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

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

Macaque Xi at interphase

Diseases

Some X chromosome disease locations

Inheritance Pattern Images

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.

• NCBI Genome View - X chromosome
• Ensembl Ensembl - X chromosome
• OMIM OMIM - Xist

Glossary Links

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols

Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G