What makes one allele dominant over another? - (Feb/16/2006 )

Hi,

What makes one allele dominant over another? I understand how dominance ect works but what is it that makes a allele dominant.

Does it just produce more product so swamps out the other alleles 'signals'? I really am curious about the mechanism of dominance, anyone got any ideas?

thanks

---

I think perhaps this link will help you

---

Dominance is a relative term.
There is no overproduction of product etc, that's called overexpression (which is dangerous).
There is no multiple copies of the gene (that's called amplificitation, which is also dangerous)

What happens is the dominant trait overpowers the recessive trait. A trait is rarely limited on one set of genes. So one trait is more powerful than the other, and it gets expressed.

Vetticus

---

I guess what i am trying to ask is: How does the dominant trait 'overpower' the recessive trait?

---

It has to do with the biochemical behavior of the proteins that are the products of those particular genes - that is, the dominant gene protein has a stronger effect than does the recessive gene protein.
The biochemical behavior changes with each protein.

Does this help?

vetticus

---

Hi,

Ru sure its at the protein level? Meaning.. that "the genes express equally" but the protein has a stronger ability to overpower the function of the recessive gene protein. But both proteins are made and are functional to a certain extent. Am i getting it right?

I have no read evidence but its just easier to beleive that it wud be better if it be at the expression level ( less waste of sources, better control?.. dunno!)

What say, all?

---

Things to remember:

1. Traits, not genes, are dominant or recessive.
2. It is traditional to refer as genes as dominant or recessive.
3. Since then, lots of advances have occured in regard to genetics.
4. Because of this new knowledge, we now know that protein gene products influence phenotype.
5. Dominant and recessive refer to the nature of inheritance of phenotypes, not to genes, alleles, or mutations (can't restate this enough).


The problem with discussing genetics in terms of dominance and ressiveness is that the traits are framed as exclusive alternatives. Only one will dominant, it's either one or the other that is expressed, not both.
This makes it hard to get people to understand that traits may be expressed together (a compound phenotype) or that the traits may actually interact with eachother. I made a bit or an error before, in saying that the dominant trait overpowers the recessive trait, (it's just the way i was taught how to explain it, until i actually got interested in genetics and looked stuff up for myself. still that description seems to stay put. my appologies).

if you look back at what mendel did with the peas, a go beyond high school text books, you can see that he knew that there was "non-Mendelian" cases (where no dominance occures). Specifically at the peas, he noted that the size and form of the leaves, stem length, sead coat colour, flowering time, and peduncle lenth, did not follow his dominant /recessive model. mendel new that dominance was not the exclusive norm. He deliberately focused his work on traits that he could work a model on.

Today, when docs decide to label human disease as dominant or recessive, this too has a slightly different meaning. dominance generally desicribes and condition found - fully or partially - in heterozygotes. The heterozygote need not resemble the homozygote at all. Dominant in this setting means, a condition where just one relevant alleleis present (don't belive me, check out the US national human genome research institute. They define dominant as: "a gene that almost always results in a specific physical characteristic, for example, a disease, even though the patient's genome possesses only one copy. With a dominant gene, the changes of passingon the gene (and therefore the disease) to children is 50-50 in each pregnancy" www.genome.gov/glossary.cfm?key=dominant).
recessive alleles are found only where two copies are present. so you cold imply here that dominant referes to the causal efficac of a single allel. What was i going on about again? oh yeah....

mendel used d/r to refer as traits. this is on appearances only, and acribes no properties to hidden alleles, which cannot be directly inspected. Traits are not always reliable benchmarks. but many conditions can be d or r, depending on different mutations (even at the same locus). MIM said that they would stop classifying autosomal traits as dom. or rec. was that so often the precise same phenotype was dominant when caused by one mutation but recessive when caused by another mutation in the same gene.
You cannot always define traits as dominant or recessive consistently.

