Genetics and Evolution
Genetics and Evolution covers Mendelian genetics, the molecular basis of inheritance, and evolution. The exam tests crosses and ratios, DNA calculations, and evolutionary mechanisms.
Core rules
- Law of segregation: alleles separate during gamete formation. Independent assortment applies only to genes on different chromosomes.
- A test cross with a heterozygote gives 1:1; a monohybrid F2 gives 3:1; a dihybrid F2 gives 9:3:3:1 if genes are unlinked. Incomplete dominance gives 1:2:1 phenotypes; codominance is seen in the AB blood group.
- DNA is antiparallel and follows Chargaff's rule: A = T and G = C. Replication is semiconservative; transcription uses one strand as template; the genetic code is triplet, degenerate, and non-overlapping.
- Evolution is change in allele frequency in a population. Natural selection acts on phenotype; genetic drift is random; Hardy-Weinberg equilibrium requires no mutation, no migration, no selection, random mating, and a large population.
Worked example
A double-stranded DNA sample contains 28% adenine. Find the percentage of guanine. Reasoning: A pairs with T, so T = 28%. A + T = 56%. The remaining 44% is G + C. Since G = C, G = 22%.
Traps
Linked genes do not give a 9:3:3:1 ratio. Dominant does not mean more common in a population. Evolution occurs in populations, not individuals. Mutation is random, not directed by need. In incomplete dominance, the heterozygote is intermediate, not a blend that disappears.