The Genetic Basis of Diseases

Genes are specific DNA sequences that are analogous to the blueprint for a human being. The human genome contains more than 22,000 genes--every gene codes for a specific protein and molecule that makes up and performs most of the body's functions. When a gene mutates, the blueprint changes--usually for the worse--and disease is the result. Five major types of genetic disorders are chromosomal, single-gene, mitochondrial, somatic mutation and polygenic.

Chromosomal Disorders

• Chromosomes are structures made up of bundled DNA. Humans have 23 paired chromosomes. Down syndrome is a common example of a chromosomal disorder where translocation (an abnormality in chromosome structure) has taken place on Chromosome 21.

Single-Gene Disorders

• Also referred to as monogenic or Mendelian disorders, single-gene disorders are caused by mutations that occur in the nucleotide sequence of a single gene. The mutated gene now produces a malformed protein that will not carry out its intended function. Examples of monogenic disorders include sickle-cell anemia and Huntington's disease.

Mitochondrial Disorders

• Rare as far as genetic disorders go, mitochondrial genetic disorders are caused by mutations in mitochondrial DNA. Examples of this type of disorder are Multiple Scelrosis-type disorders and neuropathy.

Somatic Mutations

• Somatic refers to the body--mutations occur in the DNA of any cells of the body but not in the germ cells (sperm and egg). Thus, they are not passed onto the following generation.

Polygenic Disorders

• Also called multifacorial, polygenic disorders occur due to a combination of mutations in multiple genes and environmental factors. A good example is breast cancer. Genes that influence a person's susceptibility to acquiring breast cancer occur on multiple chromosomes, and their influence is related to environmental factors such as exposure to toxins. Other examples include Alzheimer's disease, diabetes and heart disease.