(Original post by futuredoctorVSB)
I've tried searching, but no real good explanations found. I know the effects of the different allele types, but why are they different in the first place?
My hypothesis is that dominant alleles produce proteins that are specific to recessive allele mRNA, and destroy it (preventing recessive allele expression).
And in homozygous recessive, the recessive allele is expressed, since, there is no protein present to break it down?
Same with codominant alleles?
I know my theory is far fetched, but could someone please clarify the actual mechanism for me?
I will keep it as simple as possible (I am not sure if you have College or Uni experience, would assume you know A levels bio)
Say you have a gene: it has variance in its DNA sequence. This certain variance (e.g. AAGGTTCC) is called variance A or allele A and it expresses brown eye colour
Other variance (call it AAGGTTTT) is called variance a or allele a and it expresses blue eye colour
When you have AA (both homologous chromosomes have gene A sequence) you will get protein A expression or the protein that is responsible for brown eye expression
If you have Aa, one chromosome has A and the other has a (i.e. your mum had blue eyes or a and dad had brown eyes or A) and you will get brown eyes
The magic is here: DNA sequence A (not protein A) masks the expression of DNA sequence a
. This is an absolute fact that is well established and have applications over it (search balancer chromosome) the actual masking mechanism I would guess can be done in three ways (based on my year 1 and 2 uni lectures in molecular biology):
1- When the cell reads that it expresses sequence A to an mRNA and to a protein, there would be a biochemical pathway that would recruit transcription factors that would methylate and induce heterochromatin formation over sequence a
Or to rephrase it, the presence of protein A is (probably) sensed by certain transcription factors in the cell cytoplasm that would create a pathway that would eventually lead to the methylation (adding methyl molecules above certain DNA sequences) or coiling of DNA sequence a that would make its expression / translation impossible, as RNA polymerase wouldn't physically see sequence a as it is tightly coiled or hidden under methyl molecules.
2- When sequence A mRNA is transcribed, a transcription factor binds to it and says "sh!t I have to degrade similar mRNAs or stuff bloody methyl groups above similar sequences" then it continues to do the same above mechanism
3- RNA interference where antisense translation of sequence A happens (yes it can happen: from 3' to 5' DNA ends) produce a 'rogue' RNAi that search and destroy the mRNAs of sequence a by binding to them, before their translation
Hope this cleared some clouds