Multifactorial inheritance

Introduction

In epidemiological terminology, multifactorial inheritance, alternately known as polygenic inheritance describes a pattern of predisposition for a disease process which is the result of multiple genetic alterations (mutations) , multiple environmental factors, or a combination of one or more of these two.

Examples of disease processes generally considered to be results of multifactorial etiology:

• Diabetes
• Cancer
• Cleft palate
• Club foot

Multifactorially inherited diseases are said to constitute the majority of all genetic disorders affecting humans which will result in hospitalization or special care of some kind .

Heritable disease and multifactorial inheritance

The carrier gene of a disease which can be passed on to offspring is usually recessive; rarely dominant. The phenotypic expression of the disease or syndrome may even be the result of one or more genes being expressed together. When more than one gene is involved in the prescence of environmental triggers, we say that the disease is the result of multifactorial inheritance.

The more genes involved in the cross, the more the distribution of the genotypes will resemble a normal, or Gaussian distribution.

A cursory look at some examples

Examples of such diseases are not new to medicine. The above examples are well-known examples of diseases having both genetic and environmental components. Other examples involve atopic diseases such as eczema or dermatitis, spina bifida (open spine) and anencephaly (open skull) are other examples.

While schizophrenia is widely believed to be multifactorially genetic by biopsychiatrists, no characteristic genetic markers have been determined with any certainty.

Is it multifactorially heritable?

It is difficult to ascertain if any particular disease is multifactorially genetic. If a pedigree chart is taken of the patient's family and relations, the pattern is usually taken is if brothers and sisters of the patient have a multifactorially inherited disease, then there is a strong chance that the patient will also be a genetic carrier. This likelihood is reduced if cousins and more distant relatives have the disease. It must be stated that while multifactorially-inherited disease tends to run in families, inheritance will not follow the same pattern as a simple monohybrid or dihybrid cross.

If a genetic cause is suspected and little else is known about the illness, then it remains to be seen exactly how many genes are involved in the phenotypic expression of the disease. Once that is determined, the question must be answered: if two people have the required genes, why some people still don't express the disease. Generally, what makes the two individuals different are likely to be environmental factors. Overall, such investigations are difficult and arduous to carry out, sometimes taking several years to reach any conclusions. It is not usually the first avenue of investigation one would choose to find etiology.

More often than not, investigators will hypothesise that a disease is multifactorially heritable, along with a cluster of other hypotheses when it is not known what causes the disease.