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Based on my understanding of Knobil and Neill's Physiology of Reproduction. For the vast majority of autosomal genes, expression occurs from both alleles simultaneously. In mammals, however, a small proportion (<1%) of genes are imprinted, meaning that gene expression occurs from only one allele (some recent studies have questioned this assertion, claiming that the number of regions of parent-of-origin methylation in, for example, the human genome, is much larger than previously thought).

I'm confused, why would expression occur from both alleles simultaneously? Ex. if it was "Rr" - the expression would only occur from "R", as opposed to both alleles simultaneously?

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Definition of dominance and recessivity

By definition, an allele is dominant over another allele if the phenotype of the homozygotes for the dominant allele and the phenotype of the heterozygote are the same but not the ones of the homozygotes for the recessive allele. In other words, let $p_{ab}$, be the phenotype of an individual with alleles a and b, then

  • $p_{rr} = P_1$
  • $p_{rR} = P_2$
  • $p_{RR} = P_2$

, where $P_1$ and $P_2$ are two phenotypic describer.

Note btw, that very very few traits have their variance explained only by the genetic variance, even fewer are explained only by variance at a single locus and yet even fewer have a locus that show a simple dominance / recessivity pattern.

Expression

If needed, please check gene expression.

The fact that an allele is dominant over another does not mean that the other allele is not being expressed. In fact, it does not say anything about the molecular mechanism by which this dominance pattern is achieved. It is possible that in some cases, if the recessive allele was not expressed at all, then the resulting phenotype would be $P_3$. It just so happen that most of the time, both alleles are being expressed.

Imagine for example a case of a locus coding for a black pigment. Let's consider haplotypes that where the gene is missing (copy number variation). Let's refer to such haplotype with 0. 00 are white. 0r individuals are light grey. rr individuals are grey. The allele R causes the expression of a lot of pigments so that both rR and RR individual are indistinguishably black. The allele r is still being expressed in heterozygotes rR even the phenotypes of rR and RR individuals are the same.

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  • $\begingroup$ So both genes are being expressed as in "Rr", but because "R" is dominant, only, "R" is demonstrated via the phenotype. $\endgroup$ – jackmann Feb 26 '18 at 1:48
  • $\begingroup$ Yes, by definition of R begin dominant, the phenotype of Rr is the same of RR but r might be expressed (or not). See added example at the end of the post. $\endgroup$ – Remi.b Feb 26 '18 at 1:57
  • $\begingroup$ Note btw, that very very few traits have their variance explained only by the genetic variance, even fewer are explained only by variance at a single locus and yet even fewer have a locus that show a simple dominance / recessivity pattern. WHAT DOES THIS MEAN? $\endgroup$ – jackmann Feb 26 '18 at 3:56
  • $\begingroup$ What part of this sentence is unclear to you? You should maybe check the word 'locus'. You probably should need to have a look at the post Why is a heritability coefficient not an index of how “genetic” something is? that offers an intro to quantitative genetics to get a sense of why I am talking about variance. $\endgroup$ – Remi.b Feb 26 '18 at 4:07
  • $\begingroup$ thanks asked a new question with my interpretation of the terms. $\endgroup$ – jackmann Feb 26 '18 at 4:12
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why would expression occur from both alleles simultaneously?

Why wouldn't it? What mechanism do you propose and have evidence for that would cause one allele to not be transcribed, based only on the phenotypic outcome of its expression?

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