It is also important to note that diseases and traits can be familial without following Mendelian principles. Empiric examples include many congenital malformations such as cleft lip and palate, club foot, pyloric stenosis, spina bifida and related neural tube abnormalities as well as many of the common disorders affecting both children and adults, such as diabetes, schizophrenia, heart disease, and high blood pressure. Several possible mechanisms for non-Mendelian inheritance can be briefly considered. There can be multiple alleles at more than one locus whose effects are additive. Height, weight, skin color are examples.
Some traits have both genetic and environmental components that contribute to their expression.
Other exceptions to Mendelian inheritance such as imprinting and trinucleotide repeats are the subject of later lectures, reading and discussion.