This Science Daily article says that in females, one of the X-chromosomes is completely and permanently inactivated during early development. This is to avoid females producing twice as many copies of proteins as males. OK, makes sense.
That is, until you consider sex-linked conditions, the classic example, taught in beginning biology classes, being hemophilia. Sex-linked conditions show up in males because they only inherit one X-chromosome. If it has a defective gene, then the condition is exhibited. In females, the condition is only exhibited if both X-chromosomes have the defective gene, a much rarer occurance.
But this contradicts the complete inactivation of one of the X-chromosomes. If one of the X’s were inactivated, then you’d expect any sex-linked conditions to occur about half as often in females. That doesn’t happen. So am I right in concluding the inactivation of X-chromosomes is not complete?
X-chromosome inactivation is random in females. The inactivation occurs early in development, but not at the time of fertilization. Hence, half of her cells will have one copy deactivated while the other half will have the other copy deactivated. This is generally enough to counteract any deleterious genes on one copy.
I think the OP’s point was that by a simplistic understanding of X-chromosome disabling, they should be very close to half as common, with women randomly having either the “good” or the “bad” chromosome active when they were bearers of a disadvantageous gene. Instead we have rates such as this, for red-green colour blindness: “Between seven and ten percent of men and 0.49% to 1% of women are affected.” X-linked recessive inheritance - Wikipedia
The explanation is simple. It’s not the same X-chromosome that is turned off in every cell.
“In the early blastocyst, this initial, imprinted X-inactivation is reversed in the cells of the inner cell mass (which give rise to the embryo), and in these cells both X chromosomes become active again. Each of these cells then independently and randomly inactivates one copy of the X chromosome.” X-inactivation - Wikipedia
In addition to what was said above, I have also read that a small part of the deactivated X is still active. Presumably this corresponds to whatever is found on the Y.
As the two different X-chromosomes are turned off randomly, a person with a defective gene for, say, colour-blindness, should I assume have a random smattering of failed cones; so I assume this means colour sense would be impaired but they would still be able to distinguish different colours, just not as intensely?
This is also mentioned in one article I read as the reason for a wider diversity of intelligence in males. Men are apparently on the distribution curve smarter and dumber than women - i.e. a flatter, more spread out distribution. A genetic attribute like intelligence (or lack) would be more pronounced if it resulted from one “X” rather than a compromise patchwork of two versions where one may be lesser.