Why aren't all our genes dominant in the first place?

[zut212]

Why aren't all our genes dominant in the first place?

Suppose that a population has 25% of it's people with genetics aa; 50% have Aa; and the remaining 25% have AA. Over time, I believe that the homozygous recessive people, aa, would be vanquished.

Assuming that the gene for AA/Aa/aa does not make one more robust or hurt our chances for survival, wouldn't the genotype for aa vanish over time in an exponential-decay like fashion?

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[PiBond]

Why aren't all our genes dominant in the first place?

Suppose that a population has 25% of it's people with genetics aa; 50% have Aa; and the remaining 25% have AA. Over time, I believe that the homozygous recessive people, aa, would be vanquished.

Assuming that the gene for AA/Aa/aa does not make one more robust or hurt our chances for survival, wouldn't the genotype for aa vanish over time in an exponential-decay like fashion?

recessive doesn't necessarily mean bad or evolutionarily disadvantageous. the allele frequencies would remain consistent if you go with hardy-weinberg argument

 

[yamaraja808]

Why aren't all our genes dominant in the first place?

Suppose that a population has 25% of it's people with genetics aa; 50% have Aa; and the remaining 25% have AA. Over time, I believe that the homozygous recessive people, aa, would be vanquished.

Assuming that the gene for AA/Aa/aa does not make one more robust or hurt our chances for survival, wouldn't the genotype for aa vanish over time in an exponential-decay like fashion?

It is because the heterzygous state in "some situations" tends to be the most advantageous than homozygous recessive and dominant. A prime example of this is malaria resistance of the Africans in Africa. The homozygous dominant individual (wildtype) is not immune to malaria and will die because of it. The homozygous recessive individual (mutant) has sickle like blood cells (sickle cell anemia) and will die due to bad transport of oxygen to the tissues. Thus the heterzygous state will give the person the immunity to malaria and will still have the blood to carry oxygen efficiently enough. This is why the recessive phenotype is still possible in the population because the heterzygous phenotype gives the population the best fitness.

 

[loveoforganic]

Adding on to this, what makes a gene recessive or dominant? Like, at the molecular level

 

[yamaraja808]

Adding on to this, what makes a gene recessive or dominant? Like, at the molecular level

Well one cannot really describe it at the molecular level that easily. If there is someone who can, by all means explain it to me too.

However, in relative terms, what makes an allele dominant and what makes it recessive is dependent on how it affects the other alleles. Lets say there is a mutation for 6 fingers. Now if this mutation is dominant then it will override the wildtype which is 5 fingers with just one allele. If the mutation is recessive then it will not be able to override the wildtype with just one allele. (putting this in simple terms)

Now lets say the 6 finger phenotype is recessive and the wildtype allele for 5 fingers is wiped out from existence. Now the 6 finger phenotype is only prevailing phenotype. Then another mutation comes in for 7 fingers. Lets say this mutation is recessive to the 6 finger mutation. So in this situation the dominant allele is the 6 fingers and the recessive allele is the 7 fingers. However, the 6 finger allele is still recessive to the 5 finger allele (this does not change).

So to put it bluntly dominance and recessiveness is relative to how one allele affects another.

 

[DrYoda]

Why aren't all our genes dominant in the first place?

Suppose that a population has 25% of it's people with genetics aa; 50% have Aa; and the remaining 25% have AA. Over time, I believe that the homozygous recessive people, aa, would be vanquished.

Assuming that the gene for AA/Aa/aa does not make one more robust or hurt our chances for survival, wouldn't the genotype for aa vanish over time in an exponential-decay like fashion?

It's because the hetrozygotes carry the recessive alleles on to the next generation. As long as Aa individuals aren't at a disadvantage compared to AA, there will always be recessive alleles in the population.

 

[Phantastic]

Adding on to this, what makes a gene recessive or dominant? Like, at the molecular level

When asking this question, we must consider all cases: codominance, heterozygote advantages, pleiotropies, epistasis...etc.etc.

Ultimately, I believe we categorize the phenotype, as the central dogma holds that one gene -> one RNA -> one polypeptide. So on a molecular level there is no difference between a gene that is "dominant" or "recessive." It is merely the effect that is somewhere in the spectrum, in my opinion.

 

[docelh]

Adding on to this, what makes a gene recessive or dominant? Like, at the molecular level

This question is too loaded.

 

[loveoforganic]

I didn't think of, but should have recognized the complexity of what I was asking. Thank you all for your explanations.

 

[Lokhtar]

Many times, a gene is dominant over another because the recessive gene doesn't have a function and a dominant gene does. So let's say you have two copies of a gene, one produces skin pigment X, the other produces no skin pigment (just a made up example). If you have both, you'll produce skin pigment. The one that produces no skin pigment is 'recessive' because it'll simply not produce a skin pigment. Usually that arises via a mutation that causes the loss of a function. Think of blood types ("o" being recessive) - it's simply the absence of antigens.

Technically, both are frequently expressed, it's just that one produces a product that can't be seen phenotypically (or doesn't produce a usable product at all). We call that gene recessive.

 

[FCBarca1990]

I read in a book somewhere an interesting example of different types of expression at different levels of an organism. I can't remember the actual gene, but it goes something like this:

Gene level: Inherited gene codes for modified protein over normal protein (Dominance)

Protein level: Modified protein is actually a subunit, which causes the entire quaternary structure to function with a minimized ability but does not disable it entirely (Incomplete Dominance)

Tissue level: Abnormal quaternary structure causes changes in phenotype seen only in cells which express high levels of the protein complex (Codominance)

Disease level: Absence of the protein complex in those crucial cells causes abnormal function and weakness of the entire body (Dominance)

So as you can see, the whole idea of "dominance" is far more complicated than the Mendelian perspective abscribes.