to dominant or not to dominant that is the question

[Paragonidae]

what makes the Dominant allele dominant???

[MrMistery]

actually a very good question. It really depends on the gene in question. Often a gene will direct the formation of an enzyme, and if the person is heterozygous half the normal quantity of product is enough to do the job.

[Paragonidae]

so does it mean that the recessive allele isn't even functioning AT ALL? that cannot be otherwise we wouldn't have homozygous recessive?

[alextemplet]

Yes, it means the recessive allele normally isn't functioning at all. Homozygous recessive means that none of the required enzyme is being produced. The lack of this enzyme causes the recessive phenotype.

[Paragonidae]

so theoretically an all homozygous recessive genotype is incompatible with life?

[mith]

No, think of Huntingtons disease which is auto dominant. In this case, the recessive gene is the "good" one.

[Paragonidae]

but if there is a gene (an allele) that doesn't produce its product (polypeptide chain) than how can it be compatible with life?

[mith]

A gene produces a protein. A mutant gene also produces a protein. But assume I didn't tell you which was which. Who is the mutant and who is the original? Who isn't producing "its" product?

Point being there's no inherent product that a gene is supposed to produce. It produces what it produces and utility of this product is selected for.

[Paragonidae]

so a recessive allele is basicly a mutant gene?
ah... well I guess that is beyond the level of my understanding, but thanks for the quick replays.

[mith]

recessiveness and dominance is more a measure of gene expression than whether or not it is mutant.

[Paragonidae]

so that brings me back to the beginning, how is it that one allele is expressed over the other while if it the recessive one on its own (with its homologous) they are expressed, is it a question of quantitative effect where the dominant is expressed stronger and the recessive expression is insignificant in comparison and when there are two of them they make enough product to be expressed phenotipicaly. sorry for repeating this but it is really hard for me to understand. maybe you can recommend some reading material on this subject.

[mith]

I am actually not that familiar with the actual mechanisms(and you probably don't need to know them) but there can be complex interactions in determining why or why not one is expressed.

For example you mentioned where there's the situation where dominant expresses a functional and recessive expresses an non-functioning

then there's one where dominant expresses a product that inhibits recessive

complete dominance is actually a simplified version of gradients of dominance.

more here

http://www.pubmedcentral.nih.gov/articl ... id=7297851