Dihybrid Cross

A dihybrid cross involves two traits, each controlled by a single gene with two alleles. Here, seed shape (R = round, dominant; r = wrinkled, recessive) and seed color (Y = yellow, dominant; y = green, recessive) are considered. Both parent plants are heterozygous (RrYy), meaning they have the genotype RrYy for both traits.
To determine the phenotypic ratio of the offspring, we analyze the inheritance of both traits using Mendel’s law of independent assortment, which states that alleles for different traits segregate independently during gamete formation. The possible gametes for each RrYy parent are RY, Ry, rY, and ry.
Using a Punnett square for a dihybrid cross (RrYy × RrYy), we can calculate the phenotypic outcomes. However, for simplicity, we can use the phenotypic ratios derived from Mendel’s experiments:
- For each trait individually, a cross between two heterozygotes (e.g., Rr × Rr) produces a 3:1 phenotypic ratio (3 dominant : 1 recessive).
- For seed shape: 3 round (RR or Rr) : 1 wrinkled (rr).
- For seed color: 3 yellow (YY or Yy) : 1 green (yy).
Since the traits assort independently, the combined phenotypic ratio for the dihybrid cross is calculated by multiplying the ratios of the individual traits:
- Round, yellow:
- Round, green:
- Wrinkled, yellow:
- Wrinkled, green:
Thus, the phenotypic ratio of the offspring is 9 round, yellow : 3 round, green : 3 wrinkled, yellow : 1 wrinkled, green, or 9:3:3:1.

The 9:3:3:1 ratio in a dihybrid cross denotes the typical phenotypic ratio that results from the inheritance of two traits, each with two alleles (dominant and recessive), where the two genes assort independently according to Mendel's law of independent assortment.

Explanation of the 9:3:3:1 ratio:

• 9: The first number (9) represents the offspring that display both dominant traits for both genes. These individuals exhibit the dominant phenotype for both traits.
• 3: The first 3 represents offspring that have the dominant phenotype for the first gene and the recessive phenotype for the second gene.
• 3: The second 3 represents offspring with the recessive phenotype for the first gene and the dominant phenotype for the second gene.
• 1: The final 1 represents offspring that exhibit the recessive phenotype for both genes.

In a typical dihybrid cross:

• Consider two genes, where each gene has two alleles (dominant and recessive).
• If both parents are heterozygous for both traits (i.e., AaBb × AaBb), the expected offspring will follow the 9:3:3:1 phenotypic ratio.
• This results from the independent assortment of alleles for each gene, meaning the inheritance of one gene does not affect the inheritance of the other gene.

Example: If we were to cross two heterozygous pea plants (YyRr × YyRr), where Y represents the dominant allele for yellow color and y represents the recessive allele for green color, and R represents the dominant allele for round seeds and r represents the recessive allele for wrinkled seeds, the phenotypic ratio of the offspring would be 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled.

This 9:3:3:1 ratio is a classic result of a dihybrid cross involving two traits, each determined by a single gene with two alleles, and is one of the foundational concepts of Mendelian genetics.