X chromosome

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Introduction

Some X chromosome disease genes
Human idiogram-chromosome X.jpg

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.[1] (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

  • FOXL2 is a female sex-determining gene in the goat[2] "The origin of sex reversal in XX goats homozygous for the polled intersex syndrome (PIS) mutation was unclear because of the complexity of the mutation that affects the transcription of both FOXL2 and several long noncoding RNAs (lncRNAs). Accumulating evidence suggested that FOXL2 could be the sole gene of the PIS locus responsible for XX sex reversal, the lncRNAs being involved in transcriptional regulation of FOXL2. In this study, using zinc-finger nuclease-directed mutagenesis, we generated several fetuses, of which one XX individual bears biallelic mutations of FOXL2. Our analysis demonstrates that FOXL2 loss of function dissociated from loss of lncRNA expression is sufficient to cause an XX female-to-male sex reversal in the goat model and, as in the mouse model, an agenesis of eyelids. Both developmental defects were reproduced in two newborn animals cloned from the XX FOXL2(-/-) fibroblasts. These results therefore identify FOXL2 as a bona fide female sex-determining gene in the goat. They also highlight a stage-dependent role of FOXL2 in the ovary, different between goats and mice, being important for fetal development in the former but for postnatal maintenance in the latter.
More recent papers  
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Search term: X chromosome | X Inactivation | X-linked | Trisomy X

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[3]


Diseases

Some X chromosome disease locations

 ICD-11

  • 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

X-Linked dominant (affected father) X-Linked dominant (affected mother)

X-Linked recessive (affected father) X-Linked recessive (carrier mother)

Codominant inheritance


Trisomy X

Trisomy X

Fragile X

External Links

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Glossary Links

Glossary: 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 | Term Link



Cite this page: Hill, M.A. (2019, July 21) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome

What Links Here?
© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
  1. Evolutionary analysis of the female-specific avian W chromosome http://www.nature.com/ncomms/2015/150604/ncomms8330/full/ncomms8330.html
  2. Boulanger L, Pannetier M, Gall L, Allais-Bonnet A, Elzaiat M, Le Bourhis D, Daniel N, Richard C, Cotinot C, Ghyselinck NB & Pailhoux E. (2014). FOXL2 is a female sex-determining gene in the goat. Curr. Biol. , 24, 404-8. PMID: 24485832 DOI.
  3. McLaughlin CR & Chadwick BP. (2011). 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. , 12, R37. PMID: 21489251 DOI.