Dominant/Recessive Genes

Brian Foley brianf at med.uvm.edu
Mon Feb 28 13:58:50 EST 1994


Mark Joseph Johnston (se040 at sna.co.umist.ac.uk) wrote:
: I am a computer science student working on genetic algorithms and 
: want to ask a "silly" question.

: Recessive genes are generally not "good" for the organism, yes?

	No.  Whether a gene is recessive or dominant does not necessarily
reflect on whether or not it is "good".  We rarely find dominant lethal
genes or dominant harmful genes in a wild population because they have
been selected against.  Another reason why recessive traits are often 
seen as detrimental is that a recessive trait is often caused by a lack
of a functional gene product.  The gene for "albino" is actually the
loss of pigmentation genes.
	Most genetic algorithms do not depend on a diploid genome do they?
It seems that life on earth existed for a few billion years before 
diploidy was introduced.  Having a diploid genome allows individuals to carry
nonfunctional alleles.  Haploid organisms, even if only a small portion 
of their life cycle is haploid, play by different rules.

: If so why do they exist? Why do two alleles exist together?

	Most people think of "Survival of the fittest." and therefor 
think that everything that exists must exist because it was selected "for".
It sometimes helps to think about "Elimination of the weakest." and 
realize that natural selection most often acts by eliminating weak 
individuals rather than by ensuring excess reproduction of fit individuals.
	There are not just "two" alleles for a gene in a population.  
There might be thousands of alleles for a given gene.  Each diploid 
individual will carry two alleles (or possibly two copies of the same 
allele), but there is no reason to think of any gene as having just two 
alleles. 	

: Is it to conserve genetic diversity (without manifesting a trait 
: with a poor survival value??

	Biological evolution has no "purpose in mind".  Life just 
happens.  With genetic algorithms you can design a gene that says "Look 
at all the other individuals in my population and steal good genes from 
the individuals who are doing better than me."  But no such gene has been 
found in biology.  It would be tough for such a gene to evolve because it 
is not easy to tell from an organisms phenotype which genes are 
responsible for a part of the phenotype (Look how long it took 
intelligent humans to find the cystic fibrosis gene.).
	With genetic algorithms there is a closer link between "gene" and 
"phenotype" so designing genes that will rob other genomes is easier.  I 
diploid organisms, mating selection rituals have evolved to accomplish 
the same thing.  An individual will look for another individual that looks 
very "fit" and is likely to carry "good" genes for a mating partner.  
Organisms that mate selectively can thus evolve much faster than 
organisms that mate by blind chance. 
	

: Forgive my ignorance.

	Thank you for at least trying to learn about biology.  Even 
though nucleic acid-based intelligent systems evolve much more slowly 
than sillicon-based systems designed by humans, they have had a few 
billion years head start and we can learn a lot by studying what they can 
and can't do.
	If we can start by looking at what is best about biology and then 
eliminating some of the "blind chance" and replacing it with "intelligent 
choices" we can come up with some very rapidly evolving systems.  
Lamarkian evoltuion would be very powerful.

: Cheers.
: Mark
: %^).

--
********************************************************************
*  Brian Foley               *     If we knew what we were doing   *
*  Molecular Genetics Dept.  *     it wouldn't be called research  *
*  University of Vermont     *                                     *
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