How does any of this information apply to Muscular Dystrophy (MD)?
Your book describes another hereditary disease-- sickle cell anemia (see page 158). With sickle cell anemia, the allele that causes the disease is a recessive allele. With many (but not all) genetic diseases, the allele that causes the disease is a recessive allele.
Most types of MD are also due to a recessive allele. However, MD is a little trickier to understand than that. The most severe form of MD is called Duchenne's MD. That type is not a case of simple dominant or recessive inheritance because in males only one MD allele is needed to have the disease.
Since most genetic diseases are embedded in recessive alleles, the diseases tend to only occur in people who are homozygous recessive. A person who is heterozygous for that gene has a normal phenotype. For example, I am heterozygous for Tay-Sachs disease (an extremely severe disease). But I do not have Tay-Sachs. You might think, then, that I am just fine. You're right. But I still have that bad, disease allele in my genetic code. And when I have children, I will give one of my two alleles to the next generation. Will I give my disease allele or my normal one? I can't predict.
People who are heterozygous for a disease carried by a recessive allele are called carriers. They do not have the disease, but they carry the allele and maintain the diseased allele within the human population. I am a carrier for Tay-Sachs. If two carriers have children together, there is a chance that their children will get the disease-allele from each parent and then have the disease. You will learn how to figure out the chances of this happening next week.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: September 05, 2007