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Modern Genetic Analysis Glossary
H
half-chromatid conversion A type of gene conversion that is inferred from the existence of nonidentical sister spores in a fungal octad showing a non-Mendelian allele ratio.
haploid A cell having one chromosome set or an organism composed of such cells.
haploidization Production of a haploid from a diploid by progressive chromosome loss.
haploinsufficient Description of a gene that, in a diploid cell, cannot promote wild-type function in only one copy (dose).
haplosufficient Description of a gene that, in a diploid cell, can promote wild-type function in only one copy (dose).
Hardy-Weinberg equilibrium The stable frequency distribution of genotypes A / A, A / a, and a / a, in the proportions of p2, 2pq, and q2, respectively (where p and q are the frequencies of the alleles A and a), that is a consequence of random mating in the absence of mutation, migration, natural selection, or random drift.
harlequin chromosomes Sister chromatids that stain differently, so one appears dark and the other light.
helicase An enzyme that breaks hydrogen bonds in DNA and unwinds it during movement of the replication fork.
helix-loop-helix protein See HLH proteins.
hemizygous gene A gene present in only one copy in a diploid organismfor example, X-linked genes in a male mammal.
hemoglobin (Hb) The oxygen-transporting blood protein in most animals.
hemophilia A human disease in which the blood fails to clot, caused by a mutation in a gene encoding a clotting protein; inherited as an X-linked recessive phenotype.
hereditary nonpolyposis colorectal cancer (HNPCC) One of the most common predispositions to cancer.
heredity The biological similarity of offspring and parents.
heritability in the broad sense See broad heritability.
heritability in the narrow sense (h2) The proportion of phenotypic variance that can be attributed to additive genetic variance.
hermaphrodite (1) A plant species in which male and female organs are in the same flower of a single individual (compare monoecious plant). (2) An animal with both male and female sex organs.
heterochromatin Densely staining condensed chromosomal regions, believed to be for the most part genetically inert.
heterodimer A protein consisting of two nonidentical polypeptide subunits.
heteroduplex A DNA double helix formed by annealing single strands from different sources; if there is a structural difference between the strands, the heteroduplex may show such abnormalities as loops or buckles.
heteroduplex DNA model A model that explains both crossing-over and gene conversion by assuming the production of a short stretch of heteroduplex DNA (formed from both parental DNAs) in the vicinity of a chiasma.
heterogametic sex The sex that has heteromorphic sex chromosomes (for example, XY) and hence produces two different kinds of gametes with respect to the sex chromosomes.
heterogeneous nuclear RNA (HnRNA) A diverse assortment of RNA types found in the nucleus, including mRNA precursors and other types of RNA.
heterokaryon A culture of cells composed of two different nuclear types in a common cytoplasm.
heterokaryon test A test for cytoplasmic mutations, based on new associations of phenotypes in cells derived from specially marked heterokaryons.
heteromorphic chromosomes A chromosome pair with some homology but differing in size, shape, or staining properties.
heteromultimer A protein consisting of at least two polypeptide subunits, with at least two of the subunits being nonidentical.
heteroplasmon A cell containing a mixture of genetically different cytoplasms and generally different mitochondria or different chloroplasts.
heterothallic fungus A fungus species in which two different mating types must unit to complete the sexual cycle.
heterozygosity A measure of the genetic variation in a population; with respect to one locus, stated as the frequency of heterozygotes for that locus.
heterozygote An individual having a heterozygous gene pair.
heterozygous gene pair A gene pair having different alleles in the two chromosome sets of the diploid individualfor example, A / a or A1 / A2.
hexaploid A cell having six chromosome sets or an organism composed of such cells.
high-frequency recombination (Hfr) cell In E. coli, a cell having its fertility factor integrated into the bacterial chromosome; a donor (male) cell.
Himalayan A mammalian temperature-dependent coat phenotype, generally albino with pigment only at the cooler tips of the ears, feet, and tail.
