Final answer:
The statement is false; a dihybrid cross yields a 9:3:3:1 phenotypic ratio, not a 3:1 ratio. The 3:1 ratio pertains to monohybrid crosses for a single trait, while the 9:3:3:1 ratio arises from the independent assortment of two separate traits in dihybrid crosses.
Step-by-step explanation:
The statement that any dihybrid cross should theoretically yield a 3:1 phenotypic ratio is false. A dihybrid cross involves two traits and typically follows a 9:3:3:1 phenotypic ratio. In a dihybrid cross, each trait is considered independently thanks to the principle of independent assortment.
For instance, when analyzing seed texture and seed color separately, we predict that for each trait, 3/4 of the F2 offspring will display the dominant phenotype and 1/4 will display the recessive phenotype. This matches the monohybrid 3:1 ratio for each trait individually.
When these two ratios are multiplied, they produce the 9:3:3:1 phenotypic ratio observed in dihybrid crosses. As a real-world example, cross AaBb (both traits heterozygous) with AaBb yields this ratio, as shown via a Punnett square:
- 9/16 display both dominant phenotypes (3/4 * 3/4)
- 3/16 display the dominant phenotype for the first gene and the recessive for the second (3/4 * 1/4)
- 3/16 display the recessive phenotype for the first gene and the dominant for the second (1/4 * 3/4)
- 1/16 display both recessive phenotypes (1/4 * 1/4)
Each phenotype's ratio results from the application of the product rule to independent events, a fundamental principle in Mendelian genetics.