Mendel's dihybrid cross and the laws of inheritance
Crossing round yellow and wrinkled green peas gives all round yellow F₁. The F₂ shows 9 : 3 : 3 : 1, proving the law of independent assortment.
The cross, step by step
- 1Parents: round yellow RRYY × wrinkled green rryy.
- 2Gametes: RY from the first parent and ry from the second.
- 3★F₁: all RrYy, all round yellow (round and yellow are dominant).
- 4F₁ plants are self-pollinated: RrYy × RrYy.
- 5Each F₁ plant makes four kinds of gametes: RY, Ry, rY and ry, in equal numbers.
- 6These combine in a 4 × 4 checkerboard, giving 16 combinations.
- 7★F₂: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
★ key step
Checkerboard for the F₂ (RrYy × RrYy)
| Female gametes ↓ / Male gametes → | RY | Ry | rY | ry |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
Counting the 16 boxes
Mendel's laws of inheritance
- Law of dominance: in a hybrid, one allele (dominant) shows and the other (recessive) is hidden.
- Law of segregation (purity of gametes): the two alleles of a gene separate during gamete formation, so each gamete gets only one.
- Law of independent assortment: alleles of different characters go into gametes independently of each other. Seed shape does not decide seed colour.
- Proof: new combinations, round green and wrinkled yellow, appear in the F₂.
- Each character on its own still gives 3 : 1: 12 round : 4 wrinkled, and 12 yellow : 4 green.
Using the 9 : 3 : 3 : 1 ratio
A dihybrid F₂ has 320 pea plants. How many are expected to be round green?
- Ratio = 9 : 3 : 3 : 1, so there are 16 parts in all.
- Round green = 3 parts out of 16.
- Number = 320 × 3/16.
- 320 ÷ 16 = 20, and 20 × 3 = 60.
- Check the whole set: round yellow 180, round green 60, wrinkled yellow 60, wrinkled green 20; total 320.
About 60 round green plants.
Rice breeders combine high yield from one parent with disease resistance from another to make a new variety. Independent assortment is what makes such new combinations possible.
1 marks: “How many kinds of gametes does an RrYy plant make?” Four: RY, Ry, rY and ry.
2 marks: “State the law of independent assortment.” When two pairs of characters are inherited, the alleles of each pair go into gametes independently of the other pair.
7 marks: “Explain Mendel's dihybrid cross with a checkerboard.” P: RRYY × rryy; F₁ RrYy, round yellow; four kinds of gametes; 16-box checkerboard; F₂ 9 : 3 : 3 : 1; new combinations prove independent assortment.
Di means two traits: 16 boxes, 9 : 3 : 3 : 1. The two 3's are the new mixes.