Complex Traits Qualitative traits. Discrete phenotypes with direct Mendelian relationship to genotype. e.g. black or white, tall or short, sick or healthy Quantitative traits. Phenotypes that require numerical description due to continuous variation. Variation often represented in a population as a normal distribution (bell shaped curve). Quantitative traits may be due to influence of multiple genes (polygenic) or Environmental influences or More often, a combination of the two (Multifactorial traits)

A Quantitative Trait

Heritability (broad sense) is the proportion of total variation that is due to genetic variation. h2=VG/VG+E In practice, heritability can be defined by response to selection. h2=R/S R= selection response S= selecton differential

Threshold Traits The measurement is qualitative but the underlying inheritance is polygenic or environmentally influenced Examples Many cancers Diabetes Cardiovascular diseases

Chumakov et al. PNAS 99: 13675-13680. 2002 Four genes for susceptibility to schizophrenia

Genome scan: Concordant segregation of QTL with mapped markers

Other complex traits? Intelligence Athletic ability Anti-social behaviors