Biology IX: Genetics
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Mendelian Genetics
Mendel's experiments with pea plants (Mendel, 1866) established the rules of inheritance. Key terms: gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive.
- Law of segregation: the two alleles for a gene separate during gamete formation.
- Law of independent assortment: alleles of different genes assort independently (for genes on different chromosomes).
- Monohybrid cross: Aa × Aa gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
- Dihybrid cross: AaBb × AaBb gives 9:3:3:1.
- Test cross: crossing an individual of dominant phenotype with a homozygous recessive reveals its genotype.
- Probability: the product rule (independent events multiply) and the sum rule (alternative outcomes add). The chance of an Aa × Aa couple having three affected (aa) children is (1/4)³ = 1/64.
- Pedigrees: identify autosomal dominant, autosomal recessive, and X-linked patterns.
Extensions of Mendel
| Pattern | Description | Example |
|---|---|---|
| Incomplete dominance | Heterozygote is intermediate | Pink flowers from red × white |
| Codominance | Both alleles expressed | AB blood group; sickle cell trait |
| Multiple alleles | More than two alleles in the population | ABO blood groups (Iᴬ, Iᴮ, i) |
| Sex linkage | Genes on the X chromosome | Haemophilia, red-green colour blindness |
| Linkage and crossing over | Genes on the same chromosome are inherited together unless separated by crossing over | Recombinant frequency gives map distance |
| Polygenic inheritance | Many genes affect one trait | Height, skin colour |
| Epistasis | One gene masks another | Coat colour in mice |
| Pleiotropy | One gene affects many traits | Cystic fibrosis, sickle cell disease |
Chi-Squared Test
To test whether observed results fit an expected ratio: χ² = Σ (O − E)² / E, with degrees of freedom = number of classes − 1. The critical values at p = 0.05 are 3.84 (1 df), 5.99 (2 df), and 7.81 (3 df). If χ² is less than the critical value, the difference is not significant, so the data are consistent with the expected ratio (Griffiths et al., 2020).
Population Genetics and Breeding
Use the Hardy-Weinberg equation (Chapter 6). Factors changing allele frequencies: mutation, selection, genetic drift, gene flow, and non-random mating. Plant and animal breeding: artificial selection, hybridization, heterosis (hybrid vigour), inbreeding depression, polyploidy; applications in crops and livestock (Chapter 22).
Common Mistakes
- Using the wrong ratio in dihybrid crosses with linked genes.
- Misreading X-linked pedigrees (a carrier mother can pass the allele to sons).
- Concluding that a significant chi-squared value shows the hypothesis is "proved" (it allows rejection or non-rejection).
CHAPTER 12