Almost all human disease has a genetic basis, even those associated with lifestyle choices such as obesity, heart disease and alcoholism. So why do genetic diseases occur in some people, but not in others? The answer lies in the human genome.
Every human body has several trillion cells, and each one of those cells contains a complete copy of the human genome. A genome contains all the genes that must work together as a master blueprint for the growth and development of the human body. Considering all the possible combinations, it's no wonder that genetic disorders exist.
Geneticists organize genetic disorders into four categories: single gene, chromosomal, multifactorial and mitochondrial, the rarest type.
Single Gene Disorders
Single gene disorders, sometimes referred to as Mendelian disorders, have a specific cause: the mutation of one particular gene. There are more than 6,000 single gene disorders. Although specific diseases caused by a single gene disorders are rare, when combined they occur in about one in 200 births. Sickle cell disease, cystic fibrosis and Huntington's disease are among the more common types of single gene disorders.
Chromosomal Disorders
Humans typically carry 46 chromosomes in each cell which in turn hold about 25,000 genes. Half of those chromosomes are inherited from the mother's egg, and the other 23 are inherited from the father's sperm.
Chromosomal disorders develop from an abnormality in the sperm or egg. The abnormality may be a missing, duplicate, broken or inverted chromosome which occurs when a portion of one chromosome is transferred to another. In some rare cases, these "translocated" chromosomes are passed from a parent to a child.
Down syndrome, also called Trisomy 21, is one of the most common chromosomal disorders. It occurs when a person is born with three copies of chromosome 21. Other examples of chromosomal disorders are Fragile X Syndrome, the most common form of mental retardation, and Turner Syndrome, a condition that occurs only in females and which may appear as deformities at birth and the absence of menses and vaginal dryness, among other symptoms, in the adult women.
Multifactoral Genetic Disorders
Sometimes called complex or polygenic, multifactoral genetic disorders result from the interaction of multiple genes with lifestyle or environmental factors.
Examples of multifactoral genetic disorders are heart disease, diabetes, asthma, cancer and mental illness. Researchers have data showing that these types of diseases appear more frequently in some families than in others, which is a clue that inheritance plays some role. The specific triggers that cause a person to develop a multifactoral genetic disorder, however, is not yet clear. That makes it difficult to predict the risk a person has of inheriting or passing along the gene for the disorder.
Mitochondrial Genetic Disorders
Mitochondrial disorders are caused by mutations in the DNA of mitochondria. Mitochondria are organelles, or diminutive organs within a cell. The roles of mitochondria are to process oxygen and convert food molecules into energy that other cells need to function properly.
Mitochondrial disorders are the rarest forms of genetic diseases. An estimated one in 4,000 children in the United States is born with malfunctioning mitochondrial DNA. The mutation can lead to diabetes, deafness, neurological diseases of the brain and central nervous system and diseases that affect the heart and muscles.
