This was written by a well respected teacher on the AP Bio listserv and relates to a question we had in class.
"What is the difference between the dominant allele and the
recessive allele that allows the dominant to show up?" In other
words the chemical basis for dominance.
You have to take it back to the fact that the genes are coding for
proteins. Proteins serve different roles in an organism: enzymes, structural proteins, hormones, regulation. Therefore a non-functional protein can have varied effects, either
manifesting as a dominant trait or a recessive trait in the heterozygous genotype. The key is which phenotype is exhibited in the heterozygote -- this then is deemed the dominant trait.
If having one allele (let's call it allele A1) that codes for a
properly functional protein is enough to not see the effects of the
other allele coding for a non-functional or an improperly functional
protein (let's call it allele A2) then A1 masks A2 and A1 is
"dominant" so we now call A1 just A and A2 just a.
But if even having one allele coding for a non-functional or an
improperly functional protein causes a change in the organism, then
A2 would be dominant and instead that would be called A.
Here are a bunch of examples:
a. enzymes: In genotype Aa, if one allele produces non-functional
enzyme and the other produces the functional enzyme then this is
usually a sufficient amount of enzyme for the individual to not
experience any deficit effect. So the effect of the non-functional
protein is masked and hence considered recessive. An example is
Tay-Sachs, a disorder in which homozygotes are lacking the enzyme,
hexosaminidase, which normally breaks down fats. Without the enzyme,
fatty acids accumulate in the lysosomes of nerve cells, disrupting
cellular function and causing a relentless deterioration of mental
and physical abilities.
b. structural proteins (loss of function effect): In genotype Aa, if
one allele produces non-functional protein like a cell membrane
channel and the other produces the functional channel protein then
the working channels usually are sufficient to allow overall
functionality and hence the mutation is considered recessive. An
example is cystic fibrosis in which the recessive allele codes for a
non-functional chloride channel. Chloride cannot flow out of the cell
which has serious osmotic consequences causing sticky mucus to clog
numerous organs, like the lungs.
c. structural proteins (gain of function effect): If the mutation in
the gene for a structural protein causes it to have a new function or
an improper function, like a cell membrane channel locked in the open
position, then this allele would be dominant to the normal allele -
genotype Aa would exhibit the disorder. An example is myotonia
congenita (Becker's or Thomsen's disease), a neuromuscular disorder
in which chloride channels are malformed and now allow sodium ions to
leak into muscle cells. This constant inflow of cations causes slow
relaxation of voluntary muscles, muscle stiffness primarily in leg
muscles, and hypertrophy (muscle enlargement).
d. hormones: In genotype Aa, one allele produces a non-functional
protein hormone and the other produces the functional hormone. The
non-functional hormone would fail to elicit the cellular response, so
- just as with enzymes - the heterozygous condition would express the
normal phenotype since the one allele would produce a sufficient
amount of hormone for the individual to not experience any deficit
effect. An example is an inherited growth hormone deficiency which
only manifests in the homozygous recessive condition and leads to
small stature since insufficient growth hormone is produced in the aa
individual.
e. regulatory proteins: The proteins produced by a gene can serve a
regulatory role like a transcription factor which turn on or off
other genes or a receptor that responds to promoters or inhibitors
from outside the cell. Mutations in these genes can have large-scale
effects on an organism and can exhibit a dominant inheritance
pattern. One example is achondroplasia (dwarfism). The mutation is in
the fibroblast growth factor receptor. The normal function of the
receptor is to slow down the formation of bone by inhibiting the
proliferation of chondrocytes, the cells that produce cartilage. This
mutation in the receptor therefore causes the growth inhibitor to
remain on. Therefore, as long as some of this mutant receptor is
produced, the individual will experience a slowing in growth.
Consequently, a heterozygous (Aa) individual will inherit dwarfism as
a dominant trait. In fact, this condition is lethal as a homozygote.
Kim B. Foglia
Biology Educator
Division Ave High School
Levittown, NY 11756