In pea, a pure tall plant (TT) is crossed with a short plant (tt). What will be the ratio of pure tall plants to short plants in the F2 generation?
- A.1:1
- B.1:3
- C.3:1
- D.2:1
Correct answer
Explanation
The correct answer is A, 1:1. The F2 genotypes come out as 1 TT : 2 Tt : 1 tt, so pure tall plants (TT) and short plants (tt) are equal in number.
This is Mendel's monohybrid cross. Crossing TT with tt gives F1 plants that are all Tt, and all tall because T is dominant. When F1 plants are self-pollinated, each parent forms T and t gametes in equal numbers, and their random union gives one TT, two Tt and one tt in every four offspring. The phenotypic ratio is 3 tall : 1 short, but the question asks only about pure tall plants, which are one in four, against short plants, also one in four. Hence 1:1.
B is wrong: 1:3 reverses the phenotypic ratio and still ignores the hybrid tall plants.
C is wrong: 3:1 is the ratio of all tall plants (TT and Tt) to short plants.
D is wrong: 2:1 is the ratio of hybrid tall (Tt) to pure tall (TT) or to short plants.
Exam tip: monohybrid F2 = phenotype 3:1, genotype 1:2:1; read whether the question says tall or pure tall.
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