Multifactorial inheritance

The concept of multifactorial inheritance implies that a disease
is caused by the interaction of several adverse genetic and
environmental factors. The liability of a population to a
particular disease follows a normal distribution curve, most
people showing only moderate susceptibility and remaining
unaffected. Only when a certain threshold of liability is
exceeded is the disorder manifest. Relatives of an affected
person will show a shift in liability, with a greater proportion of
them being beyond the threshold. Familial clustering of a
particular disorder may therefore occur. Genetic susceptibility
to common disorders is likely to be due to sequence variation
in a number of genes, each of which has a small effect, unlike
the pathogenic mutations seen in mendelian disorders. These
variations will also be seen in the general population and it is
only in combination with other genetic variations that disease
susceptibility becomes manifest.