Re: Referring to medical molecular genetics, what determines the severity of a

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.