Punnett Squares show probability of what traits the offspring can show. It crosses both parent’s genes to show the possible genotypes (genes/letters) and phenotypes (physical trait/what you see) of the offspring. A capital letter represents a dominant trait (the trait that shows when at least one gene is present). A recessive trait only shows with there are TWO recessive genes present (or lowercase letters). Purebred , or homozygous , means that the organism has two of the same genes (BB for example). Hybrid , or heterozygous , means that an organism has one of each gene, a dominant and a recessive (Bb for example).
Example)
A purebred/homozygous yellow pea plant (GG) is being crossed with a purebred/homozygous green pea plant (gg). Yellow is the dominant color.
G G g
Gg Gg
Genotype (Ratio) = 0 GG: 4 Gg ; 0 gg
Genotype (Percent) = 0% GG: 100% Gg: 0%gg
Gg Gg
Phenotype (Ratio) = 4 Yellow: 0 Green g
Phenotype (Percent) = 100% Yellow:0% Green
Let’s practice a few as a class!
1) A hybrid/heterozygous green pea plant (Gg) is crossed with another hybrid/heterozygous green pea plant (Gg).
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent:
2) A homozygous green plant is crossed with a heterozygous yellow plant.
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent:

Now try a few with your table partner!
3) A homozygous tall plant (TT) is crossed with a hybrid tall plant (Tt). Tall is the dominant trait.
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent:
4) A hybrid tall plant is crossed with a purebred short plant.
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent:
Now try two on your own!
5) A homozygous red flower is crossed with a homozygous white flower. Red is the dominant trait (R).
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent:
6) A purebred red flower is crossed with a homozygous red flower.
Genotype Ratio:
Genotype Percent:
Phenotype Ratio:
Phenotype Percent: