What you'll learn
Unit 5 covers how traits pass between generations: meiosis, Mendel's laws, and inheritance patterns. ~8–11% of the exam.
Meiosis
Meiosis produces four genetically unique haploid gametes from one diploid cell. Sources of variation:
- Crossing over (prophase I) — exchange between homologs.
- Independent assortment (metaphase I) — random orientation of homolog pairs.
- Random fertilization. Contrast with mitosis (one division, two identical diploid cells).
Mendel's laws
- Law of segregation: the two alleles for a gene separate into different gametes.
- Law of independent assortment: alleles of different genes assort independently (if on different chromosomes).
Punnett squares & probability
- Monohybrid Aa × Aa → 3:1 phenotypic, 1:2:1 genotypic.
- Dihybrid AaBb × AaBb → 9:3:3:1.
- Use the rules of probability (multiply 'and', add 'or') for complex crosses — faster than large squares.
- A test cross (× homozygous recessive) reveals an unknown genotype.
Non-Mendelian inheritance
- Incomplete dominance — blended phenotype (red × white → pink).
- Codominance — both alleles expressed (AB blood type).
- Multiple alleles (e.g. ABO).
- Sex-linked — genes on X; recessive X-linked traits more common in males.
- Polygenic — many genes (height, skin colour); continuous variation.
- Linked genes — close together on a chromosome, inherited together (recombination frequency maps distance).
Exam tips
- Show probability reasoning, not just a filled square.
- For pedigrees, determine dominant/recessive and autosomal/sex-linked from the pattern.
- Link variation in meiosis to evolution/natural selection.
Common mistakes
- Confusing genotype and phenotype ratios.
- Forgetting sex-linked dosage differences between males and females.
- Treating linked genes as independently assorting.