PKU is Recessive, Not Dominant
Name: Sandy P.
Date: July 2003
Like PKU, most neuropsychological disorders with a major genetic component are linked
to recessive genes. Why are they rarely linked to dominant genes?
Because most of the time you have another "good" copy of the gene to compensate. For instance,
if the protein coded for by a gene is needed only in small amounts, one gene producing the protein
is enough to get by. Sometimes when an organism is heterozygous, half the amount gives you a
reduced effect, i.e., pink snapdragons result from a flower that is half red, half white
(no color). The definition of a dominant trait is one that when present, its effect is seen.
In one type of dwarfism the gene codes for a protein necessary in making cartilage, which helps
to elongate the bones. When one copy of the "bad" gene is present it makes no protein. The
other copy is normal, so only half of the protein is made, and the person has shorter than
normal long bones. If both copies are dominant, no cartilage is made and this is incompatible
with life, so the embryo is miscarried. (This is also why two dwarfs have a 25% chance of
having a full size offspring-all dwarfs are heterozygous-at least for this type of dwarfism.)
In the case of Huntington disease, the dominant copy of the gene interferes with the normal,
recessive copy so its effect is seen.
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Update: June 2012