|MadSci Network: Genetics|
The varying severity in diseases such as multiple sclerosis (MS) is attributed to: "The cause of MS is not a single thing but a coming together of a certain genetic predisposition, certain environmental contacts, probably early in life, and perhaps certain other factors that we really only partially know about today. " Basically, diseases such as MS can be cause by a variety of genetic mishaps. That is mutations in several genes can lead to a similar phenotypic effect. For example, an individual may have a mutation in gene "A" and have a very severe case of a particular disease. Another person may have a mutation in another gene (gene "B") that can lead to similar symptoms of a disease but be less severe. In other words there are many diseases which can come about via different mutations or series of mutations. Also, one must take into account any environmental factors which may alleviate or contribute to a particular disease or disorder - such as nutrition, medical care etc... Thus, a single genotype can have several phenotypes, and a single phenotype can come about do to several genotypes - based on environmental factors influencing how genes are expressed. Genes that are influenced by the environment are called "multifactorial traits". References: http://www.healthtalk.c om/msen/toc/causes/01.html Hartl DL, Jones EW, (1998) Genetics: Principles and Analysis, Jones and Bartlett, Inc. Sudbury Massachusetts pp. 669-670 Admin addendum: You are also right in supposing that the level of expression or amount of residual function in a protein may play a part. Even when the disease is only caused by a single gene, the type of mutation in that gene (that is, the mutant ALLELE present), might affect the severity of the disease. Some alleles may have more residual activity than others. Whether the individual is heterozygous or homozygous may also play a part -- for some types of diseases, heterozygotes may be affected as well, though less severely than homozygotes. This is because the heterozygotes retain some wild type protein activity, while the homozygotes make all mutant protein. So there are many factors that can influence the severity of a disease, it all depends on the specific disease, what causes it, and how these causes are inherited/produced.
Try the links in the MadSci Library for more information on Genetics.