Principles of Inheritance
Dihybrid cross
Follow two traits at once, and the F₂ come out in a 9 : 3 : 3 : 1 pattern. This is the sign of two genes mixing freely.
Cross pure round-yellow (RRYY) with pure wrinkled-green (rryy). The F₁ are all RrYy, round and yellow. Self-cross them, and the F₂ split 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.
The two new mixes, round-green and wrinkled-yellow, prove that the two genes separated on their own.
Where students slip up
The 9 : 3 : 3 : 1 holds only when the two genes are unlinked and each one shows simple dominance. If the genes are linked, or if there is incomplete dominance, the ratio changes. It is not a fixed number; it depends on these conditions.
Hold onto this
9 : 3 : 3 : 1 is just two 3 : 1 crosses multiplied.
Why it helps in the exam: see it as (3 : 1) × (3 : 1), and you can rebuild the ratio without drawing all sixteen boxes.
Where this connects
The ratio is not sixteen boxes to memorise, it is two coin games multiplied. Once you see that, you can predict a three-gene or four-gene cross the same way, without drawing anything.
The rule behind it →Quick check
A dihybrid cross (RrYy × RrYy) gives an F₂ look ratio of:
- A3 : 1
- B1 : 2 : 1
- C9 : 3 : 3 : 1
- D1 : 1 : 1 : 1