Genetics and Evolution
Genetics and Evolution tests inheritance patterns, chromosomal basis, variation and evolutionary mechanisms. Expect crosses, probability and evidence for evolution.
Core rules
- Mendel: law of segregation and law of independent assortment. A test cross uses a homozygous recessive parent; a back cross uses a parent genotype.
- Incomplete dominance gives an intermediate phenotype; codominance expresses both alleles, as in AB blood group.
- Chromosomal basis: genes on chromosomes; linked genes do not assort independently.
- Evolution: Darwin proposed natural selection; evidence includes fossils, homology, biogeography and molecular similarities.
- Hardy-Weinberg equilibrium assumes random mating, no selection, no mutation, no migration and large population.
Worked example
A recessive disorder affects 1 in 10,000 people. Estimate the carrier frequency.
Reasoning: q² = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency = 2pq = 2 × 0.99 × 0.01 = 0.0198, about 2%. Carriers are far more common than affected individuals. Answer: about 2%.
Traps that cost marks
- Test cross uses homozygous recessive, not heterozygous.
- Codominance is not blending; both alleles are expressed.
- Linked genes violate independent assortment.
- Natural selection acts on phenotypes, not directly on genes.
- Homologous structures indicate common ancestry; analogous structures do not.