Alleles, d/r, is what biologists (outside the field of genetics) generally use. However, this isn't reliable either. An allele can appear to be dominant or recessive, depending on how it is paired with other alleles. ie blood type (in case anyone is wondering, i'm AB+ , thus sparked curiosity in genetics and future career path). given the blood enzymes are A, O, and B. A is dominant to O, in the sense that AO corresponds to AA. When AB is about, both alleles are expressed, here A and B are codominant. but there is also cis-AB. cis-AB can generate antigens to both A and B. when A and cis-AB are coupled, the phenotype is AB. in this case, A is recessive to cis-AB. the very same allele - seeding the type A blood antigen - can be doinant, codominant, and recessive, depending on whether it is couple with O, B, or cis-AB allele. But the behaviour of A remains fundamentally the same, regardless (ironic?). A will always yeild A antigens. Labelling it as dominant, obsucres the understanding of this.

then there is another problem in characterising phenotype. you cannot always describe all aspects of a genetic condition as undiformily d or r. think of sickle cell anemia. if you think of this trait as circulation and survival, the functional hemoglobin is dominant. however, if the trait is malarial resistance, that non-sickly hemoglobin is recessive. if te trait is, oxygen physiology, than the functional hemoglobin is incompletely dominant. so basically, a trait is d or r depending on how you interpret the trait.

but many syndromes have many disctinct traits. like with the sickle cell anemia; circulation, oxygen physiology and malaria resistance are addressed independently, alhtough they are related genetically. dominance cannot accomodate this unity of features.
tay sachs disease is another example. is is recessive, incompletely dominant, or codominant, depending on the level of the phenotype.

doninance is descriptive, not explanatory. dominance embodies a linear model of causality, which emphasizes singular causes, which obsures the context and overstates the effects. it also raises the problem of articulating the role of two parallel genomes (this issue at hand here). dominance does not cause the full phenotype, but contributes causally. that would change the way were were taught (from Aa and AA to "allele D with haplophenotype potential of D". i don't think my biology teacher would have liked that). you would have to describe that the potential effect of a single allele is not overstated. it does not yield a full phenotype on its own. it contributes to only a hplophenotype (half-phenotype).

the d/r model, punnet squares and all that, shows that a phenotype is currently conceived as singular. yet a diploid phenotype is double. alleles are active in paits. diploid organisms are diphenic, each allele functions partly independently, in aprallel with its homolog. and then the phenotype is compound, two traits expressed in tandem and variously overlapping or intersecting.

so, in all, i'll stand by my previous post that "domiance" has to do with the biochemical behavior of the proteins that are the products of those particular genes.

Vetticus
(sorry it's so long. seemed like such an easy question...)

was going to edit out some spelling mistakes, but too many.... can't be bothered.

"At more advanced levels, the concepts of expressivity and penetrance help further modify the basic all-or-none model of dominance. Most students never encounter these qualifications." so sorry to hear that.

---

the same way mess is dominant over tidyness.

An allele is dominant if it's geneproduct produces something that makes the recessive alleles geneproduct irrelevant. Ex. allele that makes a cell produce a pigment is dominant over an allele that encodes "no pigment".

Or, a more visible example; if a spouce makes a mess around him/her, visitors will not see the effect of the other spouse beeing very tidy.

---

A simpler definition may be, if the heterozygote(Aa) is showing the wild type phenotype(AA), then (A) is dominant over (a)

---

I think that if you would like i simplish definition of how it works, consider the following example,

a promoter region only needs to have a single transcription factor (or any other protein) bound to it to allow production of the relevent protein, which in turn is enough to show a phenotypic effect, in this case even if one allele expresses no protein or a slightly different version, the dominant allele can in any case already bind the promoter area and induce transcription of the relevent protein,

in this case the dominant allele gives a product regardless of the other allele. Its is a similar case in most dominant protiens where only a small amount of protein is needed give the phenotypic effect and thus should one dominant allele provide this the recessive allele is masked.

Ok i also know that there are slightly more complex scenarios than the one described but for a simple understanding that shouls suffice, as with most genetic effects this can occur at various stages during transcription, translation, protein modification, and it would be impossible to discuus each scenario individually

---