Dominant & Recessive
We can visualize the inheritance of genotypes from parents to offspring using Punnett squares. Punnett squares show the genotype of each parent along two sides of a square and all of the potential genotypes of the offspring inside the square. Recall that Mendel often observed a 3 : 1 ratio of dominant to recessive phenotypes (phenotypes included pea color, plant height, flower color, etc.) when he crossed two heterozygous peas. Using a Punnett square, we can understand why a Aa x Aa cross (heterozygous cross) yields this ratio: essentially, this cross had four possible offspring with equal probability:
1 AA genotype, dominant phenotype
2 Aa genotype, dominant phenotype
1 aa genotype, recessive phenotype
This is an example Punnett square between two heterozygous (Aa) pea plants, which yields a 3 green : 1 yellow phenotypic ratio of potential offspring.