In a monohybrid cross Aa x Aa for a trait with complete dominance, what is the expected phenotypic ratio of dominant to recessive phenotypes among offspring?

• A. 3:1
• B. 1:2:1
• C. 1:1
• D. 4:0

Answer: (A)

This question tests how a monohybrid cross with complete dominance translates genotype into phenotype. When Aa is crossed with Aa, each offspring has a 1/4 chance of AA, 1/2 chance of Aa, and 1/4 chance of aa, giving genotypes in a 1:2:1 ratio. With complete dominance, any genotype containing at least one dominant allele (AA or Aa) shows the dominant phenotype, while only aa shows the recessive phenotype. Therefore, three of the four offspring will display the dominant trait and one will display the recessive trait, yielding a 3:1 phenotypic ratio. The other ratios refer to different aspects or crosses (for example, 1:2:1 is the genotypic ratio, not phenotypic; 1:1 would occur in crosses like Aa x aa; 4:0 would be all dominant but isn’t possible with Aa x Aa).