Heredity and Evolution
Mendel's experiments, monohybrid and dihybrid ratios, sex determination and the evidence for evolution form the examinable core of this domain.
Mendel's laws
Law of dominance, law of segregation and law of independent assortment. Traits are controlled by pairs of factors (alleles); the dominant allele shows in F1. Monohybrid F2 gives 3:1 phenotype and 1:2:1 genotype. Dihybrid F2 gives 9:3:3:1.
Sex determination
Humans have 23 pairs of chromosomes; the 23rd pair is XX in females and XY in males. The father's sperm decides the child's sex, because the mother always contributes an X.
Evolution
Variation plus natural selection (Darwin) leads to speciation; genetic drift acts strongly in small populations; reproductive isolation separates species. Homologous organs share basic structure but differ in function (forelimbs of whale, bat, human) and indicate common ancestry. Analogous organs (wings of bird and insect) show convergent evolution. Fossils give direct evidence.
Worked example
Cross Tt with Tt. Each parent gives T or t, so offspring are TT, Tt, Tt, tt: three tall to one dwarf. The genotypic ratio is 1 TT : 2 Tt : 1 tt. The dwarf appears because both parents carried the recessive allele hidden behind a tall phenotype.
Traps
Writing the genotypic ratio as 3:1; saying acquired traits such as a wrestler's muscles are inherited; mixing homologous with analogous organs; claiming evolution always produces better organisms.