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: Genital - Female Development | X Inactivation | Genetic abnormality locations chromosomes 21-XY

Human Chromosomes

Human Idiogram: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | X | Y

X Inactivation

Macaque Xi at interphase^[1]

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

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Cite this page: Hill, M.A. (2024, April 19) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome

What Links Here?

↑ McLaughlin CR, Chadwick BP. Characterization of DXZ4 conservation in primates implies important functional roles for CTCF binding, array expression and tandem repeat organization on the X chromosome. Genome Biol. 2011 Apr 13;12(4):R37. PMID: 21489251 | Genome Biol.

[1] McLaughlin CR, Chadwick BP. Characterization of DXZ4 conservation in primates implies important functional roles for CTCF binding, array expression and tandem repeat organization on the X chromosome. Genome Biol. 2011 Apr 13;12(4):R37. PMID: 21489251 | Genome Biol.

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