X and Y Chromosomes: Homologous Regions


The human X and Y chromosomes have quite different deoxyribonucleic acid (DNA) and gene contents, and appear completely cytologically nonhomologous. However, they share many homologous regions, the relics of a process of gradual evolution from a homologous autosomal pair into the differentiated sex chromosome pair they are today.

Keywords: sex chromosomes; PAR1; PAR2; nonrecombining region; homologous regions

Figure 1.

Map of homologous genes on the X and Y chromosomes. Genes (black), and pseudogenes (green) of PAR1, PAR2 and the YDR (or MSY) are shown as diamonds. Novel transcripts and genes, described after the full sequencing of the sex chromosomes, are shown right of the PAR scales, and in bold for YDR shared genes. The hashed regions represent X/Y homologous blocks. The PAR scale is in kilobase, and the YDR in megabase. CEN: centromere; TEL: telomere. The Y heterochromatin is abridged (Modified from Ciccodicola et al., ; Tilford et al., ; Skaletsky et al., ; Ross et al., ; Blaschke and Rappold, ; The Ensembl website).

Figure 2.

Evolutionary layers of the X and Y chromosomes. (a) Comparative mapping of genes from the human X in marsupial mammals detected an ancient region conserved on the X in all mammals and denoted XCR, as well as a recently added region denoted XAR that is autosomal in marsupials. In monotreme mammals, XCR genes map to chromosome 6 (revealed as the ancient autosome) and XAR genes to other autosome blocks. (b) Comparative mapping of homologues of human genes in the chicken map to three autosomes, identifying three layers; two subdivide the XCR and one represents the XAR. (c) Nucleotide sequence comparisons of human genes shared on the X and Y group these into five layers, the oldest of which is named stratum 1, up to the most recent and least divergent stratum 5. (d) Comparative mapping of the human and marsupial Y indicate that the Y can also be subdivided into layers corresponding to those of the X. The ancient YCR is very small, and most of the Y evolved from a large YAR. A few genes were transposed from autosomes and are shown as hashed bands. The heterochromatin is denoted as HC, and the pseudoautosomal regions as PAR (Modified from Graves, ; Waters et al., ).



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Further Reading

Graves JA, Wakefield MJ and Toder R (1998) The origin and evolution of the pseudoautosomal regions of human sex chromosomes. Human Molecular Genetics 7: 1991–1996.

Web Links

The Human Genome. National Center for Biotechnology Information's guide to online information resources http://www.ncbi.nlm.nih.gov/genome/guide/human/

Explore the Homo sapiens genome at The Ensembl website http://www.ensembl.org/Homo_sapiens/index.html

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El‐Mogharbel, Nisrine, and Graves, Jennifer AM(Mar 2008) X and Y Chromosomes: Homologous Regions. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005793.pub2]