The smaller of the two sex chromosomes. In
male mammals it occurs paired with the other
type of sex chromosome (X), which carries far
more genes. The Y chromosome is the smallest
of all the mammalian chromosomes and is
considered to be largely inert (that is,
without direct effect on the physical body).
See also sex determination. In humans, about
one in 300 males inherits two Y chromosomes
at conception, making him an XYY triploid:
this gives added height, greater emotional
instability, and great aggressiveness. In
1989 the gene determining that a human being
is male was found to occur on the X as well
as on the Y chromosome; however, it is not
activated in the female.