Ask A Scientist© Biology Archive

name         Katie
age          16

Question -   What Causes some genes to be ressesive and other genes to be dominant?
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Think about the fact that genes code for directions for making proteins.
There are many different kinds of proteins in our body-enzymes for regulating
metabolism, structural proteins for building our bodies, hormones for
regulating processes etc.  If the gene for the protein is structural, then it
is important to have the right kind and the right amount.  If the gene is
defective (usually recessive genes are defective, but not always) the right
protein will not be made and the structure will either be defective and won't
work at all or there won't be enough to maintain the structure.  Sometimes
you need both genes to be working to get enough of the structure.  So a
homozygous person will have the strongest structure.  A heterozygote would
have one gene that is working a may produce enough of the protein to maintain
the structure, but maybe not.  So in some cases, just having one copy of the
dominant (working) gene is enough.  If it is a trait for something like eye
color, this is not going to cause a defect, just a difference.  In this case,
being heterozygous or homozgyous for the dominant trait produces the same
color of eye.  In the case of sickle cell anemia, the recessive gene changes
the protein structure of the shape of the red blood cell.  If you have one
good copy and one bad copy of the gene, some of your cells will be normal and
some will be sickle cells.  One good copy of the gene gives you enough normal
red blood cells to stay   healthy.  But if you don't have a normal copy, all
of your cells have the capability to sickle under certain conditions and this
can be fatal.  So think of dominant and recessive genes in terms of what they
produce and what that protein is supposed to do and then think of what would
happen if the dominant gene was able to overtake the effect of the recessive
gene.
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Dominant or recessive depends on the function of the
gene. The term is mostly used for genetic diseases.
Look at it this way. If a gene codes for a protein,
hormone or factor that is needed in whatever amount,
the lack of a portion of that protein would not be
disastrous. So if one of the two genes present in a
diploid cell would be damaged (mutated), but the other
one is intact, the cell or organism would still be
pretty healthy. However when both gene copies are
damaged, the cell would suffer from disease. That is a
recessive gene.
When a gene product and it's amount is essential, and
a mutant form would cause damage to the cell or the
organism, even one copy of a mutated gene would cause
damage:  the healthy second copy of that gene could
not prevent this. That gene is causing a disease that
is inherited in a dominant form.

Dr. T. Wassenaar
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