Genetics and Evolution: Inheritance, Populations and Change over Time
Genetics and Evolution tests Mendelian inheritance and its exceptions, the chromosomal basis of sex determination, population genetics and the evidence for evolutionary change.
Core rules
- Law of segregation: the two alleles of a gene pass into different gametes. Independent assortment applies only to genes on different chromosomes; linked genes give fewer recombinants.
- A test cross with a recessive parent reveals whether a dominant phenotype is homozygous or heterozygous.
- Incomplete dominance (Snapdragon: red × white → pink) gives a 1:2:1 phenotypic ratio; in codominance both alleles are expressed, as with A and B blood groups.
- Sex determination: XX–XY in humans, XX–XO in grasshoppers, ZZ–ZW in birds, haplodiploidy in honeybees, where drones are haploid and have no father and no sons.
Population genetics
Hardy–Weinberg: p² + 2pq + q² = 1, assuming no mutation, no migration, random mating, a large population and no selection. A change in allele frequency means evolution is occurring.
Worked example
A recessive disorder affects 1 in 10,000 people. Then q² = 0.0001, so q = 0.01 and p = 0.99. Heterozygous carriers = 2pq = 2 × 0.99 × 0.01 = 0.0198, about 2% of the population - roughly 198 carriers per 10,000 people.
Traps
- Dominant does not mean more common in the population.
- Recombination frequency is proportional to distance only for genes on the same chromosome.
- Analogy (wings of a bird and a butterfly) is convergent evolution; homology (forelimbs of vertebrates) is divergent.
- Fitness means reproductive success, not physical strength.