Making Use of Polymorphic DNA

Multiple alleles and noncoding polymorphic DNA are of considerable importance in gene mapping—identifying the relative positions of genetic loci on chromosomes. Gene maps are constructed by using the frequency of crossing-over to estimate the distance between a pair of loci. To obtain a good estimate, one must analyze a large number of offspring from a single cross. In laboratory organisms such as the fruit fly Drosophila, programmed crosses can be carried out so it is possible to use gene loci to construct a reliable genetic map. In humans, this is not the case. For this reason, the more highly variable noncoding regions are of considerable importance in human genetic mapping. see also Blood Type; Immune System Genetics; Mapping; Polymorphisms; Transplantation.

Andrea Bernasconi


Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland, 2002.

Strachan, Tom, and Andrew P. Read. Human Molecular Genetics. New York: Bios Scientific Publishers, 1996.

Muscular Dystrophy

Muscular dystrophies (MDs) are a group of disorders that share three characteristics: They are inherited, they cause progressive weakness and muscle wasting, and the primary defect is localized to skeletal muscle, sparing the nerves. Although selected limb muscles develop some degree of weakness in all dystrophies, to distinguish among the different types, it is critical to know the mode of inheritance, the age of onset, and whether muscles other than limb muscles are also affected. For example, some dystrophies additionally affect eye and lip closure; another type affects eye movement ability, as well as swallowing and speech.

More than thirty types of MDs are now recognized. Three of the more prevalent forms—Duchenne, myotonic, and limb-girdle dystrophies—will be discussed from the standpoint of the presenting symptoms, age of onset, inheritance pattern, causative genes, and the availability of prenatal and presymptomatic molecular testing.

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