| variable gene | <molecular biology> Those regions in the amino acid sequence of both the heavy and the light chains of immunoglobulins where there is considerable sequence variability from one immunoglobulin to other of the same class, in contrast to constant sequence (C) regions. The V regions are associated with the antigen binding areas. They contain hypervariable regions of particularly high sequence diversity. (18 Nov 1997) |
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| gap gene | <molecular biology> Segmentation genes involved in specifying relatively coarse subdivisions of the embryo. They are expressed sequentially in development between egg polarity genes and pair rule genes. In Drosophila, there are at least three such genes, for example Kruppel. (18 Nov 1997) |
| gene | <cell biology, molecular biology> Originally defined as the physical unit of heredity, it is probably best defined as the unit of inheritance that occupies a specific locus on a chromosome, the existence of which can be confirmed by the occurrence of different allelic forms. Genes are formed from DNA, carried on the chromosomes and are responsible for the inherited characteristics that distinguish one individual from another. Each human individual has an estimated 100,000 separate genes. Given the occurrence of split genes, it might be redefined as the set of DNA sequences (exons) that are required to produce a single polypeptide. (09 Oct 1997) |
| gene activation | The process of activation of a gene so that it is expressed at a particular time. This process is crucial in growth and development. (05 Mar 2000) |
| gene amplification | <molecular biology> Selective replication of DNA sequence within a cell, producing multiple extra copies of that sequence. The best known example occurs during the maturation of the oocyte of Xenopus, where the set (normally 500 copies) of ribosomal RNA genes is replicated some 4,000 times to give about 2 million copies. (18 Nov 1997) |
| gene bank | A group of genes which are coordinately controlled. (09 Oct 1997) |
| gene cloning | <molecular biology> The insertion of a DNA sequence into a vector that can then be propagated in a host organism, generating a large number of copies of the sequence. (18 Nov 1997) |
| gene cluster | A set of closely related genes that code for the same or similar proteins and which are usuallygrouped together on the same chromosome. (09 Oct 1997) |
| gene conversion | <molecular biology> A phenomenon in which alleles are segregated in a 3:1 not 2:2 ratio in meiosis. May be a result of DNA polymerase switching templates and copying from the other homologous sequence or a result of mismatch repair (nucleotides being removed from one strand and replaced by repair synthesis using the other strand as template). (18 Nov 1997) |
| gene deletion | The total loss (or absence) of a gene. Gene deletion plays a role in birth defects and in the development of cancer. (12 Dec 1998) |
| gene disorder | Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease). Compare polygenic disorders. (05 Mar 2000) |
| gene disruption | Use of both in vitro and in vivo recombination to substitute an easily selected mutant gene for a wild-type gene. (09 Oct 1997) |
| gene divergence | The difference (expressed as a percentage) in the nucleotide sequencesbetween two related genes that developed from the same ancestral gene. (09 Oct 1997) |
| gene dosage | <molecular biology> Number of copies of a particular gene locus in the genome, in most cases either one or two. (18 Nov 1997) |
| gene dosage compensation | The putative mechanism that adjusts the X-linked phenotypes of males and females to compensate for the haploid state in males and the diploid state in females. It is now largely ascribed to lyonization which compensates the mean of the dose but not its variance, which is greater in females. (05 Mar 2000) |