Females- only one X chromosome turned on?

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Aerlinn
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Females- only one X chromosome turned on?

Post by Aerlinn » Mon Oct 23, 2006 11:17 am

Is it true that in females, they have only one X chromosome turned on at a time? But then... with X linked traits, how can there be recessive and dominant traits being expressed when there is effectively only one X chromosome?
Just very confused =S
Hope someone can help!
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G-Do
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Post by G-Do » Mon Oct 23, 2006 4:58 pm

Some genes on the inactive X are still transcribable. This is especially true for genes in the pseudoautosomal region, the spot on the X that synapses with the Y during meiosis. This region is gene-rich.

Also, in general it is more correct to say that in female cells there is only one X turned on at a time. The inactive X is not picked in the zygote, but in the blastocyst stage of development. For this reason, most human females are genetic mosaics.
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Post by sdekivit » Mon Oct 23, 2006 6:37 pm

although females are carrier for X-linked disease, in some cases, they also show the disease but only in a mild form.

i also want to mention that X-chromosome inactivation occurs at random (so it's a probability case what X-chromosome is inactivated during the development in the cells of the blastocyst)

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Post by Aerlinn » Tue Oct 24, 2006 8:49 am

Ok, thanks.
The inactive X is not picked in the zygote, but in the blastocyst stage of development. For this reason, most human females are genetic mosaics.

But not too sure I understand this. Why does that mean most human females are genetic mosaics?

i also want to mention that X-chromosome inactivation occurs at random (so it's a probability case what X-chromosome is inactivated during the development in the cells of the blastocyst)

Once a cell's X chromosome becomes inactivated, does that always stay that way during the cell's lifetime?
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Post by Ultrashogun » Tue Oct 24, 2006 7:24 pm

But not too sure I understand this. Why does that mean most human females are genetic mosaics?


Which X chromosome is deactivated is decided independantly by each cell at a certain time during embryonic development, so you could say that there are "patches" of the organism which express the one and others which express the other chromosome.

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Post by sdekivit » Wed Oct 25, 2006 3:29 pm

Ultrashogun wrote:
But not too sure I understand this. Why does that mean most human females are genetic mosaics?


Which X chromosome is deactivated is decided independantly by each cell at a certain time during embryonic development, so you could say that there are "patches" of the organism which express the one and others which express the other chromosome.


that explains why calico cats are always female --> those are mosaics too

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Post by Ultrashogun » Wed Oct 25, 2006 4:42 pm

sdekivit wrote:
Ultrashogun wrote:
But not too sure I understand this. Why does that mean most human females are genetic mosaics?


Which X chromosome is deactivated is decided independantly by each cell at a certain time during embryonic development, so you could say that there are "patches" of the organism which express the one and others which express the other chromosome.


that explains why calico cats are always female --> those are mosaics too


Thats one of those examples which they have in every book.

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