Homozygote - (Gr. homos , one and the same. + zygon , yoke) an individual that has two copies of the same allele at a particular locus in its two homologous chromosomes. See allele; genotype; heterozygote.

Hormone - a specific organic product, produced in one part of a plant or animal body, and transported to another part where, at low concentrations, it promotes, inhibits or quantitatively modifies a biological process.

Host - an organism that contains another organism, or a cloning vector.

Host-specific toxin - a metabolite produced by a pathogen which has a host specificity equivalent to that of the pathogen. Such toxins are utilized for in vitro selection experiments to screen for tolerance or resistance to the pathogen.

Humoral immune response - the production of antibody by B cells of the immune system in response to the presence of a foreign antigen. See antibody mediated immune response.

Hybrid selection - the process of choosing individuals possessing desired characteristics from among a hybrid population.

Hybrid vigour - the extent to which the performance of hybrid in one or more traits is better than the average performance of the two parental populations.

Hybridisation - interbreeding of species, races, varieties and so on, among plants or animals; a process of forming a hybrid by cross pollination of plants or by mating animals of different types.

Hybridoma - a hybrid cell, derived from a B (antibody producing) lymphocyte fused to a tumour cell, which grows indefinitely: in tissue culture and is selected for the secretion of the specific antibody produced by that B cell.

Hydrate - a compound formed by the incorporation of water.

Hydrogen bond - a relatively weak bond formed between a hydrogen atom (which is covalently bound to a nitrogen or oxygen atom) and a nitrogen or oxygen atom with an un shared electron pair. Weak interactions between electro negative atoms and hydrogen atoms (electro-positive) that are linked to other electro negative atoms.

Hydrogen uptake positive - a term describing a micro-organism that is capable of assimilating (or taking up) hydrogen gas.

Hydrolysis - a reaction in which a molecule of water is added at the site of cleavage of a molecule to two products.

Hydrophobic - interactions association of non-polar groups with each other when present in aqueous solutions because of their insolubility in water.

Hydroponics - the growing of plants in aerated water containing all the essential mineral nutrients, with no soil. Also called soil less gardening or cultivation.

Hydroxyl end, 3 - The hydroxyl group that is attached to the 3 carbon atom of the sugar (ribose or deoxyribose) of the terminal nucleotide of a nucleic acid molecule.

Hyperploid - a genetic condition in which a chromosome or a segment of a chromosome is over-represented in the genotype. Opposite : hypoploid.

Hyersensitive sites - regions in the DNA that are: highly susceptible to digestion with endonucleases.

Hypertonic - a solution with an osmotic potential greater than that of living cells, leading to water loss from, shrinkage or plasmolysis of cells in a hypertonic situation.

Hypervariable segment - a region of a protein that varies considerably between strains or individuals.

Hypochlorite - generic term for aqueous solutions of sodium hypochlorite, potassium hypochlorite or calcium hypochlorite, which are oxidising agents and used for disinfecting surfaces and surface sterilising tissues, and for bleaching.