Ask A Scientist Molecular Biology Archive

name        Sandy P.
status       student
age       20s

Question -   Like PKU, most neuropsychological disorders with a major genetic component are linked 
to recessive genes.  Why are they rarely linked to dominant genes?
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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.

vanhoeck
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