Expert: Dana Krempels, Ph.D. - 8/17/2010

Question
Hello Dana Krempels,

I have a question about Barr bodies. I am a bit confused and
I want you to correct me on this concept.  I think I am
understanding it but I am not too confident.  School has not
started yet for us but I want to be ready for my class
because I am retaking it with a very difficult professor and
I want to understand the material well.

So, from what I get barr bodies are something that
inactivates an X chromosome in a female and one of the
advantages is that it can inactivate the extra chromosomes
they may have which may cause aneuploidy.  

Can you let me know if this is correct?  I have been
listening to past lectures from this professor and he is not
very good at English so I am having trouble with
understanding him.  

Some other questions I would like to know is do all women
have barr bodies or just the ones who need it?

I understand the purpose of barr bodies is not to get rid of
extra chromosomes but it is an advantage to having a barr
body.

Also, if a woman has two active chromosomes and an
inactivated chromosome, that is normal right?  Can a woman
have more than one barr body, too? I think so but I just
want to make sure.  

Sorry, I am just very nervous and I want to pass this when I
start it late August.  I have been trying to find these
answers online but I can't find it. Plus I just want an
experienced person in this subject to let me know that I am
right so that I can feel confident.

Thank you so much.

Elham

Answer
Dear Elham,

A Barr Body *is* an inactivated X chromosome.  All females with normal sex chromosome ploidy have one Barr Body per cell, if all things are normal.  Males with normal ploidy (i.e., not XXY or XXXy, etc.) lack Barr Bodies.  This is true for all mammals.

In each cell of a female mammal's body, one or the other X chromosome winds up into (mostly) untranscribed heterochromatin at some point in her embryonic development.  That inactivated X chromosome is the Barr Body.  Only one X chromosome in each cell is functional, just as in the male.  But because the female's X chromosomes inactivate randomly in each cell, she will usually be producing the products of both of her X chromosomes...just not in the same cell.

You can get a picture overview of this with a classic example:  calico coat color in cats.  Have a look here at my course web site:

http://www.bio.miami.edu/dana/dox/calico.html

Hope that helps clear things up.  Good luck!

Dana