His Histidine (an amino acid).
histocompatibility antigens Antigens that determine the acceptance or rejection of a tissue graft.
histocompatibility genes The genes that encode the histocompatibility antigens.
histone A type of basic protein that forms the unit around which DNA is coiled in the nucelosomes of eukaryotic chromosomes.
HLH proteins A family of proteins in which a part of the polypeptide forms two a helices separated by a loop (the HLH domain); this domain acts as a sequence-specific DNA-biding domain. HLH proteins are thought to act as transcription factors.
homeobox (homeotic box) A family of quite similar 180-base-pair DNA sequences that encode a polypeptide sequence called a homeodomain, a sequence-specific DNA-binding sequence. Although the homeobox was first discovered in all homeotic genes, it is now known to encode a much more widespread DNA-binding motif.
homeodomain A highly conserved family of sequences, 60 amino acids in length and found within a large number of transcription factors, that can form helix-turn-helix structures and bind DNA in a sequence-specific manner.
homeologous chromosomes Partly homologous chromosomes, usually indicating some original ancestral homology.
homeosis The replacement of one body part by another. Homeosis can be caused by environmental factors leading to developmental anomalies or by mutation.
homeotic gene complex One of the tightly linked clusters of genes controlling A/P segment determination in animals. These genes all encode homeodomain transcription factors.
homeotic genes Genes that control the fate of segments along the anteriorposterior axis of higher animals.
homeotic mutations Mutations that can change the fate of an imaginal disk.
homodimer A protein consisting of two identical polypeptide subunits.
homogametic sex The sex with homologous sex chromosomes (for example, XX).
homolog A member of a pair of homologous chromosomes.
homologus chromosomes Chromosomes that pair with each other at meiosis or chromosomes in different species that have retained most of the same genes during their evolution from a common ancestor.
homomultimer A protein consisting of two or more identical polypeptide subunits.
homothallic fungus A fungus species in which a single sexual spore can complete the entire sexual cycle (compare heterothallic fungus).
homozygote An individual having a homozygous gene pair.
homozygous gene pair A gene pair having identical alleles in both copiesfor example, A / A or A1 / A1.
hormone A molecule that is secreted by an endocrine organ into the circulatory system and that acts as a long-range signaling molecule through activating receptors on or within target cells.
hormone response element (HRE) For hormones that act by binding to receptors that can act as transcription factors, an HRE is a cis-regulatory DNA sequence that is a binding site for a hormonereceptor complex.
host range The spectrum of strains of a given bacterial species that a given strain of phage can infect.
hot spot A part of a gene that shows a very high tendency to become a mutant site, either spontaneously or under the action of a particular mutagen.
HRE See hormone response element.
Huntington disease A lethal human disease of nerve degeneration, with late-age onset. Inherited as an autosomal dominant phenotype; new mutations are rare.
hybrid (1) A heterozygote. (2) A progeny individual from any cross between parents to differing genotypes.
hybrid dysgenesis A syndrome of effects including sterility, mutation, chromosome breakage, and male recombination in the hybrid progeny of crosses between certain laboratory and natural isolates of Drosophila.
hybridization in situ Finding the location of a gene by adding specific radioactive probes for the gene and detecting the location of the radioactivity on the chromosome after hybridization.
hybridize (1) To form a hybrid by performing a cross. (2) To anneal nucleic acid strands from different sources.
hydrogen bond A weak bond in which an atom shares an electron with a hydrogen atom; hydrogen bonds are important in the specificity of base pairing in nucleic acids and in the determination of protein shape.
hydroxyapatite A form of calcium phosphate that binds double-stranded DNA.
hyperploid Aneuploid containing a small number of extra chromosomes.
hypervariable region The part of a variable region that determines the specificity of an antibody.
hypha (hyphae) A threadlike structure (composed of cells attached end to end) that forms the main tissue in many fungus species.
hypoploid Aneuploid with a small number of chromosomes missing.

© 1999 by W. H. Freeman and Company. All rights reserved.