DNA is a double helix polymer found in the nucleus, made of repeating nucleotide units. Each nucleotide consists of a common sugar, a phosphate group and one of four different bases (A, C, G or T). The sugar and phosphate groups form the backbone of each strand, and the bases on the two strands pair up (A with T, C with G) to hold the double helix together. A gene is a section of DNA that codes for a particular protein.
Alleles are different versions of the same gene. Dominant alleles (capital letter, e.g. B) are expressed when one or two copies are present. Recessive alleles (lowercase, e.g. b) are only expressed when two copies are present (homozygous recessive, bb).
Genetic diagrams (Punnett squares) predict offspring ratios. A cross between two heterozygous parents (Bb × Bb) gives a 3:1 phenotype ratio.
Sex determination: XX = female, XY = male. Sex-linked conditions (e.g. colour blindness) are carried on the X chromosome and more commonly affect males.