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At school we were taught in quite simplistic terms. We were told that there is a dominant and recessive type of every gene and that if the dominant gene was present, the protein is produced and if it isn't, then a protein is not produced. We were just told that dominance was just an intrinsic property of some genes.

However, upon researching a little, I see that some genes are dominant because their corresponding allele simply doesn't make the protein but the dominant one does (please correct me if I'm wrong).

Does this mean that if someone is homozygous dominant for a particular gene, they produce double the amount of that certain protein as someone who has heterozgous genes or is there a mechanism that suppresses the expression of one of the dominant genes?

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Dominance is defined based on the phenotype

Dominance is defined based on a phenotype of interest. Pick a phenotype, say coat color for example. If genotypes AA and Aa have the same coat color while aa has another coat color, then A is domiant over a.

The concept hold even for sequence that do not produce proteins

The concept of dominance can be applied to any locus which variance explains phenotypic variance, whether this locus codes for a protein or not.

Most sequences that affect the phenotype are regulatory sequences that do no produce any protein. In such case, your intuition that dominance is the allele that produce more proteins really does not make much sense.

Mechanism

A whole lot of mechanism can yield to such dominance relationship. It is not uncommon that the recessive allele comes from a loss-of-function mutation. In such case, yes it is likely that the AA genotype produce twice as many proteins as the Aa genotypes. But this is definitely not the only possible mechanism. Note that whether the number of proteins produced of the AA genotype is twice the number of protein produced of the Aa genotype, won't change anything to the phenotype of interest by definition of what dominant mean.

Dominance is actually pretty unusual

Most phenotypic traits actually result from the influence of several loci and allelic relationship at these loci are rarely perfectly dominant. Perfect dominance exist in reality but is definitely over represented in intro classes as they are simple to understand study cases.

Fitness

Note that there is a positive correlation between how recessive an allele is and how deleterious it is. It does not mean that the recessive allele is necessarily deleterious but there is a tendency.

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  • $\begingroup$ Suppose that the allele for a curled tail is dominant (A). So if you had Aa for particular phenotype, does that mean that the loci and protein coding genes associated with the "a" allele are expressed as well as the ones associated with "A" but only the effects of the "A" gene is seen on the outside? Or does it mean that the expression of the genes to do with "a" is suppressed, possibly epigenetically? $\endgroup$ – Aswin Abraham Aug 24 '17 at 14:37
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    $\begingroup$ There are many possible mechanisms of dominance. It is not because a is recessive that it necessarily does not express anything (but it is possible). It can be, but not necessarily. The notion "seen on the outside" is a bit misleading. It could be possible that A is dominant but Ao individuals (individuals that have lost one copy of the gene on one haplotype) differ from the Aa individuals. So really, the mechanisms are multiple, the only that matters to the definition is that AA and Aa have the phenotype while aa has a different phenotype. $\endgroup$ – Remi.b Aug 24 '17 at 14:42

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