So-called “supermales” have a superfluous Y chromosome, and are XYY (or even XYYY, etc.). In the XY pairing, the X is the longer chromosome and when paired with the shorter Y, the genes in the unpaired X are unopposed alleles. I am note sure, but I suspect that some of them may have essential functions that, if they are missing, will lead to a non-viable individual. I’m not aware of any viable YY individuals.
Supermales are vulnerable to Kryptonite.
If that’s the case, then how do we KNOW that the X and Y chromosomes cause human gender. In other words, is XY the cause of maleness in humans or is maleness the cause of XY? How do we know it isn’t some other, unknown factor that determines gender?
Lovell-Badge and Goodfellow have done some work to confirm it. Google their names along with “Sex determination” and “y chromosones” to read more.
BTW,
XO = Turner sysndrome; short, sterile females - occur in about 1/5000 births
XXY = Klinefelter sysndrome; lanky males with some female characteristics, as well as mentally retarded and sterile - occurs in 1/1000 births
XYY = ??no name in my genetics textbook??; Reasonably normal looking males, usually fertile. Males with this syndrome are hotely debated as having a predisposition towards violence… more “male” than normal. That’s still unknown ASAIK, though several lawyers have tried to use it as grounds for acquittal in crimes of violence. I beleive this condition is the human equivilant to the “supermale”… I’ve also heard, and could be completely wrong here, that there are a disproportionately high number of men with this condition in prison; then again that doesn’t really mean anything, so nevermind it. Also occurs in 1/1000 births.
The Pathology Guy has a good page debunking many myths about XYY, http://www.pathguy.com/xyy.htm
[quote]
If that’s the case, then how do we KNOW that the X and Y chromosomes cause human gender
[quote]
This doesn’t seem the case. It appears that the chromosones influence gender but it is determined by hormone levels. As I pointed out above could lead to XX males and XY females
the only essential thing is to have SOME complete copies of an X gene. in normal women the extra X is “switched off”.
the Y gene contains no important information other than maleness.
and my genetics professor said there were more variations
X0
XX
XY
XXY
XYY
XXX
XYYY
XXYY
and more.
turners, klinefelters and XYY being most common.
some of the others only occuring extremely rarely.
but no YY.
and no Y0.
The process in humans has been thoroughly studied and is well understood, though I’m not an expert. As I mentioned before, there’s a gene located on the Y chromosome (except in very very rare cases where it’s been translocated) that starts things going. From there, there’s a complicated cascade of reactions involving genes scattered throughout the genome and their products. That’s about the extent of my knowledge. If you want more, I’d suggest finding a textbook on early human development.
IIRC from my genetics tutor at college, a human embryo missing an X chromosome is simply not viable. It is probably aborted, naturally, before the mother even knows she’s pregnant.
Last week’s New Scientist (www.newscientist.com) had an article on how intelligence developed in humans. The theory offered was one of sexual selection - women like smart men and men like smart women. Some evidence was offered that some genes determining intelligence are located on the X chromosome. This was expected according to evolutionary theory.
One killer comment from the New Scientist was - suppose instead of intelligence, ancestral human females had preferred bright pink bottoms? 
The X chromosome carries genes essential to life, not merely sex characteristics. The Y chromosome is small, and while there are a couple of traits linked to it (and hence passed down in males father to son) is not (obviously) essential to life.
Therefore a YY combination would be lethal and would never develop.
The “supermales” mentioned above are XYY, and have a tendency towards antisocial, aggressive behavior.
I might point out that there are at least two well-respected posters on this board who are XY-bearing females.
Supermales are YY as supported in my cites above. Humans are not capable of a supermale but other species are. I believe sometimes humans who are XYY are refered to as supermales but the term is for YY.
y ask y?
dandr
YY = Jm J. Bullock
There are also cases where the receptors for testosterone have a mutation and are non-functional. The presence of a Y chromosome causes an embryo to produce testosterone, which normally causes the development of male sexual characteristics. But an embryo that is insensitive to testosterone would not develop male sexual characteristics and would have female secondary sexual characteristics and external genitalia. Many times they wouldn’t even be aware that they had any sort of chromosomal abnormalities. However, this person would be sterile.
I was not aware that XY female’s are sterile and what about XX males?
It’s called Androgen Insensitivity Syndrome (AIS). Outwardly, they look female. They don’t, however, ever develop a uterus, ovaries or vagina, and of course, they never ovulate or menstruate. If the abdominal testes aren’t removed, there is a high risk of testicular cancer.
The link to the site by the guy with XYY was quite interesting. I noticed that in it, he mentioned that there are XXX females. Do they have any special characteristics phenotypically? I would do a Google search for that, but I have a feeling typing “XXX females” into a search engine will not get me the type of sites I’m looking for…
Polycarp, don’t be stereotypical…
“XYY’s average only slightly lower intelligence than XY’s, and the range is the same for both groups. If XYY’s really exhibit severe behavior problems, it has resisted demonstration by the best scientific minds in the field of genetics. Here’s why – it’s something called “ascertainment bias”. Kids who are screened for chromosomal problems tend to be learning and/or behavior problems. If they come up with XYY, it’s easy to blame the karyotype. What’s more, somebody doing bad science can get up a series: “Look at all the XYY’s I’ve discovered, and most of them have mental problems!” (See the fallacy?) But to date, nobody’s shown that XYY’s are more common among kids who are screened for these problems than in the general population. And if XYY was itself a major problem, you’d think this would have been accomplished long ago.”
From The Pathology Guy
So can the geneticists among you tell me HOW any keryotype with multiple Y’s occurs (short of human intervention)? 
I can understand multiple X’s (Mom’s ova remained XX, Dad’s sperm remained XY, you get an XXXY kid), but where do the extra Y’s come from? (Three Y’s? Did the kid have 3 dads??)