Genetics and Inheritance
What you must understand
Know DNA structure, genes and chromosomes, and how genes code for proteins. Learn mitosis (growth, identical cells) and meiosis (gametes, halving chromosome number, variation). Understand alleles, dominance, genotype and phenotype, and use Punnett squares for monohybrid crosses. Know sex determination (XX female, XY male) and be able to interpret pedigrees. Understand mutation, and the difference between continuous and discontinuous variation.
Traps examiners set
- Writing genotype ratios as phenotype ratios.
- Forgetting that a recessive phenotype requires two recessive alleles.
- Confusing mitosis (diploid, identical) with meiosis (haploid, variable).
- Assuming a dominant allele is always the more common one in a population.
Worked rule of thumb
For a cross between two heterozygotes (Aa × Aa), the offspring ratio is 1 AA : 2 Aa : 1 aa, giving a 3:1 phenotype ratio if A is dominant. Always state the gametes first, then fill the grid, then read off ratios separately for genotype and phenotype.