Heredity and Evolution
What the exam tests
Heredity and Evolution tests Mendel's experiments, dominant and recessive traits, genotype and phenotype, sex determination, variation and the evidence for evolution. Punnett squares and pedigree-style reasoning are common.
Core rules
- Heredity is the transmission of traits from parents to offspring; variation is the difference among individuals.
- Mendel used pea plants. A monohybrid cross of true-breeding tall and short plants gives F1 all tall and F2 in a 3:1 phenotypic ratio.
- Dominant alleles mask recessive alleles in heterozygotes. Genotype is the allele pair; phenotype is the visible trait.
- A test cross with a recessive parent reveals whether a tall plant is TT or Tt.
- Sex in humans is determined by sex chromosomes: XX female, XY male. The father contributes X or Y.
- Acquired traits are not inherited; germline changes can be.
- Evolution is change in populations over time. Evidence includes fossils, homologous structures (common ancestry) and analogous structures (similar function, different origin).
- Natural selection favours traits that improve survival and reproduction in a given environment.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. The gametes are T and t from each parent. The Punnett square gives TT, Tt, Tt and tt. Three have at least one T and are tall; one is tt and short. The phenotypic ratio is 3 tall : 1 short, and the genotypic ratio is 1 TT : 2 Tt : 1 tt.
Traps that cost marks
- Saying acquired traits such as a weightlifter's muscles are inherited.
- Claiming the mother determines the child's sex; the father does.
- Confusing homologous with analogous structures.
- Writing the F2 ratio as 1:1 instead of 3:1 for a monohybrid cross.