Why does it cause problems to be missing an X chromosome

[RabbitWho]

http://en.wikipedia.org/wiki/Turner_Syndrome
As in being X0 rather than XX or XY

I thought that the X you get from your mother and the X you get from your father all coded for the same things. I wouldn't have imagined anything would happen rather than all of the recessive genes on the X chromosome getting expressed, but recessive genes aren't necessarily harmful ones and that can happen if they're not matched on the Y or if the other X chromosome has the same recessive trait anyway...

It must be the case recessive alleles have work to do, even if their traits are unexpressed and there is another allele there?

 

[Simon Bridge]

The extra X is usually "inactive", but still has an effect - which you can see when there is an additional one as in (47,XXY) and (47,XXX) karyotypes.

I wouldn't have thought that the X chromosomes from each parent normally code for exactly the same things though. I'll leave it alone for now - for someone closer to the subject.

Since excess X has an effect, it seems only reasonable to expect that an absence would also.
The mechanism is probably similar to how the inactive X's have an effect.

 

[Ygggdrasil]

Although one X-chromosome is usually silenced in females, many genes (perhaps even up to a quarter of the genes on the x-chromosome) escape inactivation and are still expressed despite X-chromosome inactivation. Therefore, X0 is not equivalent to one active X-chromosome and one silenced X-chromosome.

https://en.wikipedia.org/wiki/X-inactivation#Expressed_genes_on_the_inactive_X_chromosome

 

[Enigman]

Although one X-chromosome is usually silenced in females, many genes (perhaps even up to a quarter of the genes on the x-chromosome) escape inactivation and are still expressed despite X-chromosome inactivation. Therefore, X0 is not equivalent to one active X-chromosome and one silenced X-chromosome.

https://en.wikipedia.org/wiki/X-inactivation#Expressed_genes_on_the_inactive_X_chromosome

Hmmm...got me thinking-what if one X chromosome carries a hemophilia trait and the other X chromosome is normal...is it possible that the healthy chromosome gets silenced? Sorry if this is a stupid questions; never been good at Biology...

 

[Ygggdrasil]

Which of the two x-chromosomes gets silenced is essentially random and the same x-chromosome is not silenced in all cells. For hemophilia, which is most commonly caused by the inability of the liver to produce Factor VIII (a clotting factor), to occur in a female heterozygous for the mutation, all of the cells in the liver would have to silence exactly the same x-chromosome, a very unlikely occurrence (essentially, the case of getting all heads when you toss a handful of coins).

 

[Enigman]

Thanks for the reply.:)

 

[Ygggdrasil]

After some further reading, there are indeed cases of female carriers of X-linked diseases showing symptoms of those diseases. For example see the following papers:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC303834/
http://www.nejm.org/doi/full/10.1056/NEJM199801293380504

 

[FainAvis]

In kangaroos, it seems the paternal X is inactivated.
http://www.sciencedirect.com/science/article/pii/S1044578183710145

 

[RabbitWho]

Really interesting answers, thank you.

Have I got it right:

Some traits need two alleles to work properly, one from the mother's x and one from the father's x or y.. ? We assume that any of the traits that don't have an equivalent allele on the Y chromosome don't need a partner to work properly. Or am I way out?

Since excess X has an effect, it seems only reasonable to expect that an absence would also.
The mechanism is probably similar to how the inactive X's have an effect.

But having an extra y has no effect. You could say that someone X0 is missing a Y. So I think that logic doesn't work. Though it is very cogent.

 

[mfb]

But having an extra y has no effect. You could say that someone X0 is missing a Y. So I think that logic doesn't work. Though it is very cogent.

As far as I know, the Y suppresses several genes on X and replaces their function with its own genes. Therefore, X0 misses an X or Y, but one of them should be there.

Recessive disorders are a big problem in that case.

 

[Simon Bridge]

But having an extra y has no effect.

What gives you that idea?
http://en.wikipedia.org/wiki/XYY_syndrome

 

[RabbitWho]

What gives you that idea?
http://en.wikipedia.org/wiki/XYY_syndrome

Reading the page you linked me to gave me that idea:


Some medical geneticists question whether the term "syndrome" is appropriate for this condition because its clinical phenotype is normal and the vast majority (an estimated 97% in Britain) of 47,XYY males do not know their karyotype.

 

[Simon Bridge]

Yah - is "phenotype normal" the same as "no effect"?
The same page lists the effects - mostly increased testosterone and high growth, increased chance of learning difficulties and delayed development in speech and language skills.

Not as extreme as the others admittedly but not "no effect".

Note: it used to be thought that XYY karyotype males were more prone to agression ... wikipedia says "no", but the situation is just more complicated than that.
See:
http://www.scienceclarified.com/dis...one-to-aggressive-behavior-than-XY-males.html
http://www.priory.com/psychiatry/XYY_agression.htm
... i.e. "sex is complicated and fraught with misunderstanding", go figure.