The simplest genetic characters are those whose presence or absence depends on the genotype at a single locus. That is not to say that the character itself is programmed by only one pair of genes: expression of any character is likely to require a large number of genes and environmental factors. however, sometimes a particular genotype at one locus is both necessary and sufficient for the character to be expressed, given the normal genetic and environmental background. Such characters are called Mendelian. Mendelian characters can be recognized by the characteristic pedigree patterns they give. Over 6000 Mendelian characters are known in humans. Information on any such character, whether pathological or nonpathological, can be found at the OMIM database.
Most organism characters are governed by genes at more than one locus. The further away a character is from the primary gene action, the less likely is it to show a simple Mendelian pedigree pattern. DNA sequence variants are virtually always cleanly Mendelian - which is their major attraction as genetic markers. Protein variants (electrophoretic mobility or enzyme activity) are usually Mendelian but can depend on more than one locus because of post-translational modification. The failure or malfunction of a developmental pathway that results in a birth defect is likely to involve a complex balance of factors. Thus common birth defects (cleft palate, spina bifida, congenital heart disease (in humans) ect.) are rarely Mendelian. Behavioral traits like IQ performance or schizophrenia are still less likely to be Mendelian- but they may still be genetically determined to a greater or lesser extent.