Monohybrid Cross Ratio

In a monohybrid cross, two parents that are both heterozygous for one trait each pass on one allele at random, so the offspring genotype ratio comes from the combinations in a Punnett square. If one allele is dominant, then three of the four possible offspring genotypes show the dominant phenotype and only one shows the recessive phenotype, giving a 3:1 phenotypic ratio. This ratio appears when the trait follows simple Mendelian inheritance with complete dominance and the parents are both heterozygous.

Use this when predicting the offspring phenotype ratio for a single-gene cross where both parents are heterozygous and one allele is dominant over the other. It is common in genetics problems about pea plants, coat color, seed shape, or other basic Mendelian traits.

• Using the 3:1 ratio for genotype counts instead of phenotype counts
• Applying it to crosses that are not heterozygous × heterozygous
• Forgetting that the ratio only works with complete dominance and one gene at a time