|Embryology - 7 Dec 2019 Expand to Translate|
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A snapshot of the human X chromosome.
- 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.
In birds, the females have a different sex chromosome, the W chromosome. Though this chromosome does not contain genes that lead to the development of a female. (The male bird has a Z chromosome PMID: 9326938) (More? chicken)
- Links: Genital - Female Development | X Inactivation | Genetic abnormality locations chromosomes 21-XY
Some Recent Findings
|More recent papers|
This table allows an automated computer search of the external PubMed database using the listed "Search term" text link.
|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.
- LD50 Number anomalies of chromosome X
- LD51 Structural anomalies of chromosome X, excluding Turner syndrome
- LD55 Fragile X chromosome
- LD56 Chimaera 46, XX, 46, XY
Inheritance Pattern Images
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Cite this page: Hill, M.A. (2019, December 7) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G