← Biology
Commonly confused
Monohybrid vs dihybrid cross
Same idea, different size: a monohybrid cross follows one trait, a dihybrid follows two at once. The ratios tell them apart.
Monohybrid cross
One trait, one gene pair. The F₁ look the same; the F₂ come out 3 : 1 by look (1 : 2 : 1 by genotype). Example: tall × short.
Dihybrid cross
Two traits together, two gene pairs. The F₂ come out 9 : 3 : 3 : 1 when the genes separate on their own. Example: round-yellow × wrinkled-green.
The trap
Mixing up the ratios is the common mistake: 3 : 1 is monohybrid, 9 : 3 : 3 : 1 is dihybrid. A quick check: 9 : 3 : 3 : 1 is just two 3 : 1 crosses multiplied. Four look-types means dihybrid; two look-types means monohybrid.
Quick check
An F₂ look ratio of 9 : 3 : 3 : 1 tells you the cross was:
- AMonohybrid
- BDihybrid
- CA test cross
- DIncomplete dominance