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I'm in the first year of Medicine and I'm studying Genetics and Evolution. I have this doubt in the back of my mind and I'm not being able to move forward without someone explaining me what's wrong with my thinking.

So I've learned an allele is a variant form of a gene, all alleles of the same gene are slightly different from each other and code the same characteristic in different forms. We know proteins can have from 20 to 34350 amino acids. Let's take a protein with 4 thousand amino acids. The RNA that formed it had 4 thousand codons, which are 12 thousand nucleotides. I didn't study mutation yet but I assume a mutation can occur in a single nucleotide in the DNA that can or cannot code for a different protein (as some amino acids are coded by more than one codon).

Ok, so let's say a super energetic UV photon reaches one of my germinative cells and mutates a single nucleotide of my DNA that changes the provious amino acid. The protein has 4 thousand amino acids. Will a single one change its function or shape? I would say it is more likely not to change, and I don't know why this reasoning can be wrong. One amino acid in 4 thousand probably will have no effect in the overall shape and function and will form a slightly different/ almost identical protein with almost identical functions. If that is right, we should have an enormous number of alleles for the same gene (e.g. if we say less than 5 amino acids won't change the structure, that are 3,2 million different alleles), instead of 2, or 3, or 4, or some small number as I was taught.

I can think in 2 explanations: The first is that if, contrary to what I think, a small change in an unique amino acid could change the protein enough to make the individual die for example. Which I doubt. The second is if we in fact had millions of alleles, but all this alleles had similar or almost identical shapes, and we could segregate them in a small number of groups (2 for example) where the functions of the coded proteins are different from the other group but identical in the same group.

Am I going too far? Could anyone explain me why this is not possible?

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  • $\begingroup$ Hi and welcome to Bio.SE! I tried to update your title to be more relevant. Please adjust if you don't think it captures your question appropriately. speaking of which, your post is a bit rambly and the actual question is not clear -- please update to clarify to avoid receiving negative response. $\endgroup$ – theforestecologist Mar 25 at 1:48
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The protein has 4 thousand amino acids. Will a single one change its function or shape?

It very well might. High schoolers learn about the existance of sickle cell anemia, Tay-Sacks, cystic fibrosis, phenylketonuria...you must know about them too.

If that is right, we should have an enormous number of alleles for the same gene (e.g. if we say less than 5 amino acids won't change the structure, that are 3,2 million different alleles), instead of 2, or 3, or 4, or some small number as I was taught

The is the dbSNP entry for human transferrin receptor. As you can see there are a whole lot of recorded variants.

https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?geneId=7037

But that doesn't mean that all of them are clinically relevant.

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Firstable, a point mutation in your DNA could change definitively the fate of your hypothetical coding RNA (later a protein product). The reason relies on the nature of the mutations: Missense mutation, Nonsense mutation or Frameshift mutation. In the nonsense one, definitely will make your protein non-functional because of the insertion of a STOP signal inside your chain of nucleotides.

Talking about the susceptibility of some regions to be mutated, in general terms the most conserved regions on a protein are really well conserved because of their functional importance, that has been evolutionarily selected and prevents that the mutation rate will have a high probability.

And also, when you said: "I assume a mutation can occur in a single nucleotide in the DNA that can or cannot code for a different protein (as some amino acids are coded by more than one codon)." Your assumption ignores the half part of history. Mutations are not restricted to small-punctual changes, but they could include big regions that alter the chromosome organization, as translocations, duplications, inversions or even loss of alleles by loss of heterozygosity. I would suggest this resource, to get the principal concepts and ideas about this topic, but for sure this is just a general description.

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  • $\begingroup$ Your answer focuses on extreme cases when mutations are more than a single amino acid, but I think the question was most concerned with the effect of a single amino acid substitution. $\endgroup$ – Bryan Krause Mar 25 at 4:13
  • $\begingroup$ Right, but in the case that you codon, for example, codifies for Tyr (UAU) have the possibility to have U->A and results into the STOP codon (UAA) and it definitively modifies the fate of your potential protein. This is a related example, isn´t it? $\endgroup$ – Cristian Velandia Mar 25 at 20:07

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