The set of homologous parts of a pair of chromosomes that may be occupied by allelic genes. The locus thus comprises a pair of locations (except in the X chromosome in males). The concept of a locus is somewhat idealized, not taking into account accidents that may occur in meiosis such as duplication of loci as a result of unequal crossing-over, translocations, and inversions.
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Examples: glitazone, GI cocktail, etc.