Genetics and Evolution
What to know
Genetics includes Mendelian inheritance, Punnett squares, genotype versus phenotype, dominant and recessive alleles, and non-Mendelian patterns (incomplete dominance, codominance, multiple alleles, sex-linked traits). Know the central dogma (DNA to RNA to protein), DNA replication, transcription, translation, and mutation types. Understand meiosis and how crossing over and independent assortment create genetic variation. Evolution covers natural selection, variation, heritability, differential reproductive success, adaptation, speciation, and evidence from fossils, comparative anatomy, and molecular data. Know Hardy-Weinberg as a null model and the mechanisms that change allele frequencies: mutation, gene flow, genetic drift, and natural selection.
Traps
Questions often confuse genotype ratios with phenotype ratios, especially with incomplete dominance. Watch for sex-linked inheritance: a father passes X-linked alleles to daughters, not sons. A common trap is treating natural selection as goal-directed or as acting on individuals to benefit the species; selection acts on heritable variation among individuals. Distinguish homologous from analogous structures and microevolution from speciation.
Worked example
Cross two heterozygotes (Aa x Aa). The genotype ratio is 1 AA : 2 Aa : 1 aa; the phenotype ratio is 3 dominant : 1 recessive. If the trait shows incomplete dominance, the phenotype ratio becomes 1:2:1 because heterozygotes show an intermediate phenotype. Rule of thumb: always ask whether dominance is complete before predicting phenotypes.
Rule of thumb
Variation plus heritability plus differential reproduction equals natural selection; without heritable variation, selection cannot act.