Question
Explain the dihybrid cross between a pea plant with round-yellow seeds (RRYY) and one with wrinkled-green seeds (rryy). Show the F2 generation phenotypic ratio and state the Law of Independent Assortment.
(NEET 2022, similar pattern)
Solution — Step by Step
Parents: Round-Yellow (RRYY) x Wrinkled-Green (rryy)
Two traits are being tracked:
- Seed shape: R (round, dominant) vs r (wrinkled, recessive)
- Seed colour: Y (yellow, dominant) vs y (green, recessive)
Parent 1 produces only RY gametes. Parent 2 produces only ry gametes.
All F1 offspring are RrYy (heterozygous for both traits).
F1 phenotype: Round-Yellow seeds (both dominant traits expressed).
Each F1 parent produces 4 types of gametes in equal proportion: RY, Ry, rY, ry.
The 4x4 Punnett square gives 16 combinations:
| RY | Ry | rY | ry | |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
F2 Phenotypic ratio:
- Round-Yellow: 9 (any R_ Y_)
- Round-Green: 3 (any R_ yy)
- Wrinkled-Yellow: 3 (rr Y_)
- Wrinkled-Green: 1 (rryy)
Ratio = 9:3:3:1
Mendel’s Law of Independent Assortment (Second Law) states: When two or more pairs of alleles segregate during gamete formation, they do so independently of each other.
The alleles for seed shape (R/r) segregate independently of the alleles for seed colour (Y/y). This produces the four gamete types (RY, Ry, rY, ry) in equal frequency — which is why we get the 9:3:3:1 ratio.
Why This Works
Independent assortment occurs because the two gene pairs are located on different chromosomes. During meiosis I, the orientation of one homologous pair on the metaphase plate is random and does not influence the orientation of the other pair. This gives four equally likely gamete combinations.
The 9:3:3:1 ratio is actually two independent 3:1 ratios multiplied together: (3 round : 1 wrinkled) x (3 yellow : 1 green) = 9:3:3:1. This mathematical relationship confirms that the two traits segregate independently.
NEET loves to modify the standard 9:3:3:1 ratio. When you see ratios like 9:3:4, 9:7, 12:3:1, or 15:1 in questions, these are modifications of the dihybrid ratio due to gene interaction (epistasis). Always check if the ratios add up to 16 — if they do, it is a dihybrid cross with epistasis.
Common Mistake
The most common error: assuming independent assortment always holds. It does NOT apply when two genes are linked (located on the same chromosome). Linked genes tend to be inherited together, giving ratios that deviate from 9:3:3:1. Mendel was lucky — the seven traits he chose in pea plants happened to be on different chromosomes (or far apart on the same chromosome).
Another mistake: getting the gamete types wrong. For RrYy, students sometimes write R, r, Y, y as the four gametes. These are individual alleles, not gametes. Each gamete must contain one allele from EACH gene pair: RY, Ry, rY, ry.