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theories of dominance

Toby Bradshaw toby at stein.u.washington.edu
Thu Mar 25 01:25:16 EST 1993

In article <BRIANF.93Mar24160949 at dna.uvm.edu> brianf at dna.uvm.edu (Brain Foley) writes:
>I was mislead earlier, in part because I was not familiar with the way
>in which you use the terms "dominance" and "recessive".  In my
>training, "dominant" means only that one copy of the gene will produce
>the phenotype being looked at (i.e. yellow/green heterozygote pea pods
>will be green; green is dominant over yellow), it says nothing about
>whether the dominant or recessive allele provides better "fitness".
>It is in this regard that I feel that new knowledge has changed the
>way we should look at genetics.  Using green and yellow pods as an
>example, we might now find out that yellow peas lack the gene for
>green pigments, or we might find out that they produce a yellow
>pigment.  Using visual analysis, green is the dominant gene, but using
>an assay that detects only yellow pigments, yellow is the dominant
>gene.  This is where I have a problem: dominance or recesiveness in
>these cases is in the eye of the beholder.

Not so.  It is as you described in your opening paragraph; the
phenotype of the heterozygote determines dominance.  In the
example immediately above, yellow is not dominant to green no
matter how you look at it.  In the yellow/green heterozygote there
is yellow pigment, and in the yellow/yellow homozygote there is
yellow pigment.  So far, so good.  The difficulty is that there
is yellow pigment in the green/green homozygote, too

>Many students in molecular genetics are confused about what "dominant"
>and "recesive" mean.  Most often, recessive means that some gene
>product is lacking, so if a cell-cell hybrid is made, the cell with
>the dominant phenotype provides the lacking enzyme.  But this is not
>always the case.

OK.  It's late and my memory isn't the best.  I'm having a hard
time coming up with an exception to the above.  Care to help me

>I guess I was sticking my neck out to post to molbio.evolution when I
>am not a student of population biology, but I think that is important
>for all biologists to use the same definitions for the same terms.  I
>think that "dominant" can be a fuzzy term in some cases now.

You'll have to provide an example to convince me that dominance
is a fuzzy term.  I think you understand the definition, but you
think it doesn't apply all the time.  Your example above is not
an example of the failure of the definition, IMHO.

>*  Brian Foley               *     If we knew what we were doing   *
>*  Molecular Genetics Dept.  *     it wouldn't be called research  *
>*  University of Vermont     *                                     *


Around here, we like to think we know what we're doing, even though
we might not know what the answer will be :)

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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