Inheritance and Variation
Genes and chromosomes
Inheritance is controlled by genes on chromosomes in the nucleus. Each body cell has pairs of chromosomes; gametes have half the number. Alleles are different forms of a gene; dominant alleles show even when only one copy is present, recessive only when two are present. A frequent trap is confusing genotype (the alleles, e.g. Bb) with phenotype (the appearance, e.g. brown eyes).
Genetic crosses
Use a Punnett square to predict offspring ratios. For a cross between two heterozygotes (Bb x Bb), the ratio is 1 BB : 2 Bb : 1 bb, giving 3 dominant : 1 recessive phenotype. Sex determination: XX female, XY male; the father determines the sex of the child.
Variation and mutation
Variation may be genetic (from genes) or environmental (from conditions). Mutation is a change in a gene or chromosome and is a source of new variation; most mutations are neutral or harmful, occasionally useful. Natural selection acts on variation: organisms better adapted survive and reproduce.
Worked example
Two brown-eyed parents (both Bb) can have a blue-eyed child (bb), with a one-in-four chance. Rule of thumb: a recessive phenotype can appear from two carrier parents, never from two homozygous dominant parents.
Practical trap
When interpreting pedigree or family data, remember ratios are probabilities, not guarantees for small families.