Date: September 2004
I am wondering how x-linked diseases like haemophilia or
color blindness affects women, since half of the cells should express the
"sick" allele from the X chromosome (since the barr body inactivation is
Related to this: do boys always have their X from the mother?
Ohhhh, what an excellent question! It is true that in females, one X
chromosome is inactivated in each cell at random. If a female is heterozygous, in
other words, has one "normal" copy of an Xlinked trait and one affected copy,
these are inactivated at random. Probability says that half should be
normal and half should be affected. Usually, in this case there is enough of the
normal protein that is needed to override the effect of the missing protein.
But it IS possible that more of the normal X's could be inactivated leaving
more of the abnormal X's active. In theory, there is a chance that ALL the
normal X's could be inactivated at random, although this is highly unlikely.
Try doing the math yourself! There are cases where a female can experience a
degree of color blindness or hemophilia because of this.
In answer to your second question, boys by definition get their Y from their
father, so yes, they always get their X from their mother.
Women who are heterozygous for X-linked, recessive genes coding for a protein or
enzyme would be expected to produce only half as much as women who do not carry
the gene in question. In fact, women who carry the gene for hemophilia, for example,
have been shown to have increased clotting times due to decreased levels of Factor
VIII. Whether or not they would be partially colorblind, I'm not sure.
Men always inherit their X chromsome from their mother. If they inherit their
father's X, they would be females.
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Update: June 2012