MadSci Network: Genetics

Re: Barr body and missing an X chromosome

Date: Tue Sep 6 13:03:14 2005
Posted By: Sanjida Rangwala, Graduate Student, Washington University
Area of science: Genetics
ID: 1125946780.Ge

Would there be a barr body in an individual with a single X chromosome?

Before I give the answer, let's review why any cell would have a Barr Body.
Mammalian females have two X chromosomes while males have an X and a Y. In
order to balance out the gene dosage of genes on the X chromosome between
males and females, females silence all but one of their X chromosomes. The
key phrase there is "all but one", as opposed to one of their chromosomes.
So if a female had three X chromosomes due to non-disjunction in the
parent, she would silence 2 Xs in all her cells, leaving one active. When X
chromosomes are silenced, they condense into a dense ball of protein and
DNA which is what we see under the microscope as a Barr Body. 

So, the answer is, if a person (male or female) has only a single X
chromosome, then no, it would not be silenced and packaged as a Barr Body.
It needs to be active, since the person needs their genes on the X in order
to develop and live and be healthy. 

Now, how exactly the X becomes silenced and how the cell knows to count Xs
and leave a single one unsilenced, and which one, are all exciting areas of
current research. 

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