Unit 2: How Is Inheritance Explained and How Does It Change?
From cells to gametes
Unit 2 connects cell division to inheritance. You must describe the cell cycle, mitosis and its role in growth and repair, and meiosis and its role in producing genetically variable haploid gametes. Compare the two: mitosis produces genetically identical diploid cells; meiosis produces four genetically different haploid cells through crossing over in prophase I and independent assortment in metaphase I. Know the difference between haploid and diploid, homologous chromosomes, chromatids and centromeres, and how karyotypes are interpreted.
Genotypes and phenotypes
Understand the relationship between genes, alleles, genotype and phenotype. Be able to complete monohybrid and dihybrid crosses, use Punnett squares, state expected ratios, and interpret pedigrees, including sex-linked inheritance. Distinguish complete dominance, incomplete dominance and codominance, and explain multiple alleles using blood groups. Know that phenotype results from genotype interacting with environment.
Traps examiners set
- Confusing meiosis I (homologues separate) with meiosis II (sister chromatids separate).
- Forgetting that independent assortment applies only to genes on different chromosomes.
- Writing ratios where proportions or percentages are asked, or omitting the phenotype in a cross answer.
- Assuming a recessive allele is "rare" or "weaker" - it is simply not expressed when a dominant allele is present.
- Misreading a pedigree: carriers are often unaffected heterozygotes.
Worked example and rule of thumb
Two heterozygous parents (Aa × Aa) for a completely dominant trait give a 3:1 phenotypic ratio and 1:2:1 genotypic ratio. Rule of thumb: set out gametes first, then fill the grid - the gametes each parent can produce determine the cross, not the parents' phenotypes.
Variation and change
Describe sources of genetic variation (mutation, meiosis, fertilisation) and how sexual reproduction generates diversity. Explain natural selection as differential survival and reproduction acting on heritable variation, and distinguish it from acquired change within an individual's lifetime.