Principles of Inheritance
Monohybrid cross
Follow one trait for two generations, and a clear pattern appears: three with the dominant look, one with the recessive look.
Start with two pure parents, tall (TT) and short (tt). The F₁ are all Tt, and all tall. Let them self-cross, and the F₂ come out 3 : 1, three tall to one short.
The short plants come back because the hidden t copy can pair up again. Behind the 3 : 1 look is a 1 : 2 : 1 genotype ratio: one TT, two Tt, one tt.
Where students slip up
Many students write ‘3 : 1’ for the F₂ and stop. But the exam often wants the genotype ratio below it: 1 : 2 : 1. Two of the three tall plants are carriers (Tt), not pure. The look and the genotype are two different questions.
Hold onto this
3 : 1 is what you see. 1 : 2 : 1 is what is really there.
Why it helps in the exam: the F₂ 3 : 1 is the most-tested Mendelian result. Know both the look ratio and the 1 : 2 : 1 below it.
Where this connects
Once you see that 3 : 1 is really 1 : 2 : 1 underneath, the dihybrid 9 : 3 : 3 : 1 stops being a new thing to learn. It is just this cross running twice at once.
Two crosses at once →Quick check
In a monohybrid cross, the F₂ look ratio is 3 : 1. The genotype ratio behind it is:
- A3 : 1
- B1 : 2 : 1
- C9 : 3 : 3 : 1
- D1 : 1