I really haven't anything to contribute regarding heterosis, but
I can address gene duplication in the beta-globin gene cluster.
Unequal crossing-over resulting in an increase or decrease of
beta-type globin genes actually does occur, and the most common
outcome, for some reason, is a deletion. One example is the so
-called Hb Lepore (hemoglobin Lepore), which is a fusion between
the minor adult beta-type globin, known as delta, and the major
adult gene, beta. There are several classes of Lepore genes, as
distinguished by the point of crossover. The most common class
has been found in a variety of ethnic groups, some quite isolated,
such that this unequal crossover has likely to have happened
independently multiple times in evolution. Interestingly, the
complementary result, a triplication (Hb Miyada) is extremely rare.
Another crossover, a fetal beta globin A-gamma fusion with the beta
gene, called Hb Kenya has been observed in a family with the sickle
gene assorting independently, but the complementary result, a
fusion of beta with A-gamma plus an extra delta and an extra beta
gene, has never been observed despite huge population screening
efforts in many ethnic groups for aberrant globin genes.
Anyway, there may be some selection against increasing the numbers
of adult beta-type globin genes, possibly having to do with the
likely resulting imbalance of beta versus alpha chains that would
probably cause a thalassemia, or hereditary anemia.
I have published a paper on various beta-type globin gene fusions.
In case you might find it of interest, here is the reference:
Metzenberg, A.B.; Wurzer, G.; Huisman, T.H.J.; Smithies, O. (1991) Homology requ
irements for unequal crossovers in humans. Genetics 128:143-161.
I look forward to the outcome of this interesting discussion on heterosis.
Univ. of Calif. San Francisco
aida at cgl.ucsf.edu