Barr's body is a spiral X chromosome. If I'm not mistaken, one random chromosome (a healthy woman) is inactivated in each cell. Which X chromosome is inactivated in a given cell is random. I've heard the example of a tricolor cat. The patched coloring is caused by the fact that cells responsible for the color of each patch contain another inactivated X chromosome.

But what if a girl is a carrier of the Daltonism gene, why isn't she sick if a chromosome with this gene can be inactivated in some cells?

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Like you said the sexual X chromosome inactivation is random in each different cell, this means women have some cells with the maternal x chromosome active and others with the paternal x chromosome active. Now if a woman carries the daltonism gene mutation (heterozygous for color blindness) she can actually manifest signs of the disease if the retinal cells randomly silenced the healthy x chromosome instead of the mutated one in more cells compared to the opposite pattern of silencing (meaning she can present daltonism out of pure bad luck).

It is worth mentioning that the magnitude of this disease will never be as strong as it presents itself in males since women (even the unlucky ones who present signs of daltonism) will always have the protection of a certain percentage of retinal cells with the healthy x chromosome active. The only way a woman could have classic daltonism to the same extent as a male is if she presents the color-blindness gene mutation in combination with Turner syndrome.

In conclusion recessive X-linked diseases aren't "everything or nothing" since woman could present signs in function to the number of cells that randomly inactivated the healthy X chromosome. They will fully express themselves in males all the time though.


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