Now, let's consider Mendel's plants. There are two possibilities for having a tall plant. The plant can be a purebred tall plant (homozygous tall), or a hybrid (heterozygous). The alleles are illustrated as follows:




homozygous tall


homozygous short

The physical traits that we observe are called the the plant's phenotype. For instance, Mendel's plants possessed two possible phenotypes for height, tall and short.

The two genes that determine an organism's traits are called the genotype. Mendel's plants could have one of three possible genotypes, TT, Tt, or tt. Because the tall allele is dominant, both TT and Tt genotypes produce a tall phenotype. However, a plant with a short phenotype must possess both recessive genes and therefore has only one possible genotype, tt.

Mendel's plants were found to have one of two possible pod shapes, dominant-smooth, and recessive-puckered. Let "S" represent a smooth pod and "s" represent a puckered pod. Move your cursor over the question marks to discover the possible phenotypes produced by the following genotypes:



Consider another example. Mendel's plants had two possible colors for pods, green pods and yellow pods. The green pod allele is dominant over the yellow pod allele. What are the possible genotypes of a plant with a green pod phenotype? What are the possible genotypes of a plant with a yellow pod phenotype? Let the letter "G" symbolize the dominant green allele and "g" symbolize the recessive yellow allele.

Phenotype: Green Pod

Phenotype: Yellow Pod