Dihybrid cross explained simply (with a worked example)
Table of contents
A dihybrid cross is a breeding cross that follows two different genes at once. Each gene controls a separate trait, such as seed colour and seed shape. The word comes from di (two) and hybrid (a mix of parental types).
It’s the natural next step after a monohybrid cross, which follows just one gene. Adding a second gene raises a new question. Do the two traits travel together, or are they inherited separately?
The rule that makes it work: Independent assortment
Gregor Mendel crossed pea plants that differed in two traits and noticed something. Knowing a seed’s colour told him nothing about its shape. This is the law of independent assortment. When gametes form, the allele a gamete gets for one gene doesn’t affect which allele it gets for the other.
This holds when the genes are on different chromosomes, or far apart on the same one. Genes that sit close together are linked and break this rule. Introductory dihybrid problems assume independence.
A worked example
Use Mendel’s own traits:
| Gene | Dominant allele | Recessive allele |
|---|---|---|
| Seed colour | A = yellow | a = green |
| Seed shape | B = round | b = wrinkled |
1. P generation
Cross a pure-breeding yellow round plant (AABB) with a pure-breeding green wrinkled plant (aabb). The first can only make AB gametes and the second can only make ab gametes. Every offspring is therefore AaBb, and all of them are yellow and round.
2. F1 × F1
Now cross two of those AaBb plants. Each one can make four kinds of gamete in equal numbers: AB, Ab, aB and ab. Four gametes from each parent give a 4 × 4 grid of 16 equally likely combinations. Here’s how to fill that Punnett square.
3. Count the phenotypes
| Phenotype | Boxes | Fraction |
|---|---|---|
| Yellow, round (A_B_) | 9 | 9/16 |
| Yellow, wrinkled (A_bb) | 3 | 3/16 |
| Green, round (aaB_) | 3 | 3/16 |
| Green, wrinkled (aabb) | 1 | 1/16 |
That’s the famous 9 : 3 : 3 : 1 ratio.
Why 9:3:3:1? A shortcut
Since the genes are independent, you can treat each one as its own monohybrid cross:
- Colour:
Aa × Aagives ¾ yellow and ¼ green. - Shape:
Bb × Bbgives ¾ round and ¼ wrinkled.
Multiply the probabilities:
- Yellow and round = ¾ × ¾ = 9/16
- Yellow and wrinkled = ¾ × ¼ = 3/16
- Green and round = ¼ × ¾ = 3/16
- Green and wrinkled = ¼ × ¼ = 1/16
This product rule is faster than drawing the square, and it scales to three or more genes.
Other dihybrid crosses worth knowing
- Test cross
AaBb × aabb: gives a 1 : 1 : 1 : 1 ratio. It shows whether a dominant-looking individual carries hidden recessive alleles. AaBb × Aabb: gives 3 : 3 : 1 : 1.AABb × aaBb: everything is yellow, and shape splits 3 round : 1 wrinkled.
When the ratio doesn’t come out 9:3:3:1
Real data rarely match exactly. Mendel counted 315 : 108 : 101 : 32, which is close but not perfect. That’s just chance. Large departures point to something else, such as linked genes, epistasis (one gene masking another), lethal allele combinations or incomplete dominance.
Key terms
- Genotype is the alleles an organism carries, such as
AaBb. Phenotype is the trait you can observe, such as yellow and round. More on the difference. - Homozygous means two identical alleles (
AAoraa). Heterozygous means two different ones (Aa). More on these.