XX is female, XY is male. What is YY?
Some references here: http://nethealthbook.com/chromosomal-abnormalities-2/47xyy-syndrome/
A person with YY chromosomes would not be born. The X chromosome has genes essential to life, so an embryo with just YY will not develop.
A person with XYY, however, would be male, just as a person with XY.
47,XYY Syndrome affects 1 in 1000 boys; it has a special karyotype (an extra Y chromosome). Another name for it is XYY Syndrome.
Their physical and mental makeup does not differ from the normal XY male.
On average 47,XYY men may grow several centimeters taller than their siblings or parents. There’s a myth in pop culture that aggression is associated with the extra Y chromosome. Not true.
Fertility is normal.
The reason for the extra Y chromosome is that an error occurred during father’s sperm production where there was a so-called “no disjunction” during metaphase I or II. This led to the production of sperm cells with the extra Y chromosome. After impregnating an egg this results in a boy where all the somatic cells contain the 47,XYY constellations. This is called the 47,XYY syndrome.
The signs or symptoms of this syndrome are not too obvious. There are some detailed studies with respect to intelligence which found that about 50% of boys with this syndrome have a learning disability, however 10% of normal 46,XY boys also have this, so there is a 40% more frequent effect. Some experience delayed speech. Testosterone levels are identical with that of normal XY boys and men.
This syndrome was detected in 1961, when Avery Sandberg from the Roswell Park Memorial Institute in Buffalo, New York, was karyotyped because he had a daughter with Down’s syndrome. This was only an incidental finding of no consequence.
This is a genetic variation of no consequences as these men father normal children and the males are normal with normal intelligence. They tend to choose occupations where practical skills are required.