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The gene is the unit of inheritance. The word was suggested in 1909 by the Danish geneticist Wilhelm Johannsen as an abbreviation of the term pangene (Greek, ‘source of everything’), which was originally used by Hugo de Vries in 1889. Johannsen thought of the genes as the characteristics specified in the gametes, but he deliberately avoided the temptation to hypothesize as to their nature. Gregor Mendel\'s work, in the 19th century, showed how characteristics were inherited as discrete ‘factors’, without blending. As the 20th century progressed, the relationship between genes and chromosomes was elucidated, and chromosome maps appeared, showing the positions of individual genes, calculated according to the degree of genetic linkage between them. As biochemical techniques improved, it was revealed that genes were made of a large molecule called DNA, contained within chromosomes.
Molecular biology was founded on the ability to determine the sequence of nucleic acid subunits which make up the DNA molecule. This sequence is codes for an amino acid sequence in a protein, which is the gene product. The gene is the genetic information which specifies a protein; by its modern definition, one gene codes for one enzyme (protein) and it is by this pathway only that the genetic information contained in the DNA can exert control over the living cell. Thus the concept of the gene refers to both a structure and a function. The gene is the unit of transfer of genetic material between generations; typically, the offspring bear two copies of each gene (two alleles), one derived from each parent. Where the alleles are different, the individual is said to be heterozygous (with respect to the gene concerned) and one of the genes is dominant and is expressed in the individual—this is the most common situation in normal populations. Where the two alleles are the same, the individual is said to be homozygous. RB
See also blending inheritance; genetic code; genetic linkage; genetics; Mendelism; polymorphism.Further reading Bruce Alberts, Molecular Biology. |
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