ANEUPLOIDY The presence of extra copies, or fewer copies, of some chromosomes.
CHIASMA (pl. chiasmata). The cytological manifestation of genetic exchange between chromosomes, indicating that a crossover has occurred between homologous chromosomes.
CONJUGATION In prokaryotes, the transfer of DNA from a donor cell to a recipient cell that is mediated by direct cell–cell contact.
DIRECTIONAL SELECTION Selection that favours one allele over all other alleles of a gene. The frequency of this beneficial allele can rise or can be held in check by recurrent mutation.
DOMINANCE COEFFICIENT The factor by which the selection coefficient is reduced in heterozygotes relative to homozygotes.
EPISTASIS (
). A measure of fitness interactions between alleles at different loci. In haploids, we use the two-locus measure, = fitness(AB) 
fitness(ab) - fitness(Ab) fitness(aB).
EQUILIBRIUM A state in which a system remains unchanged over time.
GENETIC DRIFT (also known as random drift). A phenomenon whereby the frequency of a gene in a population changes over time because the number of offspring born to parents that carry the gene is subject to chance variation.
HAPLOTYPE A haploid genotype. A diploid genotype comprises a maternal and a paternal haplotype.
HARDY–WEINBERG EQUILIBRIUM A state in which the frequency of each diploid genotype at a locus equals that expected from the random union of alleles — that is, where the inbreeding coefficient (F) is zero.
INBREEDING COEFFICIENT (F). A measure of genetic associations between alleles at the same locus, which indicates whether homozygotes (positive F) or heterozygotes (negative F) are more common than expected. For ease of comparison with linkage disequilibrium, we write the standard measure for F as {frequency(AA) frequency(aa) - [0.5 frequency(Aa)]2}/{frequency(A) frequency(a)}.
LINKAGE DISEQUILIBRIUM (D). A measure of genetic associations between alleles at different loci, which indicates whether particular haplotypes are more common than expected. We use the two-locus measure, D = frequency(AB) frequency(ab) - frequency(Ab) frequency(aB).
MUTATION–SELECTION BALANCE The equilibrium at which selection that increases the frequency of a favourable allele exactly balances mutations that decrease the frequency.
PROTIST A eukaryote other than animals, plants and fungi; often single celled.
RECOMBINATION LOAD The difference in fitness between offspring produced without recombination and those produced with recombination.
SELECTION COEFFICIENT A term that describes the difference in average fitness between two genotypes when fitness is measured relative to the average fitness of one of the genotypes (known as the reference genotype).
SINGLE-LOCUS INTERACTION (
). A measure of fitness interactions between alleles at the same locus. We use the single-locus measure, = fitness(AA) fitness(aa) - fitness(Aa)2.
TRANSDUCTION The exchange of genetic material from one cell to another that is mediated by a virus or phage.
TRANSFORMATION The uptake of DNA by a bacterium from the surrounding environment.
