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Dihybrid inheritance involves the simultaneous inheritance of two genes, each with two alleles, located on different chromosomes. This builds on monohybrid genetics and introduces Mendel's second law — the law of independent assortment.
Mendel's second law states that alleles of different genes assort independently of one another during gamete formation, provided the genes are located on different chromosomes (i.e., they are unlinked).
Key Point: Independent assortment only applies to genes on different chromosomes. Genes on the same chromosome are linked and tend to be inherited together (covered in a later lesson).
In pea plants:
These two genes are on different chromosomes, so they assort independently.
Cross: RrYy × RrYy (both parents are heterozygous for both genes)
Gametes from each parent: RY, Ry, rY, ry (four types of gamete, produced in equal proportions)
| RY | Ry | rY | ry | |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
From the 16 possible offspring:
| Phenotype | Genotypes | Count |
|---|---|---|
| Round, yellow | R_Y_ | 9 |
| Round, green | R_yy | 3 |
| Wrinkled, yellow | rrY_ | 3 |
| Wrinkled, green | rryy | 1 |
Expected phenotypic ratio: 9:3:3:1
This 9:3:3:1 ratio is the signature of a dihybrid cross with complete dominance at both loci and independent assortment.
Key Definition: The underscore notation (e.g., R_) means "either homozygous dominant or heterozygous" — the second allele can be either R or r without affecting the phenotype when the dominant allele is present.
Rather than drawing a full 4×4 Punnett square, you can use the branch (forked-line) method:
For Rr × Rr: 3/4 round, 1/4 wrinkled For Yy × Yy: 3/4 yellow, 1/4 green
Combined probabilities:
This confirms the 9:3:3:1 ratio and is much quicker than a full Punnett square.
In guinea pigs:
Cross: BbSs × BbSs
Using the branch method:
| Phenotype | Probability | Ratio out of 16 |
|---|---|---|
| Black, short | 3/4 × 3/4 = 9/16 | 9 |
| Black, long | 3/4 × 1/4 = 3/16 | 3 |
| Brown, short | 1/4 × 3/4 = 3/16 | 3 |
| Brown, long | 1/4 × 1/4 = 1/16 | 1 |
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