Monohybrid vs dihybrid cross: Key differences
Table of contents
Both crosses predict offspring from two parents. The difference is how many genes you follow. A monohybrid cross tracks one gene. A dihybrid cross tracks two genes at the same time.
Side-by-side comparison
| Monohybrid cross | Dihybrid cross | |
|---|---|---|
| Genes followed | 1 | 2 |
| Typical heterozygous parent | Aa | AaBb |
| Gamete types per heterozygous parent | 2 (A, a) | 4 (AB, Ab, aB, ab) |
| Punnett square | 2 × 2 = 4 boxes | 4 × 4 = 16 boxes |
| F2 phenotype ratio | 3 : 1 | 9 : 3 : 3 : 1 |
| F2 genotype ratio | 1 : 2 : 1 | 1:2:1:2:4:2:1:2:1 |
| Test cross ratio | 1 : 1 | 1 : 1 : 1 : 1 |
| Mendel’s law shown | Segregation | Independent assortment |
Monohybrid cross
Cross two Aa parents. Each one passes on A or a with equal probability, so the offspring are ¼ AA, ½ Aa and ¼ aa. With complete dominance, three-quarters show the dominant trait.
This demonstrates the law of segregation. The two alleles of a gene separate into different gametes.
Dihybrid cross
Cross two AaBb parents. Each gene still segregates on its own, and the two genes also assort independently of each other. That’s why the 9:3:3:1 ratio is just two 3:1 ratios multiplied together: (¾ + ¼) × (¾ + ¼).
A dihybrid cross demonstrates the law of independent assortment, which a monohybrid cross can’t show. Read the full dihybrid explanation.
Which one should you use?
- A question about one trait, such as “Can two brown-eyed parents have a blue-eyed child?”, is a monohybrid problem.
- A question about two traits together, such as “What fraction will be tall and have purple flowers?”, is a dihybrid problem. You can still solve it as two monohybrid crosses and multiply.
- For three traits the square has 64 boxes. Use the product rule or the trihybrid calculator.
Try both
- Monohybrid cross calculator (2 × 2)
- Dihybrid cross calculator (4 × 4)