What is the expected phenotype ratio of a dihybrid cross?

The expected phenotype ratio of a dihybrid cross, which involves two traits that are each controlled by two alleles, is 9:3:3:1. This ratio results from the combination of possible allele pairs from each parent during gamete formation and subsequent fertilization.

In a classic dihybrid cross, where individuals are heterozygous for two traits (for example, AaBb x AaBb), the resulting offspring will express four distinct phenotypic categories based on the combinations of dominant and recessive alleles for each trait. The 9:3:3:1 ratio specifically represents the predicted proportions of these phenotypes:

• Nine offspring exhibiting both dominant traits,

• Three offspring exhibiting the dominant trait for the first gene and the recessive trait for the second gene,

• Three offspring exhibiting the recessive trait for the first gene and the dominant trait for the second gene, and

• One offspring exhibiting both recessive traits.

This established ratio is a foundational concept in Mendelian genetics and demonstrates the principles of independent assortment, which states that alleles for different traits segregate independently during gamete formation. Understanding this outcome helps in predicting the genetic variations in the offspring from such crosses.