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From: bhorton@aries.dpi.tas.gov.au
Subject: Re: recessive & dominance
Organization: Department of Primary Industry and Fisheries
Date: Thu, 22 Jun 1995 01:29:54 GMT
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>Subject: Re: recessive & dominance
>> >> Could anyone explain what is it?

>> >   Basically a simple thing.  Given an environment where two genes
>> >-D(dominant) and R(recessive) - are capable of controlling some single
>> >characteristic C, all attributes of C will be defined by D-dominant;
>> >R-recessive is unused.  Hence, D is said to completely mask R. This is the
>> >simplest occurrence of a genetic dominant/recessive pair.

>> *heh* I'd like a little more...specifically, what are the forms that
>> create the effect of a gene either being recessive or dominant?

>[a lot of hand waving snipped here]

In the simplest biological form each gene can be considered to code for a 
protein (usually an enzyme).  All genes come in pairs so there will be 0, 1 
or 2 doses of any given protein (enzyme).

For example a flower colour gene might code for red or white.  Two does of red 
give a red flower, 2 does of white gives a white flower and one each of red 
and white gives a pink flower.  Note that there is no dominance in this case, 
we just get an average effect.  This is sometimes referred to as partial 
dominance, particularly if the intermediate form favours one side or the other 
rather than being an exact average.

In the classic situation, a gene might code for the enzyme that produces hair 
pigment (melanin).  [We are talking about rabbits now.]  The alternative is an 
inactive gene that cannot produce melanin.  Two doses of the active form gives 
coloured hair, two doses of the inactive form gives white hair (albino), and 
one dose of each gives coloured hair because one dose produces all the pigment 
needed. In this case albino is recessive to coloured because the albino form 
actually does nothing.

In the opposite situation, in sheep - There is a gene for the enzyme that can 
break down melanin.  One dose is all that is needed to produce a white sheep.  
Black sheep only occur if they have two doses of the inactive form, so that 
melanin is not broken down.  So white wool colour is dominant to black wool.

In summary if one gene is active and the other inactive then the active form 
will be dominant or at least partially dominant.  The inactive form will be 
recessive.

If any experts are reading this please don't mess up a simple 
explanation with the facts   (:-)

So what happens in a GA.
You can have a simple average scheme, with no recessive or dominant genes.
You could use a geometric mean to favour the lower value, or some other 
scheme to favour higher values while still allowing both to contribute.
You can have a rule that the highest score is 
dominant over all lower scores (assuming they can be ranked in some way).
You can have a rule that the lowest is dominant.
You could have a separate part of the code to indicate dominance.

Or use more interesting schemes that allow all or nothing effects but in a 
more even handed way than simple dominance.  e.g the system suggested by 
Ryan in 'The degree of oneness', Proc. Europ. Conf. Artific. Intell 1994. 
Available by ftp from odyssey.ucc.ie/pub/genetic/oneness.ps.z

Brian Horton
(Sorry this grew a bit long, I started with a simple explanation and got 
carried away).
