Introduction to Genetic Diseases
There are several ways the genetic system can fail.
These diseases are usually congenital and in-born,
though depending on what parts of the body these
genetic errors affect, the actual symptoms may arise
either immediately or after a delay.
A massive error
in an entire chromosome,
affecting a large number of genes,
and usually causing massive problems. There are certain
patterns in chromosome replication that make certain chromosome
errors more common than others.
Single gene disorders:
An error in a single gene,
leading to a specific genetic disease, depending on which
gene was affected. This is the type of disease that people
usually mean when they say "genetic disease".
Multiple gene disorders (polygenic disorders):
More complex disorders that result from patterns of errors in
multiple genes. Some such diseases are well understood, but
certain diseases are believed to be "polygenic disorders"
(from multiple genes) but are poorly understood (e.g. Type 2
diabetes). In fact, these diseases are not really "genetic"
at all, but often have only some level of genetic factor in
Chromosome disorders are problems with a large sequence of DNA.
They do not involve only a single gene.
The two main types of chromosome disorders
are those that affect the sex chromosomes (X and Y),
and those that affect the non-sex chromosomes (autosomes 1..22).
Sex chromosome disorders:
Gender is based
on the X and Y chromosomes: women are XX,
men are XY. But some people have odd and rare sex chromosome
Turner syndrome (X): females, 1-in-5000, minor problems
except for sterility.
Klinefelter syndrome (XXY): male, 1-in-1000, reduced
testosterone, more female-like features, usually sterile.
Jacobs syndrome (XYY): male, 1-in-2000, mostly normal,
sometimes over-male features such as acne, very tall, and
Triple-X (XXX, also XXXX or XXXXX): female, 1-in-700,
most are apparently normal, some sterile, some have slightly
lower mental ability.
There are no known examples of the sequences Y, YY, or YYY.
Order does not matter, so that XYX is just XXY.
Non-sex chromosome disorders (autosomal chromosome disorders):
The 22 non-sex chromosomes can sometimes have major problems,
such as an extra or missing chromosome. The most well-known condition
is Down syndrome.
Down syndrome: an extra chromosome 21 occurs, 1-in-800 to
1-in-25 depending on mother's age - usually mentally retarded,
enlarged tongue, round flat facial features.
Edwards syndrome: an extra chromosome 18,
- few children survive, usually more severe mental retardation
and physical problems than Downs.
Patau syndrome: an extra chromosome 13, 1-in-5000 - severe
mental and physical problems.
There are also non-sex chromosome disorders
caused by subtractions and deletions from chromosomes.
For more detailed information, see chromosome disorders
A true "genetic disease" is one that is related to a single
gene in one part of the DNA on one chromosome.
Hence, a genetic disease is a condition caused
by a small error in the genetic code.
Some examples include:
For more detailed information, see genetic diseases
Premium articles about Genetics and inheritance:
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