Re: If one of X chromosomes is Barr-body, why aren't more women color blind?

Date: Thu Oct 28 17:04:22 1999
Posted By: Lynn Bry, MadSci Admin
Area of science: Genetics
ID: 941140710.Ge

Message:

Hi Edna -

Your question is an interesting one, and deals with the process of "Lyonization" or inactivation of one of the X chromosomes in women.

There have been studies (see reference below) suggesting subtle differences in color vision between women with only one normal gene for red/green vision, as opposed to women with two normal copies of the gene. As far as I have been able to tell no studies have associated these differences with the extent of inactivation of the X chromosome. Regardless, women heterozygous for red/green color blindness would still have cells in their retina expressing the normal gene, and so would still have the capacity to detect red/green objects.

As for the X-linked hemophilias.. Let me first talk about the two types of X-linked hemophilia, A & B:

Hemophilia A: Defect in the gene for Factor VIII (8) - one of the proteins involved in clotting of the blood.
Hemophilia B: Defect in the gene for Factor IX (9) - another protein involved in clotting.

The X-linked hemophilias are more common in men than women, since men carry only one copy of X-linked genes. However, I have seen women who present with what looks like Hemophilia A. Many of them have brothers with the disease, who produce no factor VIII, though their factor VIII levels range from 7-15% of normal. (You can actually get by with very little factor VIII, so though having only 5% of normal can cause bleeding problems, it is not as severe as someone who produces no factor VIII). The fact that these women produce factor VIII implies they have at least one functional gene. Research labs have looked at the inactivation of X- linked genes in these women, and found that (in many of them) ~90% or more of the cells in their body have inactivated the X chromosome carrying the "normal" copy of the factor VIII gene. Through "extreme Lyonization" they thus appear as mild to moderate hemophiliacs.

I hope this answers your question.

-L. Bry, MD/PhD. Department of Clinical Pathology
Brigham & Women's Hospital.
Harvard Medical School
Boston, MA 02115

References:
A study of women heterozygous for colour deficiencies. Vision Res. 1993 Jul;33(11):1495-508.

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