What is a dihybrid cross?
A dihybrid cross is a genetic cross between two individuals that involves tracking two different genes simultaneously. Unlike a monohybrid cross that examines inheritance of a single trait, the dihybrid cross allows us to observe how two traits are inherited relative to each other.
Mendel's Law of Independent Assortment
Gregor Mendel's experiments with pea plants revealed that the alleles for different genes assort independently of each other during gamete formation (assuming the genes are on different chromosomes). This is the Law of Independent Assortment, and it is the foundational principle behind the dihybrid Punnett square.
The classic 9:3:3:1 ratio
When you cross two organisms that are heterozygous for both traits (AaBb × AaBb), the expected phenotypic ratio is 9:3:3:1 under complete dominance:
- 9/16 display both dominant traits
- 3/16 display the first dominant, second recessive
- 3/16 display the first recessive, second dominant
- 1/16 display both recessive traits