What is the genotypic ratio from a cross between two heterozygotes for a single gene?

• A. 1:2:1
• B. 3:1
• C. 9:3:3:1
• D. 1:1

Answer: (A)

When two organisms that are heterozygous for the same gene cross, the offspring genotypes come in a 1:2:1 pattern. Each parent can pass on either the dominant allele A or the recessive allele a with equal probability, so the possible zygotes are AA, Aa, Aa, and aa. That yields one AA, two Aa, and one aa, or a genotypic ratio of 1:2:1.

Context helps: the phenotypic ratio for this cross is 3:1 (dominant trait to recessive trait) because Aa and AA both show the dominant phenotype. The 9:3:3:1 ratio appears in a dihybrid cross involving two genes, not this single-gene cross. A 1:1 genotypic ratio would occur if you crossed Aa with aa.