The basic biological unit of heredity. A segment of deoxyribonucleic acid (DNA) needed to contribute to a function. An official definition: According to the official Guidelines for Human Gene Nomenclature, a gene is defined as "a DNA segment that contributes to phenotype/function. In the absence of demonstrated function a gene may be characterized by sequence, transcription or homology." DNA: Genes are composed of DNA, a molecule in the memorable shape of a double helix, a spiral ladder. Each rung of the spiral ladder consists of two paired chemicals called bases. There are four types of bases. They are adenine (A), thymine (T), cytosine (C), and guanine (G). As indicated, each base is symbolized by the first letter of its name: A, T, C, and G. Certain bases always pair together (AT and GC). Different sequences of base pairs form coded messages. The gene: A gene is a sequence (a string) of bases. It is made up of combinations of A, T, C, and G. These unique combinations determine the gene's function, much as letters join together to form words. Each person has thousands of genes — billions of base pairs of DNA or bits of information repeated in the nuclei of human cells —which determine individual characteristics (genetic traits). The chromosome: Genes are arranged in precise arrays all along the length of 23 pairs of much larger structures: the chromosomes. One chromosome in each pair comes from the mother and the other one from the father. The chromosomes in any particular pair look like each other, except in a boy. There is one pair of chromosomes, which usually settles the sex of the individual. This pair has two X chromosomes in females and one X and one Y chromosome in males. The X and Y chromosomes: These chromosomes — the X and Y are always capitalized — are the sex chromosomes. All the other chromosomes in the human chromosome complement are numbered from 1 to 22 and are called the autosomes (literally, the other chromosomes). History of the gene: 1869-1970: {{}}1869 - The chemical material DNA is discovered in cells but its real functions are not known. 1909 - The term "gene" is first used and the chemical composition of DNA is discovered. 1920 - Chromosomes are proposed as the mechanism by which inherited characteristics are passed on. 1944 - DNA is first connected to the inheritance of traits. 1951 - The first sharp X-ray diffraction photographs of DNA are obtained. 1953 - Crick and Watson describe the structure of DNA. 1956 - DNA is made artificially. 1966 - DNA is found to be present not only in chromosomes but also in the mitochondria. 1969 - The first single gene is isolated. 1970 - The first artificial gene is made.
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A functional unit of heredity that occupies a specific place (locus) on a chromosome, is capable of reproducing itself exactly at each cell division and directs the formation of an enzyme or other protein. The g. as a functional unit consists of a discrete segment of a giant DNA molecule containing the purine (adenine and guanine) and pyrimidine (cytosine and thymine) bases in the correct sequence to code the sequence of amino acid s of a specific peptide. Protein synthesis is mediated by molecules of messenger-RNA formed on the chromosome with the g. acting as a template. The RNA then passes into the cytoplasm and becomes oriented on the ribosomes where it in turn acts as a template to organize a chain of amino acid s to form a peptide. In organisms reproducing sexually, normally occur in pairs in all cells except gametes, as a consequence of the fact that all chromosomes are paired except the sex chromosomes (X and Y) of the male. SYN: factor (3). [G. genos, birth]
- allelic g. See allele, dominance of traits.
- autosomal g. a g. located on any chromosome other than the sex chromosomes (X or Y).
- BRCA1 g. a tumor suppressor g. on chromosome 17 at locus 17q21, isolated in 1994; encodes p53 protein, which prevents cells with damaged DNA from dividing; carriers of germline mutations in BRCA1 are predisposed to develop both breast and ovarian cancer. SEE ALSO: BRCA2 g., carcinoma of the breast.
- BRCA2 g. a tumor suppressor g. identified in 1995 on chromosome 13 at locus 13q12–q13; a large g. consisting of 27 exons distributed over 70kb, encoding a protein of 3418 amino acid s; carriers of germline mutations in BRCA2 have an increased risk, similar to that of those with BRCA1 mutations, of developing breast cancer and a moderately increased risk of ovarian cancer; BRCA2 families also exhibit an increased incidence of male breast, pancreatic, prostate, laryngeal, and ocular cancers. SEE ALSO: BRCA1 g., carcinoma of the breast.Familial clustering of breast cancer has long been recognized. Familial breast cancers are characterized by onset before age 45 and by clustering in 3 or more close relatives and in members of more than 1 generation. About 5% of all breast cancers are due to the inheritance of dominant susceptibility genes, particularly BRCA1 and BRCA2. Whereas spontaneous mutations of the BRCA genes are uncommon, hundreds of inherited mutations have been discovered on each g.. The clinical significance of many of these is unknown. Since these are autosomal chromosomes, men as well as women can inherit and pass on the BRCA mutations. The histology of breast cancer in women with BRCA1 and BRCA2 mutations differs from that of sporadic cases. The proportion of medullary carcinomas is higher among BRCA1-associated breast cancers than among all breast cancers. BRCA1 and BRCA2 are tumor suppressor genes, inhibiting tumor development when functioning normally. Both are large genes encoding large negatively charged proteins. Inactivating mutations identified to date are distributed throughout both genes, with a predominance of 2 distinct mutations for BRCA1 and 1 for BRCA2. Despite the high penetrance of the mutant g., not all carriers develop cancer. Hormonal, environmental, reproductive, and other genetic factors may influence penetrance. Estradiol increases cell proliferation and production of the BRCA1 g. product in vitro, while the estrogen antagonist tamoxifen inhibits both cell proliferation and BRCA1 g. expression. Observed mutations are distributed throughout the g.; most are insertions, deletions, or nonsense mutations. Two common changes (185delAG and 5382insC, in exons 2 and 20, respectively) account for approximately 19% of BRCA1 mutations. The former of these is present in about 1% of Ashkenazic Jews and is responsible for about 32% of familial breast cancer in Jews. It is also found in 