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It has been estimated that there are more than 4000 human genetic disorders, and the number is increasing all the time as researchers learn more about our DNA. There are several different types; a mutation in a single gene, multifactorial, where the problems are caused by environmental factors and involve several problems with several genes, plus chromosomal and mitochondrial.
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Single Gene Disorders
This type of human genetic disorder occurs when a change happens in part of a gene's DNA sequence. Genes make proteins, and a mutation in the gene will result in the protein not being able to function properly. It's this that causes the problems we associate with disease.
Cystic fibrosis is an example of a single-gene disorder. Patients with the condition have a defective gene on chromosome 7 called CFTR (cystic fibrosis transmembrane conductance regulator).
The abnormality on the CFTR gene causes the protein produced by this gene to affect how salt (sodium chloride) moves in and out of cells. This single-gene disorder affects mainly lung cells producing an abnormally thick mucus that blocks the lungs. It can make breathing and digestion difficult, as well as causing many other complications such as lung infections.
Cystic fibrosis is also known as a recessive disorder. A recessive genetic disorder is one in which both parents pass the defective gene to their children. In this situation both parents are carriers. They have the defective gene but do not express the disease themselves. If only one parent has the affected gene then the child does not expresses it, because the good version of the gene from the other parent will make the protein that the body needs. Around 5% of the US population carries a copy of the affected gene for Cystic fibrosis.
A dominant disorder is one in which only one copy of the affected gene is needed for a disease to appear. An example of a dominant genetic disorder is Marfan syndrome. Because it is dominant, people who have inherited one affected gene (named FBN1 gene) from either parent will have Marfan's, which is a disease related to the connective tissue.
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Mitochondrial disorders refer to those genetic disorders in which the DNA affected is inside the mitochondria. A mitochondrion is a tiny organelle inside our cells that help to convert substances inside our food into energy. Mitochondria have their own DNA which is separate from the DNA inside the nucleus.
An example of this kind of genetic disorder is the Leber Optic Atrophy, a mitochondrial genetic disorder that involves the degeneration of optical nerve cells. Some mitochondrial disorders can affect many organs at the same time.
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When portions or entire chromosomes are missing, duplicated, or altered, a chromosomal genetic disorder occurs. Down's syndrome is probably the most well known example. It occurs when an extra copy of chromosome 21 is present in an individual. He or she will have 3 copies of chromosome 21. Having an extra copy of this chromosome implies that each gene is producing more protein product than normal, and this affects the body's finely balanced systems.
Another example of a chromosomal genetic disorder is Williams syndrome, where people are missing genetic material from chromosome 7. It results in the deletion of a gene that makes elastin, a protein that gives strength to blood vessel walls. Without it there are many developmental problems and some children with the condition may have learning disabilities.
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Multifactorial Genetic Disorders
These are caused by a combination of small variations and abnormalities in genes. There is also usually an environmental component as well. Heart disease and many types of cancers are examples of these disorders.
So let's look at skin cancer as a brief example. A mutation in one gene will usually not be enough to give rise to the disease. But if there a several mutations either through inheritance or by the action of UV rays on DNA (the environmental condition) then the chance of getting melanoma increases.
Multifactorial inheritance disorders are difficult to diagnose, understand, and treat.
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Rare Human Genetic Disorders: What's on the horizon?
As researchers continue to uncover the genetic basis of many diseases, new treatments or courses of actions can be devised to proactively manage, treat, and, possibly cure many of today's genetic disorders.
The solutions are not simple. The path from gene discovery to therapeutic is a long one, but the more we understand about what's going on beneath our skin, the better equipped we will be to fix things when they go wrong.