Ok. There are a variety of different issues going on here. (With the disclaimer that I’m not a biologist).
First keep in mind that genes are discrete units and population sizes are finite. This allows for genetic drift which is when random sampling allows the frequency of a gene to change in the population as a random walk. This is especially easy to occur in small populations (since the sample sizes are then smaller). So two alleles can behave completely symmetrically and still have one go to fixation in a population, and similar remarks apply to symmetricly behaving genes.
Whether a given allele is recessive or dominant can be a function of a variety of different things. Keep in mind that a lot of the traits we care about have behavior more complicated than just dominance or recessiveness. There are some examples where the heterozygous case has a middle phenotype. Another example is how many of the alleles for six fingers are dominant over alleles for five, but the alleles display variable penetrance, which is a fancy way of saying that not everyone who has the allele ends up getting the phenotype due to essentially stochastic considerations.
What makes an allele recessive or dominant can depend on what it is doing. So for example, if an allele is producing a bad protein that gets in the way of something else, then it is likely to be dominant. If an allele is just needed to at the right time in development trigger a certain pathway it is likely to be recessive. Some alleles don’t code for the production of a different protein but simply don’t code for a protein at all (if they are say missing a functional start codon or some other bit of instructional code). In those cases, whether the allele is recessive or dominant depends on what the allele does. So for example, if the allele calls for the production of a protein that has its level normally regulated to stay in some level of stasis (as many proteins in cells are) then the body will simply ramp up production and so it will be recessive. Similarly, the A and B alleles in bloodtypes are dominant because what matters is whether the display occurs on the outside of the cells. So as long as there’s enough protein for any display, a recipients immune system will recognize it as foreign.
In general, the biochemistry can be really complicated. Huntington’s disease for example is caused by mutations in the Huntingtin gene. This is one of the most studied of any single human gene but what it is causing to go wrong is still not well-understood.
Ok. There are a variety of different issues going on here. (With the disclaimer that I’m not a biologist).
First keep in mind that genes are discrete units and population sizes are finite. This allows for genetic drift which is when random sampling allows the frequency of a gene to change in the population as a random walk. This is especially easy to occur in small populations (since the sample sizes are then smaller). So two alleles can behave completely symmetrically and still have one go to fixation in a population, and similar remarks apply to symmetricly behaving genes.
Whether a given allele is recessive or dominant can be a function of a variety of different things. Keep in mind that a lot of the traits we care about have behavior more complicated than just dominance or recessiveness. There are some examples where the heterozygous case has a middle phenotype. Another example is how many of the alleles for six fingers are dominant over alleles for five, but the alleles display variable penetrance, which is a fancy way of saying that not everyone who has the allele ends up getting the phenotype due to essentially stochastic considerations.
What makes an allele recessive or dominant can depend on what it is doing. So for example, if an allele is producing a bad protein that gets in the way of something else, then it is likely to be dominant. If an allele is just needed to at the right time in development trigger a certain pathway it is likely to be recessive. Some alleles don’t code for the production of a different protein but simply don’t code for a protein at all (if they are say missing a functional start codon or some other bit of instructional code). In those cases, whether the allele is recessive or dominant depends on what the allele does. So for example, if the allele calls for the production of a protein that has its level normally regulated to stay in some level of stasis (as many proteins in cells are) then the body will simply ramp up production and so it will be recessive. Similarly, the A and B alleles in bloodtypes are dominant because what matters is whether the display occurs on the outside of the cells. So as long as there’s enough protein for any display, a recipients immune system will recognize it as foreign.
In general, the biochemistry can be really complicated. Huntington’s disease for example is caused by mutations in the Huntingtin gene. This is one of the most studied of any single human gene but what it is causing to go wrong is still not well-understood.