13% of ovarian cancer patients with no family history of breast or ovarian cancer, and in 30% of those with family histories suggesting inherited disease. BRCA1 mutations cause a 3-fold increase in the risk of prostate cancer in males and a 4-fold increase in the risk of colon cancer in persons of both sexes. The BRCA2 6174delT mutation is estimated to be present in 1.3% of Ashkenazic Jews. Earlier estimates of the risk that a women with a BRCA1 or BRCA2 g. mutation would develop breast cancer at some time in her life ranged from 10–90%. These figures were based on intensive study of families known to be at risk. Current estimates are that the risk of breast cancer may be no higher than 30%. In addition, 15–20% percent of women with the BRCA1 mutation will develop ovarian cancer. Although testing for BRCA genetic mutations is commercially available, most authorities do not recommend routine screening except in women with a strong family history of cancer. Women found to have BRCA genetic mutations have been advised to begin breast self-examination at age 18 and regular annual physician examinations and mammograms at age 25. The benefit of radiologic screening must be weighed against the possible effect of radiation on the BRCA1 or BRCA2 allele. In addition, mammograms are often difficult to interpret in young women because of the density of breast tissue. BRCA2 carriers are also advised to begin ovarian cancer surveillance, consisting of annual or semiannual screening using transvaginal ultrasound (TVS) with color flow Doppler and morphology index, and determination of serum CA-125 levels, at age 25–35. Support for prophylactic mastectomy and oophorectomy is waning as it becomes evident that these drastic procedures cannot altogether abolish cancer risk. Tamoxifen has been shown to reduce the risk of breast cancer in genetically predisposed women by as much as 45%.
- C g. the g. coding for the constant regions of immunoglobulin chains.
- codominant g. a set of two or more alleles, each expressed phenotypically in the presence of the other.
- dominant g. dominance of traits.
- extrachromosomal g. a g. located outside of the nucleus ( e.g., mitochondrial genes).
- H g. SYN: histocompatibility g..
- histocompatibility g. in laboratory animals, a g. which can elicit an immune response and thereby cause rejection of a homograft when tissue is transplanted from one individual to another; in humans, histocompatibility genes control HLA antigens. SYN: H g..
- holandric g. SYN: Y-linked g..
- homeotic genes a group of genes that regulate the development of the body parts by defining the boundaries of the several regions.
- housekeeping genes genes that are generally always expressed and thought to be involved in routine cellular metabolism.
- immune response genes genes in the HLA-D region of the histocompatibility complex of human chromosome 6 which control the immune response to specific antigens.
- jumping g. a g. associated with transposable elements. See transposon.
- lethal g. a g. that produces a genotype that leads to death of the organism before reproduction is possible or that precludes reproduction; for a recessive g. the homozygous or hemizygous state is lethal.
- microophthalmia transcription factor g. g. that when mutated causes Waardenburg syndrome type 2 and Tietz syndrome in at least some subsets of families with these autosomal dominant inherited syndromes.
- mimic genes nonallelic (independent) genes with closely similar effects, e.g., elliptocytosis.
- mitochondrial g. a functioning g. located not in the nucleus of a cell but in the mitochondrial chromosome.
- modifier g. a nonallelic g. that controls or changes the manifestation of a g. by interfering with its transcription.
- mutant g. a g. that has been changed from an ancestral type, not necessarily in the current generation. SEE ALSO: mutant, mutation.
- operator g. a g. with the function of activating the production of messenger RNA by one or more adjacent structural loci; part of the feedback system for determining the rate of production of an enzyme.
- pleiotropic g. a g. that has multiple, apparently unrelated, phenotypic manifestations. SYN: polyphenic g..
- polyphenic g. SYN: pleiotropic g..
- regulator g. a g. that produces a repressor substance that inhibits an operator g. when combined with it. It thus prevents production of a specific enzyme. When the enzyme is again in demand, a specific regulatory metabolite inhibits the repressor substance.
- repressor g. a g. that prevents a nonallele from being transcribed.
- SOS genes a group of genes involved in DNA repair, often induced by damage severe enough to cause stoppage of DNA synthesis.
- g. splicing SYN: splicing (1).
- split genes genes where the genomic sequences are interrupted by intervening sequences (introns) that are spliced out of the mRNA prior to translation.
- structural g. a g. that codes for a specific protein or peptide.
- transfer genes genes carried by a conjugative plasmid, essential for fertility and establishment of the bacterial donor state.
- transforming g. SYN: oncogene.
- tumor suppressor g. a g. that encodes a protein involved in controlling cellular growth; inactivation of this type of g. leads to deregulated cellular proliferation, as in cancer. SEE ALSO: oncogene. SYN: antioncogene.In a person born with 2 normal copies of a tumor suppressor g., both copies must be inactivated by spontaneous point mutation, deletion, or failure of expression before tumor formation occurs. An inherited mutation in a tumor suppressor g. is the basis of most familial predispositions to cancer. In a person so predisposed, malignant cellular proliferation does not occur until the remaining intact copy of the g. is inactivated by deletion of part or all of its chromosome. Of many tumor suppressor genes thus far identified, the p53 g. on chromosome 17, which encodes a phosphoprotein that suppresses cell proliferation, appears to be the most important. Mutations of p53 have been found in the DNA of more than half of all human cancers studied. Li-Fraumeni syndrome, characterized by early-onset carcinomas and sarcomas, is an inherited (autosomal dominant) mutation in the p53 tumor suppressor g.. BRCA1 and BRCA2, involved in familial early-onset breast cancer and ovarian cancer, are tumor suppressor genes.
- V g. the g. coding for the major part of the variable region of an immunoglobulin chain.
- X-linked g. a g. located on an X chromosome.
- Y-linked g. a g. located on a Y chromosome. SYN: holandric g..
- Z g. the structural g. for β-galactosidase.

