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| == Sex Determination == | | {| border='0px' |
| Mammalian sex determination is regulated by chromosomes.
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| * Females have two X chromosomes. ('''XX''')
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| * Males have a single X and a small Y. ('''XY''')
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| * The X and Y chromosome are morphologically and functionally different from each other.
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| * Evolutionary studies have shown that the Y was once the homologous pair for X.
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| * It is only in the last 5 years that we have some idea about how these two types of chromosomes may be regulated and genes of importance located upon them.
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| ===X chromosome===
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| {|
| | | [[File:Follicle 001 icon.jpg|90px|link=Development_Animation_-_Ovulation]] |
| | [[File:Human idiogram-chromosome X.jpg|Human idiogram-chromosome X.jpg]] | | | [[File:Fertilization 002 icon.jpg|90px|link=Development_Animation_-_Fertilization]] |
| | | | | [[File:Pronuclear_fusion 001 icon.jpg|90px|link=Development_Animation_-_Pronuclear_Fusion]] |
| '''In females''' - the main scientific problem was understanding gene dosage, only one copy of X chromosome is needed to be genetically active the other copy is inactivated (More? [[Molecular_Development_-_X_Inactivation|X Inactivation]].
| | | [[File:Week1 001 icon.jpg|90px|link=Development_Animation_-_Week_1]] |
| | | | [[File:Ovulation_icon.jpg|90px|link=Movie_-_Ovulation 01]] |
| About the X Chromosome
| | | [[File:Fertilization 001 icon.jpg|90px|link=Movie_-_Fertilization 01]] |
| * 155 million base pairs
| | | [[File:Human-blastocyst-day-3-6-icon.jpg|120px|link=Quicktime_Movie_-_Blastocyst_Development]] |
| * In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
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| * Genes such as Wnt-4 and DAX-1 necessary for initiation of female pathway ovary development
| | | [[Development_Animation_-_Ovulation|Ovulation]] |
| * 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''' '''i'''nactive '''s'''pecific '''t'''ranscript (XIST), that acts as RNA and does not encode a protein.
| | | [[Development_Animation_-_Fertilization|Fertilization]] |
| * 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
| | | [[Development_Animation_-_Pronuclear_Fusion|Pronuclear Fusion]] |
| * 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.
| | | [[Development_Animation_-_Week_1|Week 1]] |
| |} | | | [[Movie_-_Ovulation 01|Ovulation in the rabbit]] |
| | | | [[Movie_-_Fertilization 01|Fertilization in the mouse]] |
| ===Y chromosome === | | | [[Quicktime_Movie_-_Blastocyst_Development|Blastocyst Development]] |
| {|
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| | [[File:Human_idiogram-chromosome_Y.jpg]] | |
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| '''In males''' - the main scientific problem was understanding what on the Y chromosome determined "maleness", and how this is done.
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| About the Y Chromosome
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| * 59 million base pairs, hypervariable in length, mostly non-functional repeats
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| * Current known protein-coding genes = 48 including ''SRY''
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| * ''SRY'' encodes a 204 amino acid protein that is a member of the HMG (High mobility group) box class of DNA-binding proteins. Transcription factors bind to specific sites of DNA and regulates the transcription (expression) of other genes.
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| |} | | |} |
