Genetics
Genetics tests inheritance, linkage, mutation, gene interaction, and population-level change.
Core rules
- Mendel's laws apply to nuclear genes: segregation of alleles and independent assortment of unlinked genes.
- Linked genes do not assort independently; recombination frequency estimates map distance in centimorgans, with 1% recombination equal to 1 cM for short distances.
- Recombination frequency cannot exceed 50%; 50% indicates independent assortment or genes far apart.
- Dominance, incomplete dominance, codominance, and epistasis produce different phenotypic ratios. A testcross reveals an unknown genotype.
- Mutations may be silent, missense, nonsense, or frameshift; only germline mutations are heritable in animals.
- Hardy-Weinberg equilibrium predicts genotype frequencies when mating is random and no selection, mutation, migration, or drift occurs.
Worked example
A testcross of a dihybrid produces 1000 progeny: 420 parental, 420 parental, 80 recombinant, 80 recombinant. Total recombinants = 160. Recombination frequency = 160/1000 x 100 = 16%, so the genes are linked about 16 cM apart. If the result were 250 and 250 recombinants, the frequency would be 50% and linkage could not be inferred.
Traps
Do not treat 50% recombination as proof of linkage. Do not confuse a 9:3:3:1 ratio with a testcross ratio. Do not call a silent mutation harmless in every context; splicing and codon usage can matter. Do not apply Hardy-Weinberg to a selected population without checking assumptions.