← Genetic variation and change
Meiosis, pedigrees & selection
Crossing over + independent assortment = variation. Pedigrees: recessive skips generations.Pedigree reading rules
- Recessive trait: can skip a generation; two unaffected parents can have an affected child.
- Dominant: every affected person has an affected parent.
- Autosomal vs sex-linked: affected father with ALL daughters affected (and no sons) suggests X-linked dominant; unaffected mother + affected sons suggests X-linked recessive.
Working out an unknown genotype from a pedigree
- List EVERY genotype consistent with the individual's own phenotype (e.g. Bb or BB for a dominant phenotype).
- Eliminate any that could not have produced the OFFSPRING actually observed — a recessive child rules out a BB parent.
- State explicitly if more than one genotype remains possible — the answer "may not be guaranteed correct" is often part of the mark.
The selection chain
Variation (meiosis + mutation) → selection pressure → differential survival/reproduction → allele frequencies change. Write it as a chain; each link is a mark.| Selection type | What it favours |
|---|---|
| Natural selection | Traits improving survival/reproduction generally |
| Sexual selection | Traits improving mating success specifically, even at a survival cost |
Practice for this standard covers the whole topic — flashcards and quiz.