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Principles of complex trait genetics

Juan C Celedón, MD, DrPH
Gary M Hunninghake, MD, MPH
Section Editor
Benjamin A Raby, MD, MPH
Deputy Editor
Jennifer S Tirnauer, MD


Most human genetic traits can be classified as either monogenic or complex. Monogenic traits are strongly influenced by variation within a single gene and are recognized by their classic patterns of inheritance within families. While monogenic traits formed the basis for "classic" genetics, it has become clear that conditions whose inheritance strictly conforms to Mendelian principles are relatively rare. (See "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)" and "Genetics: Glossary of terms".)

Complex traits are believed to result from variation within multiple genes and their interaction with behavioral and environmental factors. Complex traits do not follow readily predictable patterns of inheritance.

This distinction between monogenic and complex traits, while useful, can be overly simplistic. Traits that appear to be monogenic can be influenced by variation in multiple genes ("modifier genes") [1]; complex traits can be predominantly influenced by variation in a single gene [2].

This topic will review the challenges related to the identification of complex trait susceptibility genes, the factors that contribute to phenotypic complexity, and current understanding of the genetic architecture of complex genetic traits. Genetic traits with monogenic inheritance, either with a Mendelian or a non-Mendelian pattern, are discussed separately. (See "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)" and "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)", section on 'Causes of non-Mendelian inheritance'.)


Most monogenic diseases are caused by mutations that reduce the function or stability of a single protein by altering its three-dimensional structure. These mutations include point mutations (eg, changes in single nucleotides that alter the amino acid sequence), insertions, or deletions in the DNA sequence that encodes the protein; or changes in the non-coding DNA that interfere with gene splicing. (See "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)" and "Basic principles of genetic disease", section on 'DNA sequence variation' and "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)", section on 'Causes of non-Mendelian inheritance'.)

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Literature review current through: Nov 2017. | This topic last updated: Apr 10, 2017.
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