Reading the ratios
One Punnett square gives two different ratios, and confusing them costs more marks than anything else in this chapter.
What the plants look like
Counted by appearance: three tall plants to one dwarf.
3 : 1
What factors they carry
Counted by pairs of factors: one TT, two Tt, one tt.
1 : 2 : 1
Both ratios come from the same four boxes
Nothing changes between them except what you choose to count.
| Pair of factors | Boxes | The plant looks |
|---|---|---|
| TT | 1 | Tall |
| Tt | 2 | Tall — and it is not the same as the row above, though it looks it |
| tt | 1 | Dwarf |
Two of the three tall plants are not pure
If you sowed the seed of the F2 tall plants, one in three of them would breed true and the other two would throw dwarf offspring. They are indistinguishable in the field, which is why a ratio counted by appearance can never tell you the whole story.
Assignment 20.3
Work these out on paper
- Cross a pure tall pea plant with a hybrid tall one. What are the offspring, and in what ratio?
- Cross a hybrid tall pea plant with a dwarf one. What fraction of the offspring is dwarf?
- A tall pea plant of unknown factors gives some dwarf offspring when crossed with a dwarf plant. What must its two factors be?
Appearance 3 : 1, factors 1 : 2 : 1
From Tt × Tt: counted by appearance the offspring are 3 tall to 1 dwarf; counted by pairs of factors they are 1 : 2 : 1. Read the question to see which is wanted.