Medical Health Encyclopedia

Genetics

X-linked dominant:

Only a few, very rare, disorders are classified as X-linked dominant. One of these is hypophosphatemic rickets (also called vitamin D -resistant rickets). In this case a protein in the kidneys is defective. This protein normally transports phosphate from the urinary filtrate back into the blood. Since the amount of phosphate in the blood is much lower than normal, the bones are chronically stimulated to release calcium and phosphate by hormones such as parathormone. This results in fragile and abnormally structured bones.

CHROMOSOMAL DISORDERS

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In chromosomal disorders, the defect is due not to a single gene, but to an excess or deficiency of the genes contained in a whole chromosome or chromosome segment.

Down syndrome is the most common chromosomal disorder (1 out of 800). Affected individuals have an extra copy of chromosome 21. This unbalanced set of genes results in mild to moderate mental retardation and numerous physical changes. Other common examples are Klinefelter syndrome (1 out of 1,000 males) and Turner syndrome (1 out of 5,000 females).

MULTIFACTORIAL DISORDERS

Many of the most common diseases which affect humans undoubtedly involve interactions of numerous genes, including coronary heart disease, hypertension, stroke, and various kinds of cancer. These are currently active areas of research.

MITOCHONDRIAL DNA-LINKED DISORDERS

Mitochondria are small organelles present in most of the body's cells which function in the conversion of certain chemicals in our food, in the presence of oxygen, to the common currency of energy inside cells (ATP).

Mitochondria contain their own private DNA. In recent years, more than 20 hereditary disorders have been shown to result from mutations in mitochondrial DNA. Because mitochondria come only from the egg, they are inherited exclusively from the mother.