I think that as Steve Karl said, DNA duplication occurs quite often in
genomes. Allozyme loci of Drosophila once thought to be encoded by single
genes are part of small duplicate gene families, e.g, amylase, esterase,
alcohol dehydrogenase. These gene families are in various phases of
diversification, amylase and esterase are examples of relatively recent
duplications while alcohol dehydrogenase in the melanogaster group
represents an ancient duplication. I think that one benefit of
early duplication events is to provide the organism with genetic
redundancy. Deleterious mutations in one gene is compensated by the
second copy.
Not all genes will duplicate in the genome though. Consider the case
of Down's syndrome patients who have an extra copy of chromosome 21. We
know that at least one gene on chromosome 21 causes these patients to
be sterile when it is present in three copies. Thus, I would predict that
not all genes in the genome may be duplicated to generate proteins with new
functions.
An interesting question, are there any common genetic properties
in genes that can or cannot be duplicated in the genome?
Stephen W. Schaeffer
Institute of Molecular Evolutionary Genetics
The Pennsylvania State University
