If an organism is a dominant homozygote for a trait, are both alleles expressed or just one? If just one, which one? No co- or incomplete dominance situation things, just AA or Aa resulting in one trait and aa another. Maybe it's a simple question but I couldn't find anything addressing it. Thanks!

If both are being used to make the protein which contributes to the phenotype, then I'd say yes.

Forgive me for not having all the answers.

 

A recessive allele may still be transcriptionally active, but the product is different such that it doesn't follow the same biological pathway to expression. For example sickle-cell is recessive even though heterozygotes have both types of hemoglobin.

 

However if the protein product is the same, which it should for homozygotes, then I would think that both should be expressed unless one copy is silenced genetically.

Dominance is a description of the phenotype in relation to the genotype and not to its regulation. It is commonly assumed that normally (with few exceptions) diploid organisms express both alleles at a given locus. However, there is now evidence that there are more regulatory mechanisms that may result in monoallelic expression/ This differs from locus to locus.

Edited July 24, 201510 yr by CharonY

I was going to bring up X-inactivation, but that rules out recessive-dominance since only one allele is expressed for a given region of tissue.

Edited July 24, 201510 yr by MonDie

X inactivation is a special case of high-level regulation (by compacting the chromatin) but does not really seem to be the question here. That being said. there are still genes being expressed in the inactive chromosome.

The point is that recessive-dominance almost assumes that neither is silenced, unless there is some mechanism that consistently silences the recessive copy, thus making it recessive. If the dominant allele can potentially be silenced, then it isn't really dominant.

Occam's razor would have it that both products are being produced.