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Dominant Trait Functioning
Name: Robyn
Status: Other
Grade: Other
Location: CA
Country: United States
Date: February 2006
Question:
In studying Mendelian inheritance, I understand
dominance and recessiveness by figuring out gametes and using the
Punnett square. I was curious,though,what exactly happens at the
chromosome that gives the dominance in a heterozygous. Simply put,
how does the body know to phenotypically express the dominant
allele (beyond statistics)?
Replies:
Good question! It really depends on the trait. We really should change
our definition of dominance to the allele whose phenotype is "seen".
Remember that genes code for proteins. It's really about the way the
proteins coded for by each allele and the way they interact. Also remember
that you inherit a version from your mom and one from your dad. Usually,
both are made. If we are talking about sickle cell trait for instance, both
versions of the hemoglobin are made and your blood will have a combination
of the two. If we are talking about flower color in snapdragons (classic
case of incomplete dominance), in order to get red snapdragons both
chromosomes have to make the pigment.
The recessive allele is really one
that is making no color because the directions for making the protein are
too flawed by mutation. So when one makes color and the other doesn't, you
get pink, which is really half red. If neither chromosome produces color,
the flower is white. Sometimes one version of the protein is defective and
interferes with the functioning of the normal version. This is what happens
in Huntington disease, therefore we say it is a dominant disorder. So to
make a long story short, the old definition of dominant is quite
simplified.
vanhoeck
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Update: June 2012
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