MULTIFACTORIAL INHERITANCE
This form of inheritance is responsible for many normal phenotypic differences among individuals, congenital anomalies and common diseases of childhood. The multiple factors involved are genes and the environment. It depends on the interaction of a familial nature and the environment. For example the child of tall parents is more likely to be tall, than is a child of short parents, but poor nutrition (environment) may compromise or reduce the growth of either child. Quantitative traits are those that can be measured, like for example height, weight, IQ and serum cholesterol. Qualitative traits are ones like skin colour and both these types may have a multifactorial pattern of inheritance.
Most phenotypic variations seen among normal individuals is due to multifactorial traits. For example height is one of them. It shows in a population, a range from tallest to shortest the extreme values, having between them a range, where most of the population crowds towards a mean value. Each parent contributes half the genes and normally a child's height tends to be the average of both parents, the mid parent value.
In a multifactorial disease, the sum of the genetic and the environmental disease producing factors constitutes liability, which follows a normal bell curve type of distribution. If this liability exceeds the threshold for that population, the disease manifests. If it does not the disease will not occur. A few examples of multifactorial disorders are isolated anomalies like cleft lip with or without palate, congenital heart disease and anencephaly. Common diseases in adulthood are diabetes mellitus, arteriosclerotic heart disease and schizophrenia.
Clinical Characteristics