Genetics and Evolution
Genetics and Evolution tests Mendelian ratios, non-Mendelian inheritance, pedigree analysis, Hardy–Weinberg calculations, and the evidence for evolution.
Core rules
- The law of segregation and the law of independent assortment apply to genes on different chromosomes. A test cross (F1 × recessive parent) reveals the genotype of a dominant phenotype.
- Incomplete dominance: F2 phenotype ratio 1:2:1 (snapdragon). Codominance: both alleles expressed (AB blood group). Multiple alleles: ABO has IA, IB, i. Pleiotropy: one gene, many effects (sickle cell). Polygenic inheritance: many genes, continuous variation.
- Sex determination: XX–XY (humans), XX–XO (grasshopper), ZZ–ZW (birds). Mendelian disorders: haemophilia, colour blindness, sickle cell anaemia, thalassaemia, phenylketonuria. Chromosomal disorders: Down (trisomy 21), Klinefelter (XXY), Turner (XO).
- Evolution: Oparin–Haldane and Miller–Urey; homology, fossils, adaptive radiation, industrial melanism. Hardy–Weinberg: p² + 2pq + q² = 1.
Worked example
A recessive allele frequency q = 0.2 in a population of 5,000. Then p = 0.8. Carrier frequency = 2pq = 2 × 0.8 × 0.2 = 0.32, so 0.32 × 5,000 = 1,600 carriers. Affected individuals = q² = 0.04 → 200. The carriers are phenotypically normal but can transmit the allele.
Traps
2pq is the carrier frequency, not q. Hardy–Weinberg assumes random mating, a large population, and no mutation, migration or selection. Incomplete dominance changes phenotype ratios, not allele frequencies.