Genetic Diseases and Inheritance

Human bodies are made up of trillions of cells. Each of these cells has a nucleus that accommodates chromosomes, which are made up of DNA. Segments of DNA make up genes, and these determine physical traits such as height or eye color.

Alterations in DNA cause gene mutations. Mutations can lead to diseases, and these mutations are passed from parent to child just like physical traits. Mutations can be present at birth or develop over time.

How Genetic Diseases Are Inherited

Each parent passes on one copy of each gene to their children. If one or both of those copies is mutated, the child is at risk for a genetic disorder. Inherited conditions and diseases generally fall into four categories:

• Autosomal dominant diseases: There are ordinarily two working copies of any given gene in a human being. In the case of an autosomal dominant disease, one copy of the gene is altered by mutation and causes the disorder-even if the other gene copy is healthy. With this type of alteration, a parent who displays the trait has a 50 percent chance of passing it onto children. If the child does not inherit the trait, it is not likely that they will pass it on to their children. Examples of autosomal dominant diseases include some forms of dwarfism, neurofibromatosis and Huntington disease.
• Autosomal recessive diseases: In order for a recessive disorder to be active, both parents must be carriers of the mutation, and both must pass the mutation on to the child. If the child inherits one normal gene and one defective gene, he or she will be a carrier of the disease (and will be able to pass it on to future generations), but will not be affected by it. If both parents carry a copy of a recessive disorder gene, there is a 25 percent chance of passing it on to their children. Examples of autosomal recessive diseases include sickle cell anemia, cystic fibrosis, Tay-Sachs disease and phenylketonuria.
• Polygenic diseases: These conditions occur when the disease is brought on by a combination of genetic and environmental factors. These disorders include heart disease, stroke, hypertension, cancer and mental illness.
• Sex-linked diseases are connected to genes located on a sex chromosome (X or Y). X-linked recessive disorders are caused by genes on the X chromosome. Since males only inherit one X chromosome, they will usually display any mutations inherited on the X chromosome from their mother.

Females, on the other hand, inherit two X chromosomes. If one parent's copy is defective, the other copy can cancel it out, in which case she will be a carrier of the disease, but will not be affected by it. Not surprisingly, the majority of individuals who display X-linked diseases are male. Examples of sex-linked disorders include hemophilia, muscular dystrophy and color blindness.

Gene Therapy

Many patients with genetic disorders turn to gene therapy to prevent diseases caused by family DNA traits. Gene therapy involves the use of nucleic acids as therapeutic molecules. For example, gene therapy can be used to correct defects in diseases such as cystic fibrosis. Gene therapy may also include gene transfers, which can encourage an immune response, mediate cell killing, activate a drug or create a molecular decoy to stop a virus from reproducing.

In addition, genetic testing and counseling is available to would-be parents, so that they can assess the genetic risks involved with conceiving a child. Genetic testing looks at the couple's DNA family history to determine if either of them are carriers for certain diseases, and the level of likelihood that the disorders will be passed on to their children.

Limitations of Gene Therapy

Since gene therapy is still a relatively new course of treatment, it is tightly regulated in most countries. Generally, gene therapy is restricted to life-threatening disorders that cannot be cured by any other treatment options.

Resources

Best, R. (n.d.). Inherited disorders: Chromosome abnormalities and disease. Retrieved September 16, 2008, from the Science Encyclopedia Web site: http://science.jrank.org/pages/3596/Inherited-Disorders.html.

Explore DNA Staff. (n.d.). Inherited disorders and DNA. Retrieved September 16, 2008, from the Explore DNA Web site: http://www.exploredna.co.uk/inherited-disorders-dna.html.

Genetic Futures News Staff. (n.d.). Inherited genetic disorders. Retrieved September 16, 2008, from the Genetic Futures Web site: http://www.geneticfutures.com/thegift/info/sheet2.asp.

Matthews, A. (2008). Inherited disorders and birth defects. Retrieved September 16, 2008, from the Net Wellness Web site: http://www.netwellness.org/healthtopics/idbd/3.cfm.

Mayo Clinic Staff. (2007). Slide show: How genetic disorders are inherited. Retrieved September 16, 2008, from the Mayo Clinic Web site: http://www.mayoclinic.com/health/genetic-disorders/DS00549.

Pro Choice Forum Staff. (n.d.). Ethical issues: Gene therapy. Retrieved September 16, 2008, from the Pro Choice Forum Web site: http://www.prochoiceforum.org.uk/ri6.asp.