From: bhjelle@vaxine.unm.edu (Brian Hjelle)
Subject: Re: Blood types - Mother&Father
Date: Fri, 31 Jul 92 17:46:03 GMT
Organization: University of New Mexico, Albuquerque

In article <Bs9DKG.GFC@news.cso.uiuc.edu> slkg9733@uxa.cso.uiuc.edu (Steven L. Kellmeyer) writes:
>Jennifer Brown <JLB155@psuvm.psu.edu> writes:
>
>
>>A and B are both dominant.  Neither is really dominant over the other.
>>O is recessive.
>
>>The RH factor works the same way.  You get either the Positive or the
>>Negative factor from each parent.  The Positive one is Dominant, and the
>>Recessive one is the Negative.  So, if the child has a + & +, they'll be +.
>>- & - make a -.  + and - make a +.  Ok so far?
>
>You are absolutely correct in re the ABO system, but the Rh system and
>eye color is a bit more complex.  
>
>The gene that makes you an "A" is essentially one instruction that tells
>the body "Please manufacture a protein called "A" and attach it to the
>red blood cell".  The gene that makes you a "B" gives the same instruction
>but requires the body to make a protein of type "B".
>The instruction for type "O" says "Don't bother making a protein to attach
>to the RBC" (that's why it's called type O, it stands for 0, null, nada, etc.)
>
>Thus, A and B are both dominant since they make something while O is
>recessive because it doesn't make a thing.  If mother gives you A and father

I think you are at least as guilty of oversimplifying the ABO system as
Jennifer was in simplifying Rh. First of all, "A" and 
"B" antigens are not proteins but carbohydrate moieties on the red
cell membrane. They are added enzymatically by the proteins that
are the actual products of the A and B alleles. "O" is  not truly
a null phenotype. "O" red cells express the H antigen; you don't
hear about it much because few recipients have anti-H antibodies.
You will hear about it for sure when you transfuse O blood to someone
with the rare O-Bombay phenotype, however. They lyse the transfused
cells rather promptly. They lack the ability to make H antigen
and have antibodies to same.

>For Rh, however, there are about 4 or 5 instructions, each of which has a
>null form as well.  They generally get inherited together, but that isn't
>always the case.  Thus, it's possible to have Rh positive well up the
>geneological tree and still have a child who tests Rh negative.  There is  
>also something called "D mosaic", in which part of the Rh instruction set is
>active, and the other part is null instructions.  The individual tests as
>Rh- even though they are really slightly Rh+.  

I don't know what you mean by "4 or 5 instructions", but if you
are referring to the Rh CDE "loci" (whether they are truly "loci"
is subject to debate), then you should know that only D has a
null allele (an allele that does not encode an antigen). The
null allele for D is called d but does not encode an antigen.
Alternative alleles for C and E are called c and e. c and e are
antigens in their own right. The inheritance of CDE loci is
as a block (haplotype). Recombination, if it occurs, is not what
allows two Rh (D)+ parents to give birth to an Rh (D)- child.
It is simply that any Rh(D)+ person can, in fact, be genotypically
Rh D/d.

There are people who are Rh D+ who test Rh D-. Most of these are
not expressing a mosaic antigen but have the Rh Du antigen. This
allele encodes a weakened form of D antigen, but it can be
detected with modern serologic reagents available in all U.S.
blood banks. No blood is permitted by FDA to be released as
"Rh negative" unless it has been tested for Du.

The genetics of Rh are much more complex than indicated above.

Brian

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