The 9:3:3:1 ratio is a phenotypic ratio observed in the offspring of a dihybrid cross. This means that two individuals who are heterozygous for two genes are being crossed. For example, AaBb x AaBb. This ratio indicates the different combinations of traits expressed in the offspring.
Here's how it works:
- 9: Represents the number of offspring expressing the dominant allele for both genes.
- 3: Represents the number of offspring expressing the dominant allele for one gene and the recessive allele for the other.
- 3: Represents the number of offspring expressing the recessive allele for one gene and the dominant allele for the other.
- 1: Represents the number of offspring expressing the recessive allele for both genes.
Example:
Consider two pea plants, both heterozygous for seed shape (Round, R, dominant over Wrinkled, r) and seed color (Yellow, Y, dominant over Green, y).
- Parental genotypes: RrYy x RrYy
- Offspring phenotypes:
- 9 Round Yellow
- 3 Round Green
- 3 Wrinkled Yellow
- 1 Wrinkled Green
Practical Insights:
- This rule is fundamental in understanding inheritance patterns and predicting offspring traits.
- It is used extensively in genetics and breeding programs, helping to select for desirable traits.
- The 9:3:3:1 ratio demonstrates the independent assortment of genes, meaning that alleles of different genes separate independently during gamete formation.
