In a classic Mendelian cross Aa x Aa, what is the typical phenotypic ratio for a trait with complete dominance?

• A. 1:1
• B. 4:0
• C. 1:2:1
• D. 3:1

Answer: (D)

Complete dominance means one dominant allele is enough to express the trait. When two heterozygotes (Aa) mate, the genotype outcomes are 1 AA, 2 Aa, and 1 aa. The dominant phenotype comes from both AA and Aa, giving three out of four offspring with the dominant trait and one out of four with the recessive trait, so the phenotypic ratio is 3:1. The other options don’t fit: 4:0 would require all offspring showing the dominant trait; 1:2:1 is the genotype ratio, not the phenotype under complete dominance; and 1:1 would occur in a cross like Aa x aa, not Aa x Aa.