X chromosome
Introduction
- 155 million base pairs
- In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
- Genes such as Wnt-4 and DAX-1 necessary for initiation of female pathway ovary development
- An early discovery (1961) was that in order to have correct levels of X chromosome gene/protein expression (gene dosage), females must "inactivate" a single copy of the X chromosome in each and every cell. The initiator of the X inactivation process was discovered (1991) to be regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST), that acts as RNA and does not encode a protein.
- The genetic content of the X chromosome has been strongly conserved between species because these genes have become adapted to working as a single dose - Ohno's law
- X inactivation occurs randomly throughout the embryo, generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs. This can be seen in the fur colour of tortoiseshell cats.
- Links: X Inactivation
X Inactivation
The presence in females of 2 X chromosome raises the issue of gene dosage, in the case of mammals this is regulated by inactivating one of the X chromosomes. To balance expression with the autosomal chromosomes the dosage imbalance is then adjusted by doubling expression of X-linked genes in both sexes. In some other species compensation occurs by increasing the expression of X in males.
- Links: X Inactivation
Fragile X
External Links
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.
- NCBI Genome View - X chromosome
- Ensembl Ensembl - X chromosome
- OMIM OMIM - Xist
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Idiogram Chromosome Banding - The term refers to the light and dark pattern, seen after staining with a dye, of individual chromosomes identified in metaphase. It is only in meiosis and mitosis during metaphase that chromosomes can be easily identified, during the normal cell life (interphase) the chromosomes are unravelled and distributed within the nucleus in chromosome territories. A band is that part of a chromosome which is clearly distinguishable from nearby regions by appearing darker or brighter with one or more banding techniques. | |
Genetic abnormality locations: 1-4 | 5-8 | 9-12 | 13-16 | 17-20 | 21-XY | sSMC | |
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Links: Genetics | Abnormal Development - Genetic |
Cite this page: Hill, M.A. (2024, April 24) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
Glossary Links
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Cite this page: Hill, M.A. (2024, April 24) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G