Now, let’s move on into X-linked dominant inheritance. Again, I’ll emphasize that it can be hard
to pick out especially because of the concepts of penetrants, how many people are actually expressing
the genotype and the phenotype. Sometimes there are issues with expressivity. Is the disorder actually
being expressed in its full range or is there a continuum of expression? It turns out, because of features
that we’ve already discussed, there can be a big range of expression. But in classic dominant
relationships for X-linked, if you have a father that’s affected as you see here, you’ll notice that
none of the sons of an affected male are affected. You check this out. You can see that only the daughters
of affected males are affected. That is because transmission from male to male is impossible
because the male gives the Y chromosome. If the disorder is anywhere in the two X’s of a female,
they will express it. Now, we’re going to examine some more complexities or things that throw a wrench
into the situation. Is it possible for an X-linked dominant trait to be only expressed in a female?
Now, as we’ve just reviewed, it can certainly show up in males. But are there conditions where it only
shows up in females? The answer is of course because anything can happen. The example here
is of male lethality. If the condition is lethal prior to term then the fetus will be spontaneously miscarried
and only the females would show up. That’s the case with Rett syndrome which we’ll examine
in a little bit more detail later. But Rett syndrome is a rapid onset of symptoms. It’s in females
because it is lethal in males. We only see Rett syndrome in females. It’s X-linked dominant
but there are no males showing up with it. This is a condition. Probably you’ll see some sorts of questions
about Rett syndrome. That’s why we’ve taken a little extra time out for that. Now we said that we can have it
only in females. We don’t have male to male transmission. That’s one of the big characteristics
of X-linked dominant traits. You do not see it passed from male to male because the male is giving the Y.
But of course, there are always exceptions to the rules. Can you guess what this one is? Well, you’ve got it.
You’ve seen the word already, pseudoautosomal. Recall those little pieces at the very tips
of the X and Y chromosome where on occasion we may see some crossing over, right? In that region,
we can actually transfer a mutant allele from an X to a Y and thus, indeed even in a dominant
inheritance pattern that is X-linked, we could see it passed from male to male. Now of course,
we have to give this a new name because it doesn’t really fit the rules and again how it’s difficult to discern
dominant from recessive. We call this a pseudoautosomal inheritance pattern because it’s on the
pseudoautosomal region of the X and Y chromosome. Dyschondrosteosis is a condition that is pseudoautosomal.
It’s a dominantly inherited skeletal dysplasia that has a much higher prevalence in females.
It has a much higher prevalence in females because it’s X-linked and dominant. But because it’s located
on the pseudoautosomal region, on occasion we see crossing over in that region. Thus on occasion,
it shows up in males. Again, it violates the general rules of not being passed from a male to a male.
Thus, it’s an exception to that rule which means really there are no rules, right?