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gene 'jēn n a specific sequence of nucleotides in DNA or RNA that is located usu. on a chromosome and that is the functional unit of inheritance controlling the transmission and expression of one or more traits by specifying the structure of a particular polypeptide and esp. a protein or controlling the function of other genetic material called also determinant, determiner, factor

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the basic unit of genetic material, which is carried at a particular place on a chromosome. Originally it was regarded as the unit of inheritance and mutation but is now usually defined as a sequence of DNA or RNA that acts as the unit controlling the formation of a single polypeptide chain (see cistron). In diploid organisms, including humans, genes occur as pairs of allele. Various kinds of gene have been discovered: structural genes determine the biochemical makeup of the proteins; regulator genes control the rate of protein production (see operon). Architectural genes are responsible for the integration of the protein into the structure of the cell, and temporal genes control the time and place of action of the other genes and largely control the differentiation of the cells and tissues of the body.

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(jēn) [Gr. gennan to produce] a segment of a DNA molecule (RNA in certain viruses) that contains all the information required for synthesis of a product (polypeptide chain or RNA molecule), including both coding and noncoding sequences where the latter occur. It is the biologic unit of inheritance, self-reproducing, and transmitted from parent to progeny. Each gene has a specific position (locus) in the genome.

Medical dictionary. 2011.

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