Genetics
Genetics items ask you to move between three representations: a pedigree, a probability, and a molecular mechanism. Expect monohybrid and dihybrid crosses, X-linked pedigrees, Hardy–Weinberg calculations, and definitions of linkage and nondisjunction.
Core rules
- Mendel's segregation and independent assortment apply to unlinked loci; a test cross against a homozygous recessive reveals an unknown genotype.
- Probability: multiply along independent branches, add mutually exclusive routes.
- Linkage: genes on the same chromosome recombine; recombination frequency never exceeds 50%, and 1% recombination = 1 centimorgan.
- X-linked recessive: males are hemizygous, so an affected father never transmits to his sons and all his daughters are carriers.
- Mitochondrial DNA is inherited through the mother only.
- Hardy–Weinberg p² + 2pq + q² = 1 assumes a large, randomly mating population with no selection, mutation or migration.
- Nondisjunction in meiosis I or II produces aneuploidy (trisomy 21, 47,XXY).
Worked example
Two carriers of cystic fibrosis (autosomal recessive, Aa × Aa) have a healthy child. What is the probability the child is a carrier? Offspring genotypes are AA, Aa, aA, aa in equal proportions. The child is known to be unaffected, so aa is excluded and the sample space shrinks to three outcomes. Two of the three (Aa, aA) are carriers: probability = 2/3, not 1/2.
Traps
- Confusing carrier frequency with disease frequency: for 1/10,000 affected, q = 1/100 and 2pq ≈ 1/50.
- Treating dominance as commonness - a rare dominant allele is still rare.
- Forgetting the conditional denominator in "given that the child is healthy".
- Applying recombination frequencies to genes on different chromosomes (independent assortment, 50%).