Genetics, Variation and Evolution
What this domain tests
Inheritance (monohybrid crosses, test crosses, sex determination, ABO blood groups), sources of variation, mutation, natural selection and evidence for evolution.
Core rules
- A gene is a length of DNA coding for a protein; alleles are different versions of a gene.
- Genotype is the alleles present; phenotype is the observable characteristic.
- In a cross, write the parental genotypes, the gametes, then a Punnett square - never skip the gametes.
- Complete dominance gives a 3:1 phenotypic ratio only when both parents are heterozygous for the same gene.
- Continuous variation (height) is polygenic and strongly environmental; discontinuous variation (blood group) is usually single-gene.
Worked example
An unknown tall pea plant is crossed with a dwarf plant, and 48 tall and 51 dwarf offspring are produced. Reasoning: dwarf must be tt, so each offspring receives t from it. About half the offspring are dwarf, so they must also receive t from the tall parent. The tall parent is therefore Tt (heterozygous). If it had been TT, all offspring would be tall.
Traps that cost marks
- Writing a 3:1 ratio from a test cross - that cross gives roughly 1:1.
- Confusing dominant with “more common”; dominance describes expression, not frequency.
- Forgetting that sex is determined by the sperm (X or Y), not the egg.
- Describing natural selection without mentioning inherited variation and differential survival.