Two traits at once
A true round-yellow pea line is crossed with a true wrinkled-green line. Every F1 seed is round and yellow. Cross the F1 plants. Four looks appear, each a shape with a colour. Letters stand for gene copies: R is round, r is wrinkled. Y is yellow, y is green.
- Round-yellow (RRYY) crossed with wrinkled-green (rryy).
- F1: every seed is round and yellow (RrYy).
- Cross two F1 plants: shape genes and colour genes pass on separately.
- Sixteen equally likely combinations appear.
- Nine round-yellow, three round-green, three wrinkled-yellow, one wrinkled-green.
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Count the boxes for each look. Tap the number.
The rule
R hides r, and Y hides y. So RrYy seeds are round and yellow.
Each F1 plant has two pairs: Rr for shape, Yy for colour. It passes on one shape copy, R or r. It passes on one colour copy, Y or y. So the kinds are RY, Ry, rY and ry. Two parents give 4 × 4 = 16 combinations.
Two gene pairs are passed on separately. That gives sixteen equally likely boxes. In big counts, the looks come near nine, three, three and one.
A round-yellow seed has at least one R. It also has at least one Y. That is why nine boxes are round-yellow.
Take 160 F2 seeds. A sixteenth of 160 is 10. So one box holds about 10 seeds. Wrinkled-green fills 1 box: about 10 seeds. Round-yellow fills 9 boxes: 9 × 10 = 90. Round-green fills 3 boxes: 3 × 10 = 30. Wrinkled-yellow fills 3 boxes: 3 × 10 = 30. Check: 10 + 90 + 30 + 30 = 160.
Round seeds of any colour are 9 + 3 = 12 boxes of 16. Two new looks appear, round-green and wrinkled-yellow. No parent had them. They appear because shape and colour copies are passed on separately.
Watch out!
Round-yellow is the biggest group, but not because it is stronger. It fills nine boxes because more pairs hold a dominant copy. Every box is equally likely.