Maternal effect in multiple sclerosis.

The genetics of complex diseases is a fascinating but complicated issue. Often, many genes might be involved in a single disorder, each exerting a small effect. To fully understand the pathogenic mechanisms, a comprehensive knowledge of the effect of the individual genes is needed. However, before that, one should try to ascertain how important genes are compared with environmental factors. Such an assessment can be made by examining the rate of concordance in identical versus dizygotic twins or the extent of clustering in families.