Essay on Sex Chromosomes
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In humans, the sex of an individual is determined by the type of sex chromosomes a person has. Most men have an X
chromosome from their mother and a Y chromosome from their father. Most women receive an X chromosome
from their mother and an X chromosome from their father. There are exceptions to this rule, such as an XO woman
(Turner's syndrome), who receives only a single X from one of her parents and fails to receive a second X because of
nondisjunction in her parent's sex chromosomes during meiosis. This might occur, for example, when in egg
formation the mother's XX chromosomes fail to separate, resulting in one gamete receiving two X chromosomes
and another gamete receiving no X chromosome. If a sperm fertilizes the egg that has no X chromosome, then the
resulting zygote becomes an XO girl. Turner's syndrome affects about 1 out of every 3,000 female births. These
females are characterized by having skin flaps on the back of their necks, and are sterile.
On the other hand, if a sperm containing a Y chromosome comes into contact with the XX combination in the
mother's egg, then an XXY male will be produced. This is Klinefelter's syndrome, which is characterized by a sexually
underdeveloped boy who has rudimentary testes and prostate glands, often no pubic or facial hair, long arms, and in
some instances will develop breast tissue. About one of every 500 males born has Klinefelter's syndrome.
If a male gamete containing the Y chromosome fertilizes an egg containing no X chromosome, then that embryo will
fail to develop because it is essential that every human must have at least one X chromosome. There appears to be
just too much important genetic information on the X chromosome not to have one.
There are also special cases where, because of non-disjunction again, a woman can have 3 X chromosomes. These
women are called triplo-X and can show tallness and have menstrual irregularities. Men who receive an X from their
mother and a double Y from their fathers have an XYY combination and have a condition called Jacob's syndrome.
When Patricia Jacobs first described this condition in 1965, she proposed the suggestion that the extra Y might
cause increased aggression in these men that might land them in trouble with the law. In the early 70's, special
counseling was given to these boys and their families to help them to avoid trouble. With continued research it was
found that 96% of men with Jacob's syndrome are quite normal, although some may have acne, be a little taller, or
may have speech and reading problems. By telling these boys and their families that they might become aggressive
to the point of becoming criminals, they often fulfilled these expectations.
There are even special cases where a woman who is XXX and a man who is XYY can make all sorts of combinations in
the production of their gametes. It is possible, for example, for an XXX woman to make an XX egg that could join
with an XY sperm from an XYY male to produce a child who is XXXY. As you can see, human sex determination and
possible polygenetic variations can cover a broad spectrum.
One of the most interesting types of variation in sex chromosomes are those people who show sex reversal in their
chromosomal make-up. These are men who appear to be normal men, but have an XX chromosomal combination,
and women who appear to be normal women but have the XY combination. These chromosome patterns are
reversed from the standard pattern usually seen in mammals, and with your understanding of the crossover
phenomenon, you can now understand this unusual situation. On one end of the short Y chromosome, there is a
region thought to contain a gene called TDF (testes determining factor) that starts the process of an embryo
becoming a boy and also releases an anti-Mullerian hormone (AMH) that suppresses the development of feminine
structures. This amounts to only about 1/2 of 1% of the total Y chromosome. This region is called the SRY (sex
determining region on the Y chromosome). All humans start off with the same basic body plan (the evolutionist
Stephen J. Gould, writing in Bully for Brontosaurus, concludes that this conserves energy within the species and also
explains why males have nipples). The genital bud can become either a clitoris or a penis and the gonadal ridge can
become either ovary or testes. If the SRY is present, then the process of development leads to a male. If the SRY is
absent, then the embryo will become female. If during meiosis the SRY ends up crossing over to the X chromosome,
then an XX offspring (normally a female) will end up developing into a male because it has the SRY that causes
maleness. Likewise, a Y chromosome that has lost the SRY during crossover will result in an XY individual (normally a
male), who because the SRY is lacking will become female. Even if only a small snippet of the Y chromosome crosses
over to the X, this sex reversal can occur, provided that the piece that crossed over contained the SRY.
1.
Write the sex chromosomal make-up for the following genetic types:
a. Turner's syndrome:
b. Klinefelter's syndrome:
c. Jacob's syndrome:
d. triplo-X:
e. Any polygenetic sex chromosome combination:
2. What happens to the chromosomes during non-disjunction?
3. Some people have speculated that criminal behavior is caused by the genes you inherit. Explain how you
could set up an experiment to test that genetics and not the environment causes criminal behavior.
4. Give two examples of situations where a male could receive two X chromosomes.
5. Suppose that during the Olympics a certain female athlete wins the big race. After genetic testing, it is
determined that she has the sex chromosomes XY. Do you agree or disagree that it would be proper for the
Olympic committee to remove her medal because she had male sex chromosomes? Explain.
6. Suppose that you knew a person whose chromosomal make-up was XXXY. How would this person be
categorized according to sex? Support your answer.