|Embryology - 3 Mar 2021 Expand to Translate|
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Humans have 23 pairs of chromosomes, 22 autosomes and a pair of sex chromosomes. Females have a pair of X chromosomes (46, XX) and males have one X and one Y Chromosome (46, XY). This section of notes introduces the X chromosome and its role in development.
There is a separate page discussing X Inactivation that occurs in all female development with one of the X chromosomes to provide the correct gene dosage. There is a separate page discussing Trisomy X, a genetic disorder where there is an additional copy of the X chromosome. There is a separate page discussing Fragile X Syndrome, a genetic disorder where part of the X chromosome is lost.
- Genes such as WNT4 (Wingless-Type Mmtv Integration Site Family, Member 4), NR0B1 (Nuclear Receptor Subfamily 0, Group B, Member 1; 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.
|Chromosome territories (interphase)||Chromosome (Chromatin) structure (mitosis)|
X Chromosome Overview
- 1400+ genes
- 150 million base pairs
- Contains about 5% of the haploid genome.
- Genes encode house-keeping and specialized functions.
- Conserved in gene content between species.
- In females, one of the X-chromosomes is inactivated in each and every cell (known since 1961).
- This inactivation occurs during embryogenesis.
- X Inactivation appears to be random in somatic cells. (mosaic pattern)
- The process starts at the "X inactivation centre" and spreads along the chromosome.
May 2012 (EST)
|Known Protein-coding Genes||812|
|Novel Protein-coding Genes||24|
|Misc RNA Genes||52|
Ohno's law is a genetic evolutionary theory that suggests that the mammalian X chromosomes are conserved among species. Named after Susumu Ohno (大野 乾 (1928 – 2000) a Japanese-American geneticist and evolutionary biologist, and seminal researcher in the field of molecular evolution.
Contrary to the above theory, the human X chromosome long arm genes are found on the monotreme X chromosome while the short arm genes are found distributed on the autosomes.
The follow cartoons show how genes located on the X chromosome can have different potential inheritance patterns.
X-Linked dominant (affected father)
X-Linked dominant (affected mother)
X-Linked recessive (affected father)
X-Linked recessive (carrier mother)
- Inheritance Pattern images: Genetic Abnormalities | autosomal dominant | autosomal recessive | X-linked dominant (affected father) | X-Linked dominant (affected mother) | X-Linked recessive (affected father) | X-Linked recessive (carrier mother) | mitochondrial inheritance | Codominant inheritance | Genogram symbols | Genetics
|Table - Human Bmp Family|
|BMP15||bone morphogenetic protein 15||GDF9B||Xp11.22|
|Links: Developmental Signals - Bone Morphogenetic Protein | OMIM BMP2 | HGNC | Bmp Family | Sox Family | Tbx Family|
|Human BMP Family|
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.
- Ensembl X Chromosome
- 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
|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|
|Links: Genetics | Abnormal Development - Genetic|
Cite this page: Hill, M.A. (2021, March 3) Embryology X Chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_Chromosome
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G