What is the expected phenotypic ratio from a monohybrid cross?

In a monohybrid cross, involving a single trait that is controlled by two alleles, the expected phenotypic ratio is 3:1. This ratio arises when two heterozygous individuals are crossed (e.g., Aa x Aa).

To understand why this produces a 3:1 ratio, consider the possible genotypes resulting from this cross. The offspring could inherit the following combinations:

• AA (homozygous dominant)

• Aa (heterozygous)

• aa (homozygous recessive)

When these genotypes are expressed phenotypically, both AA and Aa individuals will exhibit the dominant trait, while only the aa individual will exhibit the recessive trait. Since there are three combinations that produce the dominant phenotype (AA and Aa) and one combination that produces the recessive phenotype (aa), the overall resulting ratio is 3 dominant phenotypes to 1 recessive phenotype, hence the 3:1 ratio.

This ratio is a fundamental expectation of Mendelian genetics and serves as a basis for understanding inheritance patterns in traits governed by a single gene with two alleles.