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Introduction to Pedigrees

Author(s): Lisa M. Meffert, PhD

Marfan’s Syndrome: An Example

Marfan's Syndrome is an example of an inherited disorder that can be followed in a pedigree. People expressing Marfan's Syndrome have hyper-elastic joints and elongated bones (among other features), as depicted in the photo. Note that in this pedigree, the expression of the trait has no sex-specific pattern. Both males and females express Marfan's syndrome (depicted by black circles or squares) at approximately equal frequency. Thus, we can assume the gene that causes Marfan's syndrome is autosomal.

Next, note that the trait is expressed in every generation. Therefore, we assume that an allele for Marfan's syndrome is dominant to a normal allele. An allele is an alternate form of a gene. In the this case, for example, one form (or allele) of the gene is expressed as Marfan's Syndrome and the other allele produces normal joints and bone length.