The 9:3:3:1 ratio observed in a dihybrid cross indicates the characteristic phenotypic distribution arising from the independent inheritance of two genes, each possessing a dominant and a recessive allele, in accordance with Mendel's law of independent assortment.
Breakdown of the 9:3:3:1 ratio:
- 9: This segment signifies offspring exhibiting the dominant phenotype for both traits under consideration.
- 3: This segment represents offspring displaying the dominant phenotype for the first trait and the recessive phenotype for the second trait.
- 3: This segment denotes offspring showing the recessive phenotype for the first trait and the dominant phenotype for the second trait.
- 1: This segment corresponds to offspring inheriting the recessive phenotype for both traits.
Context of a typical dihybrid cross:
- This scenario involves two distinct genes, with each gene having two allelic forms: one dominant and one recessive.
- When both parent organisms are heterozygous for both traits (e.g., AaBb crossed with AaBb), the resultant offspring phenotypes are expected to conform to the 9:3:3:1 ratio.
- This outcome is attributable to the independent segregation of alleles for each gene during gamete formation, meaning the inheritance pattern of one gene is unaffected by the inheritance pattern of the other.
Illustrative example: A cross between two heterozygous pea plants (YyRr × YyRr), where 'Y' denotes the dominant allele for yellow seed color and 'y' the recessive allele for green color, and 'R' signifies the dominant allele for round seeds with 'r' representing the recessive allele for wrinkled seeds, would yield offspring with a phenotypic ratio of 9 yellow and round : 3 yellow and wrinkled : 3 green and round : 1 green and wrinkled.
The 9:3:3:1 ratio is a fundamental observation in dihybrid crosses involving two traits, each controlled by a single gene with two alleles, and stands as a cornerstone of Mendelian genetics.