The Genetics of
Sex
by Tamar
Love
Have you ever
wondered why you're a boy or a girl? Most of us know it has
something to do with genetics, but few of us understand the
principles by which genetics
determines our sex.
The human body
is composed of billions of cells.
Most of these cells contains 46 chromosomes,
microscopic structures that determine physical and biological
characteristics such as hair and eye color, body type,
predisposition to hereditary
disease, and a number of other traits. The only cells that do
not contain 46 chromosomes are the sex cells (also known as gametes):
the sperm
and the ova.
Each of these cells only has 23 chromosomes, half the number
contained in normal human cells.
When
fertilization occurs and a sperm and ovum unite, the 23 chromosomes
in the sperm pair
up with the 23 chromosomes in the ova, creating a zygote,
which develops into a fetus.
One chromosome on each pair is contributed by the father and the
other is contributed by the mother. Each chromosome is composed of a
unique pattern of genes, which determines how that cell will
function. No two people, with the exception of identical
twins, share the same genetic information. Twenty two of the
chromosomes direct the development of the zygote and the traits it
will inherit. However, the very last pair of chromosomes—Pair
23—determines an individual's gender.
At the moment of
fertilization, each gamete contributes one chromosome to Pair 23.
The female's egg contains two X
chromosomes. The male sperm contains one X and one Y chromosome.
The female will always contribute an X chromosome and the male may
either contribute an X chromosome or a Y
chromosome. In other words, the male gametes are responsible for
determining the genetic sex of the fetus.
It's important
to remember that sex chromosomes only determine a person's sex.
Gender
is an altogether different story. It's possible that homosexual
orientation may be influenced by biological or genetic factors, but
the nature of Pair 23 doesn't determine sexual orientation: gay males have
an X and a Y chromosome and lesbian
females have two X chromosomes, just like everyone
else.
Adapted from an article by
Carolyn
Gerdes
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