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Multifactorial diseases are not confined to any specific pattern of single gene inheritance and are likely to be associated with multiple genes effects together with the effects of environmental factors.
In fact, the terms ‘multifactorial’ and ‘polygenic’ are used as synonyms and these terms are commonly used to describe the architecture of disease causing genetic component. Although multifactorial diseases are often found gathered in families yet, they do not show any distinct pattern of inheritance. It is difficult to study and treat multifactorial diseases because specific factors associated with these diseases have not yet been identified. Some common multifactorial disorders include schizophrenia, diabetes, asthma, depression, high blood pressure, Alzheimer’s, obesity, epilepsy, heart diseases, Hypothyroidism, club foot and even dandruff.
The Multifactorial threshold model assumes that gene defects for multifactorial traits are usually distributed within populations. Firstly, different populations might have different thresholds. This is the case in which occurrences of a particular disease is different in males and females. The distribution of susceptibility is the same but threshold is different. Secondly, threshold may be same but the distributions of susceptibility may be different. It explains the underlying risks present in first degree relatives of affected individuals.