The origins of disease can take many forms.

We can think about environmental causes, which means basically anything outside of genetics. These could be things like pollution, a virus/bacteria/fungus infection, mal- or under-nourishment, injury… the list goes on and on.

There are also genetic causes. Understanding which malfunctioning genes lead to diseases is an ongoing effort in genetics. Genetic causes can be broken down into two broad groups.

First, there are polygenic diseases. Polygenic literally means “many genes”, and are sometimes called multifactoral or complex diseases. The cause of these diseases is difficult to trace because there are many compounding factors. A combination of certain alleles, for example, or how a persons responds to their environment, which may lead to disease in some cases but not in others. These diseases are often difficult to trace in a family because they have have a low inheritance. For example, the chance that your sibling has the same polygenic disease as you is much lower than expected if it was cause by a single gene.

That takes us to…

Monogenic diseases, the so-called Mendelian disorders. These diseases are more straight forward because the pattern of inheritance is very clear. When we talk about inheritance we refer to Gergor Mendel, the father of genetics. He used specific traits in peas (e.g. colour and texture) to uncover patterns of inheritance. The genius in his methodology is that he followed traits which were differentiated by single gene. Different alleles in a single gene made a pea wrinkly or smooth, and different alleles in another gene made peas yellow or green. In the same way, we can follow diseases in human and if they are inherited in a Mendelian fashion, it is likely there is mutation in a single gene to blame.

There are thousands of genetic diseases caused by mutations in a single gene, but discovering which gene is the culprit is still a daunting task. In upcoming articles, I’ll explore some studies which are using novel approaches to dissect the causes of Mendelian disorders.

Pic Credit: Michael James Maxwell via Flickr