁ crosses: Every organism has a genetic makeup as well as a set of observable characteristics. All of the tall pea plants had the same phenotype, or physical traits. They did not, however, have the same genotype, or genetic makeup. Look again at Figure 11–7 and you will find three different genotypes among the F₂ plants: TT, Tt, and tt. The genotype of an organism is inherited, and the phenotype is largely determined by the genotype. Two organisms may share the same phenotype but have different genotypes.

Using Punnett Squares One of the best ways to predict the outcome of a genetic cross is by drawing a simple diagram known as a Punnett square. Punnett squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses. Constructing a Punnett square is fairly easy. You begin with a square. Then, following the principle of segregation, all possible combinations of alleles in the gametes produced by one parent are written along the top edge of the square. The other parent's alleles are then segregated along the left edge. Next, every possible genotype is written into the boxes within the square, just as they might appear in the F₂ generation. Figure 11–8 on the next page shows step-by-step instructions for constructing Punnett squares.

In Your Notebook In your own words, write definitions for the terms homozygous, heterozygous, phenotype, and genotype.

Table of Contents