In a dihybrid cross, what is the typical genotype ratio?

In a dihybrid cross, what is the typical genotype ratio?

• A. 1:2:1
• B. 9:3:3:1
• C. 3:3:1:1
• D. 2:2:2:2

Answer: (B)

When two genes are involved and segregate independently, this setup usually produces a 9:3:3:1 distribution among phenotypes for a dihybrid cross of two heterozygotes. That means: most offspring show both dominant traits, a smaller group shows the first dominant trait with the second recessive, another small group shows the first recessive with the second dominant, and a single small group shows recessive for both traits. This pattern arises from the independent assortment of the two gene pairs and the dominance relationships. If you instead look at the actual genotypes, the distribution is more complex. For each gene, the genotypic ratio is 1:2:1 (AA:Aa:aa), and when you combine two genes, you get nine genotype classes with frequencies that add up to 16 offspring: 1, 2, 1, 2, 4, 2, 1, 2, 1. So the neat 9:3:3:1 figure is most commonly cited for phenotypes, not for genotypes.

When two genes are involved and segregate independently, this setup usually produces a 9:3:3:1 distribution among phenotypes for a dihybrid cross of two heterozygotes. That means: most offspring show both dominant traits, a smaller group shows the first dominant trait with the second recessive, another small group shows the first recessive with the second dominant, and a single small group shows recessive for both traits. This pattern arises from the independent assortment of the two gene pairs and the dominance relationships.

If you instead look at the actual genotypes, the distribution is more complex. For each gene, the genotypic ratio is 1:2:1 (AA:Aa:aa), and when you combine two genes, you get nine genotype classes with frequencies that add up to 16 offspring: 1, 2, 1, 2, 4, 2, 1, 2, 1. So the neat 9:3:3:1 figure is most commonly cited for phenotypes, not for genotypes.