Genetics and Evolution
Genetics and Evolution joins inheritance arithmetic - ratios, pedigree logic, allele frequencies - to the argument that populations change over time. Expect numericals on test crosses and Hardy–Weinberg, diagrams of sex determination, and short notes contrasting Mendelian with chromosomal disorders.
Inheritance rules
- Law of segregation is universal; independent assortment applies only to genes on different chromosomes. Linked genes give fewer recombinants (Morgan's Drosophila work).
- Deviations from 3:1: incomplete dominance (Antirrhinum, 1:2:1), codominance with multiple alleles (ABO: I^A, I^B, i), pleiotropy (phenylketonuria), polygenic inheritance (skin colour, height).
- Test cross: cross with the recessive parent to reveal an unknown genotype. Back cross: cross with either parent.
Sex determination and disorders
- XX–XY (human), XX–XO (grasshopper), ZZ–ZW (birds).
- Mendelian: haemophilia and colour blindness (X-linked recessive), sickle-cell anaemia (autosomal recessive, glutamate to valine at position 6 of the beta chain), thalassaemia (reduced chain synthesis), PKU.
- Chromosomal: Down syndrome (47, trisomy 21), Klinefelter (47, XXY), Turner (45, X0).
Worked example
An autosomal recessive disease occurs in 1 of 10,000 people. Then q² = 0.0001, so q = 0.01 and p = 0.99. Heterozygote frequency = 2pq = 2 × 0.99 × 0.01 = 0.0198, about 2%, or 1 in 50. Reasoning: take the square root first to get q; never treat 0.0001 as q.
Traps that cost marks
- Giving carrier frequency as q instead of 2pq.
- Assuming independent assortment for linked genes.
- Calling Klinefelter a female phenotype - XXY is male, sterile, with gynaecomastia; Turner is female and sterile.
- Confusing analogous structures (convergent evolution, no common ancestry) with homologous ones.