Now, let’s consider disorders associated with
an XX karyotype. Chromosomally, the individual is XX
but it may not come out there phenotypically.
Most of all, as you would expect, we will see
that there is a translocation of the SRY gene
from the Y chromosome and onto the X chromosome.
If the X chromosome has an SRY gene just as
we would expect from normal sexual development,
the primordial gonad will become a testis because
there’s the SRY gene being expressed
as testis determining factor. We will see development
of testis and ideally testosterone
and then production of male external genitalia.
In the cases where we have an SRY gene,
generally everything proceeds as normal male
sexual development. However, we call them
XX testicular DSD's because we see that there
are testes developing. Now, there are some cases
where we don’t see an SRY gene present at all.
So, it wasn’t translocated from the Y chromosome
to the X chromosome. We essentially have two
normal X chromosomes. Some of these patients have
ambiguous genitalia that suggest evidence of
male sexual development. But how is that even possible
when there’s no SRY gene? That’s a really great
question, a pretty active field of research presently.
But the evidence that we see is that even
though the genitalia may be ambiguous,
they have cryptorchidism. So, there are
definitely testes but they’re undescended or hypospadias
in which the urethra hasn’t made its way
all the way through the penis. So, it’s located
just below the penis and it’s just not normal.
Male development looks ambiguous. One of those
who knows what. Another condition we see,
even smaller percentage, will have both testicular
and ovarian tissues. So, this is where we
might answer the question about true hermaphroditism
because surely if you have one ovary and
one testis, it sort of looks hermaphroditic.
But the problem there is that we actually
as humans, as you know cannot reproduce with our self
in order to have one offspring where you
self-fertilize. Leave that for other organisms.
But it’s completely possible that we have the gonad,
the one and the other on the other side
or you could have an ovotestis which the testis is
made up of both ovarian and testicular tissue.
Another means that could help this happen.
So, there are a couple of different things
that could be going on. As I said, there’s
a lot of research going on to how does this
not SRY gene XX karyotype have male characteristics.
It could be a result of too much product
from other genes. You don’t need to know those genes.
At this point in time, we’re getting
kind of a lot of detail but those other genes
that can actually override the SRY. Clearly, as we learn more,
we know more things are involved in sex determination,
SRY is a major one. But if there’s too much
of these other genes then perhaps we can
accidentally stimulate male development.
One area that we do have a little more understanding
about is CAH or congenital adrenal hyperplasia
in which we have a disruption in the production
of sex hormones that results in a disruption in the production
of sex hormone such that we may get more androgen production.
If that happens during critical periods in development,
it’s completely possible
to have an XX individual display, at least
partially as a male because of overproduction
of particular hormones at certain points during development.