from gloria stephens

Max Cates cates at biochem.purdue.edu
Mon Jul 17 18:27:45 EST 1995


>I deal with domestic feline genetics. May we send a messenger to the cell 
>to make - chocolate. In the cat world chocolate is recessive to black.
>We now have one chromosome from the mother with the recessive 'b' and one 
>chromosome from the father with the recessive 'b' gene.
>To which chromosome does the messenger go?
>If it goes to the maternal chromosome first, is mRNA formed from that 
>chromosome. Is this enough for a complete transcription?
>Suppose, on the other hand the paternal chromosome has the dominant gene 
>B. If the messenger goes to 'b' first, is mRNA formed from 'b' or does 
>the messenger go looking for the dominant trait?
>I understand that if the dominant gene is present, then it is read and 
>mRNA is formed.
>On a molecular basis, what happens?
>You say 'that only one strand of the two DNA strands is actually 
>transcribed!!'

Nope.  Both strands are transcribed and translated.

>I understand that. As far as I know, it is not known which strand acts as 
>a template and which strand is actually transcribed. But, again, my 
>question concerns both chromosomes, not the strands or the two strands of 
>the double helix.
>Sorry to be brain dead, but this is a question that has bugged me for a 
>long time.
>May I return to B and b? In B - no problem, unless there is codominance? 
>If that is the case, then B and b would have to be transcribed to make a 
>polypeptide?
>In the case of b and b, then two polypeptides are transcribed and bonded 
>to form one polypeptide?
>I understand the importance of genetic material coming from both parents 
>in meoisis and crossing over.
>I just want to understand more about mRNA and recessive genes - again - 
>on a molecular basis.
>No, I do not have a Ph D or an MD. Degree in Biology - some time ago. So 
>what I know, I have taught myself. The only thing, the more I think I 
>know, the more I find I do not know.
>I wish to thank you for helping me to understand.
>Most sincerely,
>Gloria Stephens
>

In most cases, dominance and recessiveness aren't a matter of 
differential gene expression, but are only due to protein function.
 
Example: in the white-tailed deer, brown (B) is dominant over black(b), 
and is controlled by a single allele.  The protein coded for by the B 
gene is an enzyme that converts a black-colored pigment precursor to the 
brown-colored final product.  Deer with bb are black because, while the 
gene's copy of the enzyme is translated at the same level as in the brown 
deer, both the black deer's enzymes are defective and can't turn the poor 
critter brown.  The heterozygote (Bb) is brown because large amounts of 
both enzymes (B-product and b-product--effective and defective) are made; 
more than enough for the active enzyme to convert the black pigment to 
brown without the help of the lazy b gene product.  

Your cat example would work in similar fashion, with the colors reversed. 
 The reason your bb cats are chocolate is that they're only making it 
halfway to black, thanks to their non-functioning b gene proteins.

 
 With VERY few exceptions, this is the way one-allele dominance/recessive 
cases work.  Incomplete dominance (like in Mendel's pea flowers: 
red (RR) + white (WW) =pink(RW)) arises when one copy of the gene 
producing functional protein isn't quite enough to make all the necessary 
protein, or when the inactive protein can competitively inhibit the 
active one. 
 
Hope this helps!  




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