theories of dominance

Toby Bradshaw toby at stein.u.washington.edu
Tue Mar 23 16:11:23 EST 1993


In article <1onpe4INNi7r at shelley.u.washington.edu> bishopj at botany.washington.edu writes:

[snip]

>	Foley's other objection seems to be that there are loci which exhibit  
>overdominance (the heterozygote has higher fitness, or metabolic flux than  
>either homozygote) or incomplete dominance (one homozygote has higher fitness  
>than the heterozygote or other homozygote),  or where different genotypes  
>perform differently in different environments ("conditional fitness").  With  
>regards to types of dominance,  overdominance is not commonly found.  

But see Stuber et al. (1992) Identification of factors contributing
to heterosis in a hybrid from two elite maize inbred lines using 
molecular markers. Genetics 132:823-839.  Granted, at coarse resolution
it is impossible to tell true overdominance from dominance of closely
linked loci in repulsion.  Also granted that starting with maize inbreds
stacks the deck.  However, of 14 QTLs for grain yield (about as close
to a fitness estimate as you're likely to get in a crop), 13 showed
apparent overdominance, measured in six environments in four states.

I would suggest that overdominance has not been commonly found in
the past because the tools to find it were lacking.  In fact, 
marker coverage of genomes has been so poor (with obvious
exceptions for some genetic model organisms) that it is dangerous
to conclude that any explanations for most genetic phenomena are
uncommon.

>Furthermore, among mutations arising in a population recessivity is clearly the  
>rule.

I doubt this is true.  For visible mutations, maybe.  If it were 
_generally_ true, narrow sense heritability estimates for quantitative
traits would always be low, and that's not the case.


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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