In a classic monohybrid cross with complete dominance, what is the typical phenotypic ratio?

In a classic monohybrid cross with complete dominance, what is the typical phenotypic ratio?

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

Answer: (A)

In a monohybrid cross with complete dominance, only two phenotypes appear for a single gene: dominant and recessive. When true-breeding dominant and true-breeding recessive parents are crossed, all offspring are heterozygous and show the dominant phenotype. If those heterozygous offspring are then crossed with each other, the offspring genotypes follow a 1:2:1 ratio (AA : Aa : aa). Since both AA and Aa express the dominant trait, three out of four progeny show the dominant phenotype and one out of four shows the recessive phenotype. That yields a phenotypic ratio of 3:1.

In a monohybrid cross with complete dominance, only two phenotypes appear for a single gene: dominant and recessive. When true-breeding dominant and true-breeding recessive parents are crossed, all offspring are heterozygous and show the dominant phenotype. If those heterozygous offspring are then crossed with each other, the offspring genotypes follow a 1:2:1 ratio (AA : Aa : aa). Since both AA and Aa express the dominant trait, three out of four progeny show the dominant phenotype and one out of four shows the recessive phenotype. That yields a phenotypic ratio of 3:1.