There are about 35,000 genes in the nucleus of human body cells. These genes are packaged into 23 pairs of chromosomes, numbered 1 through 22, with the 23rd pair being the sex chromosomes (X and Y). Each of the genes has a specific role in the body's function. Inherited or sporadic changes or mutations in these genes can cause a host of genetic conditions, affecting a wide variety of body systems. In addition, alterations in the number of chromosomes in each cell can cause various abnormalities, both physical and mental.
Autosomal Dominant Conditions
Our genes come in pairs, one inherited from our mother and one from our father. For some genetic diseases, a change in one gene of the pair is enough to cause symptoms of the condition. These types of conditions are called autosomal dominant. Autosomal dominant conditions can be inherited, and a parent with an autosomal dominant condition has a 50 percent chance of passing on that condition to each child. Autosomal dominant conditions can also occur sporadically, when a mutation occurs in a gene in the egg or sperm cell. In that case, the parents are both healthy, but their child develops a genetic condition seemingly out of the blue.
One common example of an autosomal dominant genetic condition is achondroplasia. Achondroplasia is characterized by abnormal bone growth and is the most common form of dwarfism. The gene for achondroplasia, called fibroblast growth factor receptor 3 (or FGFR3) is located on chromosome No. 4. Over 99 percent of individuals with achondroplasia have one of two mutations in FGFR3, and, according to a study published in a 2000 issue of "Endocrine Review," in more than 90 percent of cases of achondroplasia the mutation occurs sporadically.
Autosomal Recessive Conditions
Autosomal recessive conditions are ones in which a person needs to inherit two gene changes, one from each parent, to develop a disease. One of the most common autosomal recessive diseases is a category of diseases called hemoglobinopathies. These conditions affect the production or structure of hemoglobin, the molecule that transports oxygen in our blood.
A common example of a hemoglobinopathy is sickle cell anemia, caused by a mutation in the hemoglobin subunit beta, located on chromosome No. 11. Approximately 7 percent of the world's population are carriers of sickle cell anemia and about 1 in 5,000 people in the United States have it. The disease causes the red blood cells to take on a sickle shape, leading to pain in various body parts, organ damage and infections.
Chromosome Abnormalities
Some disorders are caused not by a change in one gene, but by an entire extra or missing chromosome. Only a handful of chromosome abnormalities result in a live birth, as most are too severe to allow survival. Chromosome abnormalities generally occur sporadically, and the risk for these conditions in a baby increases with the mother's age.
The most common chromosome abnormality is Down syndrome. Caused by an extra chromosome No. 21, Down syndrome is associated with mental retardation, characteristic facial features, heart defects and other health problems. People with Down syndrome have a wide range of abilities, with the most severe being unable to care for themselves and the highest functioning memorizing lines for television roles.
References
- "Endocrine Reviews"; The Molecular and Genetic Basis of Fibroblast Growth Factor Receptor 3 Disorders: the Achondroplasia Family of Skeletal Dysplasias, Muenke Craniosynostosis, and Crouzon Syndrome with Acanthosis Nigricans; Z. Vajo et al.; Feb. 2000
- Genetests: Sickle Cell Disease
