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?