Read Dihybrid Ratios by Building Them
Combine single-locus probabilities to solve dihybrid crosses when loci assort independently.
Dihybrid Inheritance is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Build a cross one locus at a time
For Tt × Tt, tall:dwarf is 3:1. For Yy × yy, yellow:green is 1:1. Multiplying probabilities gives 3:3:1:1 across the four combined phenotypes.
Count genotype classes
AaBb × AaBb produces nine genotype classes with ratio 1:2:1:2:4:2:1:2:1.
Multiply independent locus probabilities
For a specified multilocus genotype, find the probability at each independent locus, then multiply. In the PpQQRr × PPQqrr cross, Pp, Qq and Rr each occur with probability one-half, giving (1/2)³ = 1/8.
Multiply a specified multilocus genotype
Find each locus, then multiply: Pp×PP gives 1/2 Pp, QQ×Qq gives 1/2 Qq and Rr×rr gives 1/2 Rr. Independent loci give 1/8. Choose B.
OlympIQ exam lens
- ✓Compute each locus separately.
- ✓Multiply probabilities for combined phenotypes.
- ✓Distinguish phenotype and genotype ratios.
- ✓Compute the desired genotype probability separately at each locus.
- ✓Multiply probabilities across independent loci.
- ✓Independent locus probabilities multiply: 1/2 × 1/2 × 1/2 = 1/8.
The familiar 9:3:3:1 is a phenotype ratio, not the nine-class genotype ratio.
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