I think the most interesting example, though I may be biased, of
genes which function and live together is the vertebrate Major
Histocompatibility Complex (MHC). Here we see functionally and
structurally similar genes (called class I and II) which bind
antigenic peptides and present them to T cells. These genes
are highly polymorphic (over 100 alleles for some genes in
some wild populations). This polymorphism appears to be generated
by recombination of functional genes and pseudogenes. Linkage may be
required for the generation of polymorphism.
Within the MHC, we also find the class III genes, a group of misc.
structurally unrelated genes for which linkage to the MHC was thought
to be unimportant and possibly accidental. The recent discovery of
peptide transporter genes (TAP) and proteosome subunit genes (LMP)
within the MHC is bringing into question the importance of MHC linkage
for the class III genes. Both TAP and LMP gene products appear to be
involved in the antigen processing and presentation pathway. This
pathway breaks down proteins into peptides which are then transported
into the RER and bound to MHC class I molecules which then present the
peptides on the cell surface.
So here we have structurally and fucntionally related genes which may
require linkage for the generation of polymorphism but we also find
structurally unrelated genes involved in a pathway. In addition to
TAP and LMP genes, there are other immunologically important genes
that are also in the MHC and may be linked for a reason. There are
also some MHC-linked genes that appear to be functionally completely
unrelated. What's it all mean???
U. of North Carolina at Wilmington
Dept. Biological Sciences