Topic 7 of 8 · Form 6
Inheritance, selection and evolution
Meiosis and genetic variation (crossing over, independent assortment, random fertilisation); monohybrid and dihybrid crosses; codominance, multiple alleles, sex linkage, autosomal linkage, epistasis; the chi-squared test.
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Inheritance
Meiosis and genetic variation (crossing over, independent assortment, random fertilisation); monohybrid and dihybrid crosses; codominance, multiple alleles, sex linkage, autosomal linkage, epistasis; the chi-squared test.
Worked example: A dihybrid cross gives observed numbers 90, 34, 28, 8 (total 160) against an expected 9 : 3 : 3 : 1. Calculate χ².
Expected: 90, 30, 30, 10 → χ² = 0 + 0.533 + 0.133 + 0.4 = 1.07 (below 7.82, 3 df, p = 0.05 → not significant)
Gene control
Lac operon in bacteria; transcription factors in eukaryotes.
Selection and evolution
Variation (genetic and environmental); natural selection (stabilising, directional, disruptive); genetic drift and founder effect; Hardy–Weinberg principle; artificial selection (e.g. high-yield maize); speciation (allopatric and sympatric).
Check yourself
- What is epistasis?
- Give the Hardy–Weinberg equations.
- What is allopatric speciation?
Show answers
- Interaction between genes at different loci, where one gene affects the expression of another.
- p + q = 1 and p² + 2pq + q² = 1
- Formation of new species after populations are separated geographically.
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