Next, Mendel took the F1 progeny and allowed them to self-fertilize. In the resulting F2 generation, 3/4 showed the dominant phenotype, and 1/4 showed the recessive phenotype. For example, when a round seed line was crossed to a wrinkled seed line, the F1 generation was all round, and the F2 generation showed a phenotypic ratio of 3 round : 1 wrinkled. In plants or animals that cannot self-fertilize, the F2 generation is produced by crossing F1s to each other.
| P | round seed line x wrinkled seed line | ||
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| F1 | round seed progeny | ||
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| F2 | 3 round : 1 wrinkled | ||
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| Conclusion: F2 progeny occur in a ratio of 3 dominant : 1 recessive | |||
From these results, it’s clear that there are two types of round
peas: those that are true-breeding and those that are not. Mendel
suspected that the round peas in the F2 generation consisted of both types. To
test his hypothesis, Mendel allowed the round pea plants from the F2 generation
to self-fertilize and scored the progeny from each plant. 1 out of
3 round pea plants from the F2 generation were true-breeding and produced only offspring with round peas. The
remaining 2 out of 3 produced a ratio of 3 round : 1 wrinkled, just like
the hybrids in the F1 generation. In contrast, all the wrinkled peas
in the F1 generation were true-breeding. So hidden in the F2 ratio
of 3 round : 1 wrinkled, there’s a ratio of 1 true-breeding round
: 2 hybrid round : 1 true-breeding wrinkled.
| P | round seed line x wrinkled seed line | ||||
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| F1 | round seed progeny | ||||
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| F2 | 1 true round | : | 2 hybrid round | : | 1 true wrinkled |
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| F3 | round | 3 round : 1 wrinkled | wrinkled | ||