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Toby Bradshaw toby at stein.u.washington.edu
Thu May 6 09:46:59 EST 1993

In article <9305031546.AA10476 at map.marc.usda.gov> keele at MAP.MARC.USDA.GOV (John Keele) writes:
>In a recent message Dr. Foley indicated that there are many genes that
>increase an individual's fitness only when present in the heterozygous
>state.  Would someone be so kind as to list papers or reviews that 
>document this?  Thanks in advance.  John Keele 

_Demonstrated_ single-locus overdominance is a rare bird.  It is
impossible to separate true overdominance from pseudo-overdominance
(loci with dominant inheritance tightly linked in repulsion) without
resorting to single-gene substitution or its biochemical equivalent
(as in the case of HbS in malaria country).  But, see a recent paper
from Stuber et al. (1992) Identification of genetic factors contributing
to heterosis in a hybrid from two elite maize inbred lines using
molecular markers. Genetics 132:823-839.  In studying QTLs for
grain yield (about as close to a fitness parameter as you're likely
to get from an agronomic study) virtually all QTLs showed apparent
overdominance.  Bear in mind that this is the result of crossing
inbreds derived from an outcrossing plant, and that maize breeding
takes advantage of known "heterotic groups" of inbreds, which, when
mated, produce superior F1s.  Are these "heterotic groups" the result
of pseudo- or true overdominance?  Nobody knows.

Toby Bradshaw                       |
Department of Biochemistry          |  Will make genetic linkage maps
and College of Forest Resources     |            for food.
University of Washington, Seattle   |
toby at u.washington.edu               |

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