MODEL
Dihybrid cross studio
Choose any two-locus parents, form gametes and compare expected with sampled offspring.
Two genes. Two parents. Many possible outcomes. Each parent contributes one allele at each locus.
Start with the gametes
Parent 1: AB, Ab, aB, ab.
Parent 2: AB, Ab, aB, ab.
Each listed gamete is equally likely. In AaBb, P(A) × P(B) = ½ × ½ = ¼ for AB. Homozygous loci reduce the number of distinct gametes.
Assumes independent assortment, complete dominance at both autosomal loci, random fertilisation and equal viability. A_ means AA or Aa. Linked genes need the separate linkage model.
| Gametes | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Expected phenotypes
See genotype probabilities
Explain what you observed
When is the 9:3:3:1 ratio expected?
Your study notes
Take the next step
Connect this activity with your lesson notes and chapter practice.
Read a related learning guide →Practise Inheritance →Open the student notes folder ↗Concepts and sources
Curriculum connections: 10.1, 10.2. Selected concepts.
Read the supporting source ↗PMDC curriculum ↗Questions and explanations are original teaching material; these are not official exam questions.