Two X and one Y chromosome. Image: Shutterstock
We’ve heard it all so many times before: humans with an XY pair of sex chromosomes are male and those with an XX pair are female. Sexual Characters is a series of brief articles on how things are slightly more complicated.
X chromosome special! Buy 2, get 3!
A rather common genetic curve ball, affecting some 1-2 out of every 1,000 males, is Klinefelter Syndrome (KS), in which an XY male gets a bonus X chromosome.
This happens because at some point during a parent’s cell division, one half of a dividing cell got a little greedy. Normally, each time a cell divides, each new cell gets 1/2 of the chromosomes from the original cell. This greediness is called nondisjunction.
Typically, an egg will contain one X and a sperm will contain either an X or Y. To wind up with KS, though, you need either an egg with two X’s or a sperm carrying both an X and a Y, as illustrated below.
Symptoms
There is a range of symptoms associated with KS and those who have the syndrome can display them in varying degrees. The symptoms can be so subtle, in fact, that many people may not know they have KS until puberty or later.
The main symptoms are infertility and small testicles. Other physical symptoms can include weaker muscles, greater height, poor coordination, less body hair, breast growth, and low sex drive.
Sometimes the syndrome is accompanied by reading and speech difficulties.
Treatments
Often, people with KS are given hormone treatments, breast reductions and speech and physical therapies, as called for by their particular symptoms.
Blame the parent
KS isn’t a heritable syndrome. Obviously, if you have KS, you can argue that you inherited it from your parents, but this doesn’t mean that you can transmit it to your kids, in turn.
Given the earlier mention of infertility, it may seem like people with KS can’t transmit anything to any offspring, but people with KS can actually reproduce, thanks to the modern wonders of assisted reproductive technology.
One of the more biologically interesting aspects of KS is the fact that symptoms can vary in specific ways, depending on whether a person gets their extra X chromosome from their mother or their father.
In cases of autism, for instance, if the bonus X chromosome came from the father, the autistic child will have a greater chance of displaying more pronounced impairments in social interactions. If it came from the mother, the autistic child tends to show more repetitive behaviors and stereotyped patterns.
Speech and motor skills seem to be more impaired and body size slightly larger, if a child inherits the extra X from their father.
Interesting side note: there are actually cases of people getting an XXXY karyotype and even an XXXXY karyotype. Biology is amazing. Messy, but amazing.
We are not alone
Humans are not unique in their ability to acquire more X chromosomes. The condition is common enough in mice, that they make useful model animals, in which to study KS. Cats have a much more adorable version of Klinfelter’s, which results in male calicos.
Sex and gender in medical literature
Medical literature calls this Klinefelter Syndrome and refers to those with it as male.
The NHS, the United Kingdom’s health service, states unequivocally that an X chromosome does not make one female, since we all have at least one, but that “the presence of a Y chromosome denotes male sex.”
Very clear, unless you’ve read the past two posts on androgen insensitivity syndrome and Swyer syndrome, in which people with XY chromosomes, who develop typically female features are referred to as “women”, by doctors, researchers and lay people alike.
For all that they strive for precision and clarity, scientists suddenly waffle when what’s under the microscope doesn’t match that, which we see by eye. Get your shit together, scientists.
For their part, people with KS seem to experience as wide a set of gender identities as any other group of people. Reliable statistics on gender identity among those with KS are hard to find, but associations dedicated to those with the syndrome claim that most identify as male.
Questions? Stories about Klinefelter Syndrome? I’d love to hear them in the comments. Like this post? Share it!
Stay tuned next time, for the bonus Y chromosome.