The double karyotype of mosaic Turner Syndrome. Credit: Wang, et al.
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.
Greed and cell division
Sharing is hard, even for cells.
Generally, whenever a cell splits into two new cells, all the chromosomes (like books, in which our DNA code is written) get copied and each new cell gets a copy.
Sometimes, both copies of a given chromosome end up in one cell, leaving the other cell a bit short. This is called nondisjunction and can happen in any type of cell, even eggs and sperm.
Which brings us to bumping uglies.
Because each sperm can have either an X or a Y chromosome, if nondisjunction of the X chromosome happens during sperm formation, you wind up with a swimmer without any sex chromosomes at all.
This little sperm is still capable of infiltrating an egg (which only carries an X chromosome), resulting in an embryo with only one single X chromosome.
Welcome to Turner Syndrome.
Turner me right round, baby
It’s not easy to be born with only one X chromosome and for those who are, certain aspects of living can prove challenging as well. Women with Turner syndrome tend to be short and their uterus generally fails to fully develop. Many can’t undergo puberty without hormone therapy and some 30% of those with Turner’s have webbed necks, swollen hands and feet, low hairlines, skeletal abnormalities, kidney problems and heart defects. Like a stocking full of all the worst Christmas gifts.
Webbed neck.
Credit: Elements of Morphology, National Human Genome Research Institute
Low posterior hairline.
Credit: Elements of Morphology, National Human Genome Research Institute
Swollen foot.
Credit: Elements of Morphology, National Human Genome Research Institute
There’s something very odd about this syndrome, however.
How much X do we need?
In women with the usual complement of two X chromosomes, one of them is always inactivated, leaving her with only one functional X chromosome. So if only one is working anyway, then why does it even matter if someone is born with only one?
This, as it turns out, is a great question, because a lot remains unknown. Fun fact: unknown, in science-speak means “left for grad students.” Seriously.
It turns out that at least one specific region on the inactive X chromosome does remain active as do a few other scattered genes that escape inactivation (referred to as “escapees.”). Precisely which genes these are and what they do, though, is only partially known.
It also turns out that there is a second face to Turner Syndrome.
Mosaic people
Not all women with Turner’s have a complete loss of an X chromosome. Many, perhaps half, are “mosaic,” meaning that some of their cells have only the one X chromosome and others have two.
Rather than an empty-handed sperm weaseling its way into an egg, these women experienced an episode of greedy celledness (the aforementioned nondisjunction) very early on in development.
All the cells in your body split off from that first fertilized egg, so you can imagine that when you only consist of, say, four cells and one of them refuses to share its X chromosomes, yet the X-less one persists in dividing, then pretty soon you look (genetically speaking) like a sweatshirt made of two partial sweatshirts sewn together.
Biology never fails to amaze me.
Never would have thought…
The study of Turner syndrome has resulted in some rather unexpected findings. Foremost among them is this conclusion, from 1987, regarding gender and sexuality among Turner syndrome women: “[T]he tendency to greater femininity in TUS women cannot be explained solely by short stature.”
Surely, no matter where our attitudes towards gender identification lie, we can all agree that we would be very surprised to find that they were influenced by our height.
Nothing gets a biologist excited like having another human turn out a bit different and in that respect, Turner Syndrome has given biologists a lot to work with. I hope that after reading this, you can appreciate just how wild and diverse the things we call humans are.
