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 delay.
- Chromosome disorders: 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 what gene got 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 their cause.
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 disorders.
- 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 behavioral aggression.
- 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: occurs 1-in-800 to 1-in-25 depending on mother's age, extra chromosome 21, usually mentally retarded, enlarged tongue, round flat facial features.
- Edwards syndrome: an extra chromasome 18, occurs 1-in-3000, few children survive, usually more severe mental retardation and physical problems than Downs.
- Patau syndrome: an extra chromasome 13, 1-in-5000, severe mental and physical problems.
There are also non-sex chromosome disorders caused by subtractions and deletions from chromosomes.
- Cri-du-chat syndrome (cat's cry): deletion in chromosome 5, 1-in-50,000,
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:
- Hemophilia (familial)
- Duchenne muscular dystrophy
- Cystic fibrosis
- Some immune failure disorders: Severe Combined Immunodeficiency Disease (SCID, bubble boy disease), DiGeorge syndrome (missing T-cells from thymus failure), hypogammaglobulin anemia (missing B-cells)
- Achondroplasia (dwarfism)
- Phenylketonuria (PKU)
- Sickle Cell Anemia
- Tay Sachs
For more detailed information, see genetic diseases.
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