Huntington's disease is a dominant trait. It is not a “better” allele than a non-Huntington allele. It is “dominant” in that it only takes one bad copy stacking up in your brain cells and eventually driving you mad and killing you.
Brown eyes are not “better” than blue eyes - but the gene that says “put a lot of brown (melanin) in there” is dominant to the gene that says “don't put that much brown in there”. It is as if you instruct one worker to make a hill and the other to only fill in a hole- the one who fills it up “dominates” over the one who doesn't in that you end up with a hill. He isn't a “better” worker - both followed instructions.
Sometimes a bad copy is recessive in that it doesn't function - but one good copy does the job plenty well. In that case the “dominant” gene is better - but for every example of that there is a contrary example where it only takes one bad copy to mess something up!
If by “cast in stone” you mean determined by the molecular biology of the genes and the proteins they code for - then yes - it is cast in stone.
If by “cast in stone” you mean for every example where a dominant allele is better than the recessive allele there is probably an example where a recessive allele is better than a dominant allele; or neither is objectively better than the other - then no - it is NOT “cast in stone”.
Thank you for the reply. You at least confirmed that this is largely beyond my basic biology education.
By “expressed” I did mean this in terms of the appearance or behavior of the organism. By “better” I meant of survival benefit.
It seems counter-intuitive to me that the appearance or behavior most likely to appear in offspring would be detrimental to its survival, at least if the concept of natural selection is to be believed. I take your word for Huntington’s, but I expect that this dominant/recessive gene phenomena is overall selecting for survival in most instances.
Brown eyes (and skin pigment) may be “better” for survival. Too much Sun may be far worse than too little. Our view of what is “better” may be subjective, but I expect that survival in the natural world is cold and pragmatic.
By “cast in stone” I meant that I thought (guessed) that the gene seen dominantly expressed, say in current man, may not always be so. Your example of the digger/filler is what I was thinking of. Both genes are there and are trying to signal the action they are coded for, but something else in the overall organism makes one more effective than the other. I expect that this can change.