In a cross Aa x Aa for a trait with complete dominance, what phenotypic ratio is expected?

• A. 1:2:1
• B. 3:1 in favor of the dominant phenotype.
• C. 1:1
• D. All offspring show the recessive phenotype.

Answer: (B)

In a trait with complete dominance, the heterozygote Aa shows the same phenotype as AA, so dominant phenotype appears whenever the dominant allele is present. Crossing Aa with Aa produces genotypes AA, Aa, Aa, and aa in a 1:2:1 ratio. The dominant phenotype comes from AA and Aa (three individuals), while the recessive phenotype comes from aa (one individual). So the phenotypic ratio is 3:1 in favor of the dominant phenotype. The 1:2:1 figure refers to genotypes, not phenotypes, and other crosses (like Aa × aa) would give different ratios.