Mendelian inheritance

Disorders caused by a defect in a single gene follow the
patterns of inheritance described by Mendel and the term
mendelian inheritance has been used to denote unifactorial
inheritance since 1901. Individual disorders of this type are
often rare, but are important because they are numerous. By
2001, over 9000 established gene or phenotype loci were listed
in OMIM. Online Mendelian Inheritance in Man (TM).
McKusick-Nathans Institute for Genetic Medicine, Johns
Hopkins University (Baltimore, MD) and National Center for
Biotechnology Information, National Library of Medicine
(Bethesda, MD), 2000. World Wide Web URL:
http://www.ncbi.nlm.nih.gov/omim. Risks within an affected
family are often high and are calculated by knowing the mode
of inheritance and the structure of the family pedigree.