In a Punnett square crossing two heterozygous individuals for a trait with complete dominance, what is the expected phenotype ratio?

• A. 3:1 recessive to dominant.
• B. 1:2:1.
• C. 3:1 dominant to recessive.
• D. 1:1.

Answer: (C)

With complete dominance, having one dominant allele is enough to display the dominant trait. When two heterozygotes (one dominant and one recessive allele each) mate, the offspring genotypes appear in a 1:2:1 ratio (AA, Aa, Aa, aa). Because both AA and Aa show the dominant phenotype, while only aa shows the recessive phenotype, the expected phenotypes appear in a 3:1 ratio: three with the dominant trait for every one with the recessive trait. The other patterns described come from either the genotypic ratio or different crosses, so the 3:1 dominant to recessive phenotype is the correct result.