Herbicide resistance - the ability of a plant to withstand herbicide. Herbicide resistance has been one of the early targets of plant genetic engineering. If a herbicide is sprayed onto a field planted with such resistant crops, then all the plants except the crop would be killed, thus providing an effective method of weed control without having to develop herbicides specific to each weed type. There is substantial concern in some quarters about the widespread use of this technology, which is essentially giving the plant kingdom the ability to evade man's most effective herbicides. The concern are that, firstly, such engineering will lead to increased use of the herbicides, at a time when it is generally accepted that the use of chemicals should be kept as low as possible, and that, secondly, there is the possibility that resistant crop plants will escape to become weeds, or that their resistant genes could be transferred to other species, including weeds.

Heteroduplex - a double-stranded DNA molecule or DNA-RNA hybrid, where each strand is of a different origin, and consequently containing one or more mismatched (non complementary) base pairs. A DNA duplex is prepared by the hybridisation of single-stranded DNA molecules derived from two different sources. Where the two DNAs have identical or very similar sequences, a double-stranded molecule will be established, whereas where the two DNAs differ in sequence, single stranded regions will remain. A heteroduplex will be revealed as single-strand bushes when DNA is observed electron microscopically. A map of homologous and non-homologous regions of the two molecules may thereby be constructed. This process is known as heteroduplex mapping.

Heterogeneous nuclear RNA - large RNA molecules, which are unedited mRNA transcripts, or pre-mRNAs found in the nucleous of a eukaryotic cell.

Heteroplasmy - a cellular condition in which two genetically different types of organelles are present. cf homoplasmy.

Heteroploid - term given to a cell culture when the cells comprising the culture possess nuclei containing chromosome numbers other than the diploid number.

Heteropyknosis - property of certain chromosomes, or of their parts, to remain more dense during the cell cycle and to stain more intensely than other chromosomes or parts.

Heterozygote (Gr. heteros, different + zygon, yoke) an individual that has different alleles at the same locus in its two homologous chromosomes.

Hfr - high-frequency recombination strain of Escherichia coli; in such strains, the F episome is integrated into the bacterial chromosome.

Histocompatibility - the degree to which tissue from one organism will be tolerated by the immune system of another organism.

Histocompatibility complex - the collection of genes coding for peptides present on the surface of nucleated cells; these pep tides are responsible for the differences between genetically non-identical individuals that cause rejection of tissue grafts between such individuals. These peptides were originally called histocompatibility antigens. They are now called histoglobulins, reflecting their structural similarity to immunoglobulins.

Hollow fibre - a tube of a porous material, having an internal diameter of a fraction of a millimetre, and so its ratio of surface area to internal volume is very large. This has had two types of application. Firstly, hollow fibres can be used as filters. Because they have a huge surface area, they take much longer to clog up than normal filters. Secondly, they are used in the hollow fibre bioreactor, in which cells are kept inside the hollow, porous fibres, and the culture medium is circulated outside the reactor. The, fibres let nutrients in and products out (as they are in solution), but do not allow the passage of cells. Hollow: fibre bioreactors are very effective for maintaining mammalian cells in culture because. they have a very large surface: area for the cells to grow on without needing a large reactor to hold them, and because the nutrient reaching the cells can be kept fresh. The reactor also provides an easy way of removing the product that the cells are making: such as monoclonal antibodies. Hollow fibre reactors are less use when the cells themselves have to grow, because it is hard to get at the inside of the fibre to remove surplus cells.

Holometabolous - an insect that undergoes complete metamorphosis to the adult from a morphologically distinct larval stage.

Homeobox - a DNA sequence found in several genes that are involved in the specification of organs in different body parts in animals; they are characteristic of genes that influence segmentation in animals. The homeobox corresponds to an amino acid sequence in the polypeptide encoded by these genes; this sequence is called the homeodomain.

Homeotic mutation - a mutation that causes a body part to develop in an inappropriate position in an organism, such as the mutation in Drosophila that causes legs to develop on the head in place of antennae.

Homoalleles - mutations that are both functionally and structurally allelic; mutations at the same site in the same gene.

Homodimer - a protein with two identical polypeptide chains.