Discussion
What would be the number of genotypes and phenotypes, respectively, from a cross between genotypes AaBBCcDd and AaBBCcDd ? Assume independent assortment and simple dominant-recessive relationship in each gene pair.
Answer: C
The cross gives 27 genotypes and 8 phenotypes.
With independent assortment, count each gene pair separately and multiply:
- Aa × Aa: 3 genotypes (AA, Aa, aa), 2 phenotypes.
- BB × BB: 1 genotype (BB), 1 phenotype.
- Cc × Cc: 3 genotypes, 2 phenotypes.
- Dd × Dd: 3 genotypes, 2 phenotypes.
\[ \begin{aligned} \text{genotypes} &= 3 \times 1 \times 3 \times 3 = 27 \\ \text{phenotypes} &= 2 \times 1 \times 2 \times 2 = 8 \end{aligned} \]
8 and 4 count only phenotypes and miss one gene; 12 and 4 and 14 and 8 do not come from multiplying the per-gene counts. The common slip is to treat BB × BB as a segregating pair, which would give 81 genotypes and 16 phenotypes.
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