What would happen if two Y chromosomes were fused together? If XX is female, and XY is male, what would YY be?
-- SB at college
You'd think that a YY baby would be a potential WWF star, but no such luck. So let's talk fish, where this YY tinkering seems to have paid off in real cash. Aquaculturists raising food fish called tilapias, perch-like things, noticed the fish reached maturity fast, reproduced like crazy, crowded the ponds, and therefore never grew to profitable size. What to do, what to do. After five years of fiddling and pondering, they hit upon the idea of using fish sex hormones to reverse the gender of some male tilapias so they could be mated with other males. (Sexual identity in some of the fish world is tenuous, at best.) Researchers persisted in this gender bending, checking chromosomes in the offspring, until they had a small stock of fish that were YY males. When these superfish were mated with your ordinary XX females, about 95% of the babies were male (ordinary XY types). The females of this weird union remained sexually immature, so the pond eventually became a scaly stag party. With no overcrowding, the fish got big and fat and marketable.
In people and most other animals, the Y chromosome is the weak link in the genetic chain. If you look at a portrait of Ma and Pa Chromosome, you'll note that Pa has an amputated leg and he's real deficient in the DNA department. Ma, on the other hand, is packed with genetic information. Men and women get most of their characteristics from their X chromosomes; the Y kicks in after a few weeks in utero to turn the hermaphroditic baby into a boy. And that's pretty much all the Y is about.
There is a genetic condition in which males get an extra Y chromosome (XYY). Some wrongheaded research suggested this made males aggressive and good candidates for a criminal career. Later, it was determined that XYY might cause a slightly lower IQ, but not the urge to rob a bank.