Dihybrid Cross
Cross two genes at once and read off the 16-box grid and phenotype ratio.
| × | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
16 offspring boxes, tallied by phenotype class (dominant shown as a blank underscore).
A dihybrid cross tracks two genes at once. Crossing AaBb × AaBb, each parent makes four gametes (AB, Ab, aB, ab), filling a 4×4 = 16-box grid. Tallying phenotypes gives the classic 9:3:3:1 ratio — 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb when both genes show simple dominance.
What a dihybrid cross is
A dihybrid cross follows the inheritance of two different genes simultaneously — for example seed shape (A/a) and seed color (B/b). Each parent carries two alleles per gene, so a genotype like AaBb packs four letters. Because each gamete gets exactly one allele from each gene, a parent that is heterozygous for both genes produces four kinds of gametes: AB, Ab, aB, and ab. Pairing every gamete of one parent with every gamete of the other fills a 4×4 grid of 16 boxes.
Independent assortment: each gene segregates on its own, so two 3:1 ratios multiply into 9:3:3:1.
Worked example: AaBb × AaBb
Cross two parents heterozygous for both genes:
- 1 Write each parent’s gametes. Pick one allele from each gene. AaBb gives four gametes by FOIL: AB, Ab, aB, ab.
- 2 Build the 4×4 grid. Put one parent’s gametes across the top, the other’s down the side — 16 offspring boxes in all.
- 3 Fill every box. Combine the column gamete with the row gamete, ordering each gene dominant-first (e.g. AB × ab → AaBb).
- 4 Tally the phenotypes. Group boxes by whether each gene shows the dominant trait: 9 A_B_, 3 A_bb, 3 aaB_, 1 aabb.
- 5 Read the ratio. That is the classic 9:3:3:1 dihybrid phenotype ratio.
AaBb × AaBb phenotype classes
The 9:3:3:1 dihybrid ratio. “A_” means at least one dominant A allele; “_” is a blank that can be either allele.
| Phenotype class | Boxes (of 16) | Fraction | Traits shown |
|---|---|---|---|
| A_B_ | 9 | 9/16 | Both dominant |
| A_bb | 3 | 3/16 | Gene 1 dominant, gene 2 recessive |
| aaB_ | 3 | 3/16 | Gene 1 recessive, gene 2 dominant |
| aabb | 1 | 1/16 | Both recessive |
Independent assortment and how it differs from a 3:1 cross
Mendel’s law of independent assortment. When two genes sit on different chromosomes (or far apart on the same one), the allele an offspring inherits for one gene is independent of the allele it inherits for the other. That independence is exactly why the ratio factors: a single-gene Aa × Aa cross gives a 3:1 phenotype split, and two independent 3:1 splits multiply — (3 + 1) × (3 + 1) — into the 9 : 3 : 3 : 1 pattern.
Dihybrid versus monohybrid. A monohybrid cross tracks one gene, fills a 2×2 grid of four boxes, and yields a 3:1 phenotype ratio. A dihybrid cross tracks two genes, fills a 4×4 grid of sixteen boxes, and yields 9:3:3:1. The dihybrid ratio is simply the monohybrid 3:1 ratio applied to each gene at once. If the two genes are linked rather than independent, observed offspring depart from 9:3:3:1 — that deviation is what reveals linkage.