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Introduction to Pedigrees

Author(s): Lisa M. Meffert, PhD

Marfan’s Syndrome: Genotype the Normal Individuals

The first step for working out the pedigree for an autosomal dominant trait is to genotype the normal individuals. Here, we are using an arbitrary coding system of "m" for the recessive normal allele and "M" for the dominant allele that confers Marfan's syndrome. As noted in the previous slide, an allele is an alternate form of a gene.

All normal individuals (purple circles and squares) must be homozygous for the "m" allele. Thus, we write the appropriate genotype below each normal individual in the pedigree. In general, genotype refers to the genetic makeup of an individual. Geneticists also use the term to refer to the process of identifying the alleles present in an individual. Phenotype refers to the observable characteristics of an organism.