Each chromosome consists of a short arm and a long arm, which are joined at the centromere. The paired autosomes of the human genome are numbered in the order of decreasing length, 1 being the longest and 22 the shortest. Genes are arrayed in linear sequence along both arms of each chromosome. Differential staining permits their grouping in regions, bands, subbands, and sub-subbands. The standard designation of a gene locus consists of (1) chromosome number; (2) p for short arm, q for long arm; (3) region and band numbers, written together as a 2-digit numeral; (4) a period; (5) subband and sub-subband numbers, written together as a 2-digit numeral. 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.