In a monohybrid cross between two heterozygous individuals for a trait with complete dominance, what phenotypic ratio is expected among offspring?

• A. 3:1
• B. 1:1
• C. 1:3
• D. All Dominant

Answer: (A)

When two heterozygotes for a trait with complete dominance are crossed, the dominant phenotype will appear in three of the four possible offspring. In this Aa x Aa cross, the offspring genotypes are AA, Aa, Aa, and aa. The dominant phenotype shows up in AA and Aa (three out of four), while the recessive phenotype only appears in aa (one out of four). So the expected phenotypic ratio is 3:1 in favor of the dominant trait.

The other options don’t fit this cross: a 1:1 ratio would require a different pairing (for example, Aa x aa), and a 1:3 ratio would imply more recessive phenotypes than possible from this cross. All dominant would require no recessive allele to be passed on, which isn’t the case here.