popa0206 at PO-Box.McGill.CA
Wed Jul 12 22:30:16 EST 1995
> gloria stephens <stephens at hq.jcic.org> writes:
> May I please ask for more information concerning dominant and recessive
> 1. Is there enough information contained in the 'dominant' gene that the
> recessive gene is not read at all?
Well, not really.... First I assume you are aware that you have two copies of a gene (alleles)
for every gene (can code for protein, enzyme etc)
A dominant allele is a copy of a gene that exhibits an effect regardless
of whether it is in one copy or two. Where a recessive allele requires two copies to
to exhibit its effect.... this is as straight forward as it gets....
So the recessive allele may be "read"(transcribed and perhaps translated to protein)
as you put it but it doesn't change the outward appearance or physical characteristics
of the organism (phenotype) from the state deemed to by "dominant" when it is only present
in one copy. IF you have two copies and therefore no dominant allele you have the
outward expression of that allele as it is reflected in another phenotype deemed to be
> 2. Is there any influence from a recessive gene?
Sometimes you can have incomplete dominance in which you get a quantitatively
intermediate phenotype (ex. say recessive is white and dominant is red then you would
get pink as the incomplete dominant phenotype)
Also you can have codominance (nothing to do with prophylactics mind you <grin>)...
in which you get expression of both phenotypes... really no dominance here at all.
ex. M-N blood grouping where you can have M, N or MN blood type.... you have an
immunological antigen M and N. So if you have MN blood type your blood cells
have both M and N antigens on there surface.
> 3. Are both the dominant and the recessive gene read?
Generally yes, as in the M and N.... other cases may be different. Remember
that expression of the recessive allele may not produce the "recessive" phenotype
because of the presence of the dominant.
Ex. say you have an enzyme that you need to live... the recessive allele may
code for an enzyme that functions improperly and you could die.... and the
dominant allele codes for the proper form of the enzyme. SO when you
have one dominant and one recessive allele (heterozygous for that allele) you
can live as the dominant protein (enzyme) does its job. When you have
two recessive alleles (homozygous) for that enzyme you die... this case
would be an example of a lethal recessive.
In humans one example of a lethal recessive trait is one type of
dwarfism... when you are heterozygous for this allele you are a dwarf
and when you are homozygous it is lethal.
> 4. In dealing with the recessive gene, are the genes from both
> chromosomes read? I.E. are polypetides formed from both chromosomes?
> Basically, are both chromosomes read or just one?
Yes in most cases, but here is something to chew on.... there is evidence that
only one strand of the two DNA strands is actually transcribed!!
I think an introductory Genetics text like "Introduction to Genetic Analysis" Suzuki, Griffiths....
would be a good start.
> Gloria Stephens
> stephens at hq.jcic.org
Graham Dellaire Snail Mail:
Red Cross, Research
McGill University Montreal Blood Services
Faculty of Medicine 3131 Sherbrooke St. East
Div. of Experimental Medicine Montreal, QC, Canada
E-mail: popa0206 at po-box.mcgill.ca H1W 1B2
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