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Say I have a disease that is autosomal recessive. If one was heterozygous for this trait, could the recessive gene still be expressed?

I know sickle cell anemia has a heterozygous advantage so it must be possible but what are the conditions for this to happen and also to what degree?

EDIT: This isn't asking how a gene is recessive or dominant in a molecular level.

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    $\begingroup$ Possible duplicate of What makes a gene dominant or recessive $\endgroup$ – Tyto alba Apr 10 '17 at 17:22
  • $\begingroup$ I disagree that the post is a duplicate. This post has to do with the semantic of dominance interaction. I think the OP did not realize that for talking about dominance interaction one must chose a phenotypic trait of interest. The other post has to do with the molecular mechanisms yielding to dominance interactions between alleles. $\endgroup$ – Remi.b Apr 10 '17 at 21:28
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    $\begingroup$ Retracted my vote. $\endgroup$ – Tyto alba Apr 11 '17 at 7:57
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Short answer

The concepts dominance, additivity, epistasis, recessivity and others are all specific to a given phenotype. Here you are getting confused because you are considering two two different phenotypes at once.

Slightly longer answer

By definition, if an allele is recessive, then in the heterozygote state, the phenotype of interest is just like the homozygous dominant. An important point in the above sentence is "the phenotype of interest". A given locus may show pattern of simple dominance-recessivity for a given phenotype but may show a completely different pattern for another phenotypic trait.

Let's consider your example of sickle cell anemia. For the phenotype which is the disease, sickle cell anemia has a simple dominance-recessivity relationship. However, for the phenotypic trait which is fitness (yes, in quantitative genetics, fitness is often modelled as a simple phenotypic trait), sickle-cell anemia shows (environment dependent) heterozygote advantage. Also fitnesses of the two homozygotes differ greatly.

Note that, in reality, very few loci show cases of perfect recessivity dominance.

EDIT

You are considering two (or even three) phenotypes.

First Phenotype: Fitness

Fitness shows overdominance (with highly unequal fitness for both homozygotes).

Second phenotype: Disease

Double mutant are sick, the others aren't. Perfect dominance-recessivity (although if I am not mistaken heterozygotes individuals have some restriction when it comes to scuba diving if I am not mistaken)

Third phenotype: Shape of hemoglobin

If the heterozygote hemoglobin is shaped just like the healthy homozygote, then there is dominance-recessivity. If the shape differs (which might be causing malaria resistance, I don't know), then there is some partial dominance going on.

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  • $\begingroup$ Ok, so the phenotype in interest (again in sickle cell anemia) is having the disease, which is having the point mutation causing hemoglobin to change shape, and this is simple dominance-recessivity like you said. What I am still confused about is why are heterozygous individuals more fit, because if the phenotype in interest is the change in shape of hemoglobin then in a heterozygote all cells should be normal, and doesn't protect against malaria. $\endgroup$ – Michael Zheng Apr 10 '17 at 23:38
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    $\begingroup$ This is a bit simplified but, in the heterozygotes the cells are not 'normal'. Both types of hemoglobin are expressed, it just doesn't result in the disease. Restated, one healthy allele is sufficient to prevent the disease phenotype. At the molecular level both alleles are expressed and that's why the recessive allele protects against malaria, it's still expressed. $\endgroup$ – Artem Apr 11 '17 at 0:00
  • $\begingroup$ @MichaelZheng See edit $\endgroup$ – Remi.b Apr 11 '17 at 0:01
  • $\begingroup$ Ok I get it, Disease ≠ Shape ≠ Fitness Thanks $\endgroup$ – Michael Zheng Apr 12 '17 at 23:29
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Others have already explained. I add some clarifications. You said that you know the molecular mechanism of dominance; revisiting them will answer your queries.

Gene expression means the formation of the gene product in whatever form it is active – protein or RNA. If an allele is recessive it can:

  • Not form the product
  • Form a non-functional product
  • Form a hypo-active product
  • Form less amount of product

In all but the first case the gene is expressed although it may not give rise to the product of your interest.

Phenotypes of homozygotes and heterozygotes can differ in these different cases but in a strictly Mendelian case, the expression (or non-expression) of the recessive allele has no effect on the phenotype if the dominant allele is present.

Now, one can also dig deep on what phenotype is. Phenotype is an observable manifestation of a trait but with latest technology we can observe even the DNA sequence. In my opinion (in a contemporary perspective), an allele is strictly dominant if a recessive allele present along with it does not perturb the regulatory/metabolic/signalling network at all.

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There are a couple of distinctions to make here. You can have an allele which is 'Phenotypically Expressed', that is it's visible at the organism level by some feature. In that case, no, recessive alleles by definition do not express that particular phenotype.

Now at the molecular level things can be different. You have the same two alleles and both can produce equal mRNA and protein levels, but they differ by a nucleotide/codon, which gives rise to the distinct functions and thus produce a distinct phenotypes.

At this molecular level both alleles are expressed and are co-dominant for the phenotype of measuring mRNA variants by sequencing and at the same time the organism presents only the dominant phenotype.

So it's important to consider in which context one is discussing 'expressed'. If a research paper is about a genetic disease, you'll often hear discussion of the 'recessive allele' and still have measurements of it's expressed mRNA. Everyone just knows by convention that recessive in the context of a human genetics disease paper means the particular disease the paper is on or a phenotype that is being discussed.

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