dominant/recessive genes

Mary K. Kuhner mkkuhner at phylo.genetics.washington.edu
Thu Nov 16 18:33:39 EST 1995

>In simple discussions of genetics one hears of dominant and recessive
>genes.  Dominant genes are expressed, while recessive ones are not.

>What is it exactly that makes one of a complementary pair of genes
>dominant and the other recessive?  Also, is the domination complete
>(i.e. dominant gene expressed 100%, recessive gene expressed 0%) or
>is it more of a 90%/10% or 80%/20% situation?

>I know that the laws of thermodynamics must be obeyed, so if you
>can explain this phenomenon using thermodynamic arguments I would
>appreciate it.

>Greg King

A recessive gene is very frequently one that produces no gene product,
or a gene product that does not do anything:  the gene is broken.
Whether a broken gene appears recessive or not depends on whether a
working copy is able to provide enough of the gene product.  In many
cases it can.  If I have even one gene for the protein that breaks down
phenylalanine, I will not suffer from phenylalanine poisoning, because
one gene can produce plenty of the protein to meet my needs.  If I have
even one gene that produces blood factor A, my blood will behave like
type A blood.  Most proteins are either produced at levels greater than
the body needs, or controlled by feedback to keep them at the necessary
level.  In such cases, having one broken gene will not show.

It's not that being recessive is causing the gene not to be expressed.
The gene is broken and can't be usefully expressed no matter what.  In a
person with two broken copies, you see the absence of whatever that gene
did.  In a person with one working and one broken copy, you may see
nothing, or you may see a slight lack of the product.  (Sickle cell
heterozygotes are mostly healthy, but may experience some sickling at
high altitudes.)

A good example to think about is albinism.  Albinism is recessive
because if any gene making melanin is working, there will be melanin and
the person will be at least somewhat brown.  The "albino gene" is not a
gene that makes you an albino, it is a gene that does nothing, and if
that's all you have you will lack melanin and be an albino.

Mary Kuhner mkkuhner at genetics.washington.edu
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