On Wed, 30 Jul 2003 15:41:08 +0000 (UTC), eastmond{at}yahoo.com (John
Eastmond) wrote:

>I understand the priciple of dominant and recessive alleles which is
>the basis of basic Mendelian inheritance of characteristics like eye
>colour.
>
>Are there situations in which heterozygous individuals display a
>phenotype which is intermediate between the two alleles so that in
>some sense the dominance of the dominant allele is incomplete?
>
>Are such cases in fact more common than the case of complete dominance
>of one allele over the other?

This is one of the major problems with teaching introductory genetics:
most students think exactly the way you do about dominance.  And no
wonder, dominant/recessive is a major topic of treatment.

The fact is that there are really very few characters that show
complete dominance.  Some form of intermediate inheritance is the
rule.  Some characters show incomplete dominance, where the
heterozygote is somewhat intermediate in phenotype.  Others show
codominance, where the heterozygote has aspects of both phenotypes.  

The fact is also that almost every phenotype at the organismal level
is the result of a large number of genes acting and interacting
together.  Look at eye color or hair color, two features commonly used
in introductory course problem sets.  "Dark" is supposed to be
dominant over "light" in both cases.  Now look at peoples' eyes and
hair.  Both come in all sorts of shades and colors.

If a trait can be identified with a single protein, as in sickle cell
anemia, then that trait behaves according to simple Mendelian rules.
Most traits involve complex developmental pathways, controlled by many
proteins.  Even most biochemical traits (the concentration of melanin
in hair, eyes, or skin) depends on complex biochemical pathways with
many enzymes involved.  These traits are influenced by many genes and
the genetics is quite complex.

Mendelian inheritance is a good starting point for understanding
genetics.  It was very important in the history of science.  It really
does apply to specific (usually artificial) variation like what we see
in fruit flies.  But the simple form with one gene and dominant and
recessive alleles isn't that useful for everyday life.
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