haploid
The condition in which a cell or individual has one copy of every chromosome.

haplotype
The combination of alleles at loci that are found on a single chromosome or DNA molecule.

Hardy–Weinberg principle
The principle that allele and genotype frequencies will reach equilibrium, defined by the binomial distribution, in one generation and remain constant in large random mating populations that experience no migration, selection, mutation, or nonrandom mating.

Hardy–Weinberg proportion
A state in which a population’s genotypic proportions equal those expected with the binomial distribution.

hemizygous
A term used to denote the presence of only one copy of an allele due a locus being in a haploid genome, on a sex chromosome, or only one copy of the locus being present in an aneuploid organism.

heritability
The proportion of total phenotypic variation within a population that is due to individual genetic variation (HB; broad sense heritability). Heritability is more commonly referred to as the proportion of phenotypic variation within a population that is due to additive genetic variation (HN; narrow sense heritability).

hermaphrodite
An individual that produces both female and male gametes.

heterochromatin
Highly folded chromosomal regions that contain few functional genes. When these traits are characteristic of an entire chromosome, it is a heterochromosome or supernumerary chromosome.

heterogametic
The sex that is determined with different sex chromosomes (e.g., the male in mammals (XY) and female in birds (ZW) ).

heteroplasmy T
he presence of more than one mitochondrial DNA haplotype in a cell.

heterosis
A case when hybrid progeny have higher fitness than either of the parental organisms. Also called hybrid vigor.

heterozygosity
A measure of genetic variation that accounts for either the observed, or expected proportion of individuals in a population that are heterozygotes.

heterozygote
An organism that has different alleles at a locus (e.g., Aa).

heterozygous advantage
A situation where heterozygous genotypes are more fit than homozygous genotypes. This fitness advantage can create a stable polymorphism. Also called overdominance.

heterozygous disadvantage
A situation where heterozygous genotypes are less fit than homozygous genotypes. Also called underdominance.

HFC
Heterozygosity–fitness correlation.

Hill–Robertson effect
An effect where selection at one locus will reduce the effective population size of linked loci; increasing the chance of genetic drift forming negative genetic associations that reduce the ability of associating loci to respond to selection. See also genetic draft.

hitchhiking
The increase in frequency of a selectively neutral allele through gametic disequilibrium with a beneficial allele that selection increases in frequency in a population.

homogametic
The sex that possesses the same sex chromosomes (e.g., the female in mammals (XX) and male in birds (ZZ)).

homoplasmy
The presence of a single mitochondrial DNA haplotype within a cell.

homoplasy
Independent evolution or origin of similar traits, or gene sequences. At a locus, homoplasy can result from back mutation or mutation to an existing allelic state.

homozygosity
A measure of the proportion of individuals in a population that are homozygous; it is the reciprocal of heterozygosity.

homozygote
An organism that has two or more copies of the same alleles at a locus (e.g., AA).

HW
Hardy–Weinberg.

hybrid sink
The situation where immigration of locally unfit genotypes produces hybrids with low fitness that reduces local density and thereby increases the immigration rate.

hybrid swarm
A population of individuals that are all hybrids by varying numbers of generations of backcrossing with parental types and matings among hybrids.

hybrid vigor
See heterosis.

hybrid zone
An area of sympatry between two genetically distinct populations where hybridization occurs without forming a hybrid swarm in either parental population beyond the area of co-occurrence.

hybridization
Mating between individuals of two genetically distinct populations.