I am doing some practice exams and came across this question. Perhaps the answer is a lot simpler than my reasoning, but after discussing it with others, I still disagree why the answer is clearly and simply what it is, based on the information given. I apologise that this is quite long.
-- Question starts below --
During very early development of mammalian females, one of the two X-chromosomes normally present in each somatic cell becomes deactivated. Deactivation is random, with maternal and paternal X-chromosomes equally likely to be deactivated. All cells deriving from this cell carry the same deactivated X-chromosome, creating local patches of tissue with either maternal or paternal features.
Anhidrotic ectodermal dysplasia (AED) is an inherited skin condition caused by the recessive allele (s) of a gene that has its locus on the X-chromosome. In some individuals with this condition, sweat gland distribution is patchy, with some areas lacking glands. In other individuals with this condition, all sweat glands are absent.
Q) Which one of the following is consistent with the information given?
A. Patchy sweat gland distribution is found in males and females but is more likely in males with the condition.
B. Patchy sweat gland distribution is equally likely in both males and females with the condition.
C. Only females with the condition exhibit patchy sweat gland distribution.
D. Only males with the condition exhibit patchy sweat gland distribution.
--/ End of question /--
The correct answer is C.
I would like to know why you should be able to tell that only females will manifest or show patchy sweat gland distribution. My reasoning is, individuals with the condition must have the recessive gene. Since it is an X-linked condition, and females have 2 X chromosomes, they must have 2 copies of the recessive gene (little r) to get this condition. Thus, females must be homozygous recessive, or rr. (Males will be just r)
If we define the phenotype of having the recessive gene, we say that it causes the individual to either have patchy sweat glands or no sweat glands at all. The thing is, they do not tell you under what conditions a person will have patchy sweat glands and when they will have no sweat glands at all.
As an X linked disease, think haemophilia, they should either have the skin condition or they don't: in other words, even though females can be carriers, carrier females will not exhibit the symptoms and the question talks about all humans with the condition (either males or females).
How* the disease manifests isn't made clear, so how are you supposed to know that of those who have patchy sweat glands vs those that have absolutely no sweat glands, females are those who have patchy sweat glands?
*For example: Angelman and Prader-Willi syndromes are both results of errors on the same region of chromosome 15, however, the important how that explains who gets PW and who gets A is, the phenotype depends on whether it was maternal or paternally inherited due to imprinting. By telling you this, if the question told you that males tend to get PWS and females tend to get Angelmann, told you some characteristics of either syndrome, then told you "given that a specific gender is more likely to get a specific symptom", you can determine whether males or females are more likely to have which disorder (PWS or A) based on the symptoms.
The question doesn't have this sort of information. It doesn't tell you how a person tend to get patchy or missing sweat glands and it doesn't tell you anything about incomplete dominance, co dominance, mosaicism etc.
Since both males and females can have the condition but it is X linked, why is it not A, but C? Why can't males have patchy sweat gland distribution too?
One answer I got while discussing this was, the information tells you about random inactivation of the X chromosome. For me, this only further rejects the idea that only females can be patchy: Suppose a female is a heterozygous with XRXr. One X chromosome gets inactivated at random. Let's pretend XR gets inactivated, leaving Xr. This is the recessive gene.
Now, we know the recessive gene produces the skin condition, with could be either patchy or completely absent sweat glands. Sadly, it does not tell you the exact symptoms of the recessive gene, see my point and analogy about PWS and A above. Since it doesn't tell you, I can only assume that like all other X linked diseases, having the recessive gene will cause the individual to show symptoms of that disease, end. of. story.
Usually when you have the recessive gene, you have the disease. Either have or not have. Thus an XRXr female with the first X inactivated will have the recessive disease, and fully show this (no sweat glands at all).
If she was XRXr with the second X inactivated, she would show a normal phenotype and have fully functional in ordinary quantity sweat glands.
Similar case with the males: They either have, or not have the faulty gene due to the fact that they only have 1 X chromosome.
Now, during my discussion with others, someone said, if you assume one copy of AED X (Xr) + one copy of normal X (XR) = carrier, then there is no way to explain how anyone could have patchy glands. They either have normal glands or no glands at all.
But I think this is exactly the point. The question doesn't ask you how anyone could have either patchy or totally absent sweat glands. Why does patchy glands exist? Where is the condition of its existence? It's not told to you in the question. The normal train of thought (at least for me) is therefore, a female either has it or they don't. No where does the text mention anything to do with mosaicism. IF the question TOLD you that AED is inherited the way it is etc, then stated that females have patchy sweat glands while males have none at all, and asked you "what is the most likely reason for this", then you can say mosaicism.
This direction of reasoning would work much more logically.
Otherwise I just don't see this.
Would appreciate input from someone specialising in genetics.