Heredity and Evolution
This domain tests Mendel's laws, monohybrid and dihybrid ratios, sex determination, sources of variation, natural selection and the evidence for evolution.
Mendel's experiments
Mendel crossed tall and short pea plants. The F1 was all tall, showing tallness to be dominant. The F2 gave roughly 3 tall to 1 short, showing that the short factor was hidden but not lost. From this came the law of dominance, the law of segregation (factors separate during gamete formation) and the law of independent assortment (two pairs of factors assort independently). A test cross with the recessive parent shows whether a tall plant is TT or Tt.
Worked example
Cross Tt × Tt. Each parent gives T or t. The Punnett square gives TT : Tt : Tt : tt, a genotypic ratio of 1 : 2 : 1 and a phenotypic ratio of 3 tall : 1 short. A cross of TT × tt gives all Tt, all tall.
Sex determination
Humans have 22 pairs of autosomes plus XX in females or XY in males. The father's sperm decides the child's sex, because the mother always contributes an X.
Variation and evolution
Heritable variation comes from mutation and recombination. Darwin's natural selection says more offspring are produced than can survive, variants differ in fitness, and favourable variants survive and reproduce, so populations change over time. Homologous organs share structure but not function (forelimb of man, bat and whale), indicating common ancestry; analogous organs share function but not structure (wing of a bird and of an insect), indicating convergent evolution. Fossils give direct evidence and show succession of forms. Speciation occurs when populations become reproductively isolated; artificial selection by humans produced cabbage, broccoli and cauliflower from one wild ancestor.
Traps
Acquired traits are not inherited; 3 : 1 is the phenotypic ratio, not the genotypic one; evolution is change in a population, not in an individual.