Heredity and Evolution
What is tested
This domain tests inheritance patterns, Mendel's experiments, sex determination, evolution and speciation. You should be able to use a Punnett square and distinguish evidence for evolution.
Core rules
- Genes occur in pairs; alleles may be dominant or recessive. Genotype is genetic makeup; phenotype is observable trait.
- Monohybrid cross Tt × Tt gives genotype ratio 1 TT : 2 Tt : 1 tt and phenotype ratio 3 tall : 1 dwarf.
- Dihybrid cross gives a 9:3:3:1 phenotype ratio when genes assort independently.
- In humans, sex is determined by sex chromosomes: XX female, XY male. The father determines the sex of the child.
- Evolution is change in inherited traits over generations. Evidence includes fossils, homologous organs and embryology. Natural selection favours useful variations; speciation follows reproductive isolation.
Worked example
Cross Tt × Tt. Possible gametes are T and t from each parent. Punnett square gives TT, Tt, Tt, tt. So 3 of 4 offspring show the dominant tall phenotype and 1 shows dwarf. The genotypic ratio is 1:2:1.
Traps
Dominant does not mean more common. Acquired traits are not inherited. Homologous organs show common ancestry; analogous organs show similar function but different origin. The mother does not determine the child's sex.