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Old 02-23-2010
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Unhappy Inheritance of Fragile X Syndrome

Kaplan said it's X linked dominant while FA said it's X linked recessive! Which one is correct?

I presume that it's X linked recessive because it's not seen in females just like hemophilia

Am I right?
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Old 02-24-2010
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its definitely XLRecessive - reference First Aid for Step 1
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Old 02-24-2010
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Default Fragile X Syndrome is an Autosomal Dominant Disorder

Fragile X syndrome is inherited as X-Linked dominant.
Here's a list of references:
Genetics Home Reference
National Fragile X Foundation

The isssue here is that in X Linked dominant disorders you either have a condition that's lethal to boys in utero so they are not born with it and therefore you only see females with the disease or the condition is not lethal to boys and in this case you see it in both males and females but of course it would be twice as common in females (two X chromosomes).

Fragile X gene is more common in females. But the disease is commonly seen in males only because females are described as carriers and they don't usually manifest deccrease intelligence. One theory to explain that is the Lyon hypothesis in which one of the X choromosomes in the female is randomly inactivated.

Since males have only one X chromosome so they'll for sure be affected and therefore phenotypically speaking Fragile X syndrome is more common in males.

In fact, the inheritance of Fragile X syndrome is believed to be more complex than that. For instance, it was found that premutation is more common in females but is milder than in males. However, the general consensus as in the references I mentioned above is to consider Fragile X syndrome as following an X linked dominant inheritance with somewhat variable penetrance.

Having said that, and because of the complexity of the issue I don't think the boards will ever ask you what is the inheritance of fragile X syndrome, instead they may present a clinical case scenario with autism, large ears, large testes, ...etc and ask you for the diagnosis. I also don't think they'll ever bring an X linked dominant pedigree in the exam.

There's another thread that touched on this issue here's the link
X linked diseases in females!


Last edited by lee-usmle; 05-06-2011 at 11:56 AM. Reason: Typo errors
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Old 05-06-2011
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fA doesn't say it's Xlinked recessive anywhere.
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Old 06-07-2011
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Default Fragile X syndrome is an X-linked dominant

Fragile X syndrome is an X-linked dominant condition with variable expressivity and possibly reduced penetrance.
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Genetics-, Syndromes-Acronyms

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