Genetics and Evolution
Core concepts
Understand Mendelian inheritance, including segregation and independent assortment, and be able to solve monohybrid and dihybrid crosses using Punnett squares or probability rules. Know the difference between genotype and phenotype, dominant, recessive, codominant, and incompletely dominant alleles, sex-linked inheritance, and the distinction between autosomal and X-linked pedigrees. Familiarity with mutation types, chromosomal abnormalities, and the Hardy-Weinberg principle is expected, along with the mechanisms of evolution: mutation, gene flow, genetic drift, and natural selection.
Traps
A frequent error is confusing the probability of a single offspring with the probability across several offspring; each fertilization is independent. Another trap is assuming a rare recessive trait must skip generations, which is not always true. In Hardy-Weinberg problems, remember that p + q = 1 describes allele frequencies, while p² + 2pq + q² = 1 describes genotype frequencies; the frequency of affected individuals equals q² only for a recessive condition in equilibrium.
Worked example
If 1 in 10,000 people has a recessive disorder, q² = 0.0001, so q = 0.01 and p = 0.99. Carriers (2pq) are approximately 2 × 0.99 × 0.01 ≈ 0.0198, or about 2%.
Study tip
For pedigree questions, first ask whether the trait appears in offspring of unaffected parents; if so, it is recessive. Then check whether affected fathers pass it to all daughters, which suggests X-linked dominant patterns.