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The cells of a human being contain 46 chromosomes. Of these, 2 are sex chromosomes while the rest consist of 22 pairs of non-sex or autosomal chromosomes. The two sex chromosomes define the person’s sex with males having XY chromosomes and females having XX chromosomes. The chromosomes are made up of strands of DNA which itself consists of a series of genes. Most genes carry information necessary for the manufacture of proteins, the building blocks of life, making the DNA a kind of blueprint that defines the structure and function of the cell.

Genetic diseases in humans are caused by abnormal DNA in an individual's cells that are caused by mutations in the genes, which is why they are also referred to as genetic mutation disorders. These disorders may range from simple mutation in one particular gene to chromosomal disorders where whole segments of chromosomes may be affected.

Genes play a role in almost all diseases but their importance may vary. Genetic diseases in humans may be classified into the following types:

• Single Gene Disorders: Also known as Mendelian disorders, these diseases are caused by a defect in one particular gene. While their occurrence may be rare, there are a multitude of such disorders making their combined impact extremely significant. Single gene disorders are classified by the manner in which they are passed down in a family. Not all people affected by these genetic disorders may develop the disease associated with the defect. Some may just end up as carriers. Examples of single gene disorders include cystic fibrosis, phenylketonuria, ADA deficiency, sickle cell anemia, hemophilia A,  Huntington’s disease and vitamin D-resistant rickets.

• Chromosomal Disorders: These disorders are characterized by either a lack or an excess of genes in either the entire chromosome or a segment of chromosome. The root of these disorders may lie in the process of cell division when the chromosomes are not divided evenly, resulting in some cells receiving partial chromosomes and some cells receiving extra chromosomes. Examples of chromosomal disorders include Down’s syndrome, Klinefelter syndrome and Turner syndrome.

• Multifactorial Disorders: These disorders involve defects in multiple genes with environmental factors playing a big role. Examples of multifactorial disorders include cancer, Alzheimer’s disease, coronary heart disease, hypertension and stroke.

• Mitochondrial DNA-linked Disorders: Mitochondria are small structures found within cells that are responsible for the production of energy. They have their own distinct DNA. Disorders in mitochondrial DNA caused by mutations are mostly passed down by the mother since mitochondria comes from the female egg. These disorders may appear at any time and include blindness, developmental delay, hearing loss, gastrointestinal problems, cardiac rhythm abnormalities, metabolic disturbances and short stature.
  

The exact causes of genetic mutations that result in these genetic abnormalities is not yet understood completely. They may occur randomly, but the finger of suspicion points towards certain environmental factor. The prime causes of these genetic mutations include exposure to ultraviolet radiation, exposure to toxic chemicals such as those found in cigarette smoke and tobacco products and exposure to electromagnetic radiation such as x-rays. Age too, may play a role.

Once the mutation has occurred, the defective gene is passed down from the parent to the child. Nowadays, several tests are available that check for genetic abnormalities. One such test that is used widely is the test for Down’s syndrome which can be performed during the pregnancy itself.  If you suspect that there is a history of some particular illness in your family, then it makes sense for you to get tested for it. Remember, having a defective gene means you are predisposed to a particular disease. It does not mean that you will develop the disease.

Reference:

1. http://www.umm.edu/ency/article/002048.htm

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