> Is that possible a disease locus has several alleles but only one of
> these alleles causing disease? Any example or refernece?
Very many genes will have harmless polymorphisms, perhaps not even in
coding areas or affecting amino acid sequence, and each of these can be
viewed as an allele. If there are also one or two harmful mutations then
the gene has some alleles causing disease and others not. I would have
thought that practically every disease locus fitted these criteria. Of
course, if the variation has to be at exactly the same site in terms of
coding sequence then the there would be fewer examples. However this is
probably unnecessarily restrictive because when multiple different
mutations cause disease they generally occur at different places within
the gene. From the point of view of the locus these are all alleles. If
you go right down to the sequence I suppose you could argue that they
represented different polymorphic systems.
For practical purposes, a disease locus which can harbour pathogenic
mutations will typically also have several harmless polymorphic systems
within it or close to it, and these can be used as genetic markers for
some applications.
