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I don't see any possible evolutionary disadvantage in situs inversus(the condition where major organs are mirrored from their normal positions) when people with situs inversus can have a normal healthy life.

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    $\begingroup$ Can you expand this question by adding a definition of situs inversus and maybe some material that backs up your claim of no adverse effects? $\endgroup$ – user137 Apr 2 '15 at 16:16
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    $\begingroup$ if there is no evolutionary advantage or disadvantage then its frequency is only the result of genetic drift, given that it is a recessive trait (according to wiki) then it is not surprising it presents rarely. $\endgroup$ – rg255 Apr 2 '15 at 16:21
  • $\begingroup$ downvoting because you give no reasonable justification for you assumption that a neutral trait should be present in half of the population. Also, some health issues appear to be tentatively (accroding to wiki) linked to situs inversus. $\endgroup$ – rg255 Apr 2 '15 at 18:56
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    $\begingroup$ I'm voting to close this question as off-topic because it is based on a faulty presuppositions (that situs inversus is completely benign, and that even neutral phenotypes should be evenly distributed throughout a population.) $\endgroup$ – anongoodnurse Apr 2 '15 at 23:09
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    $\begingroup$ The condition is autosomal recessive so not only is there only a 1 in 4 chance that offspring to two carriers will be affected, it also requires two people with the genotype (heterozygous, unaffected carriers or homozygous, affected) to mate, and that makes the chances less likely. Also there may be many more carriers in the population, so the allele(s) responsible may be in the population at a much higher level that we are aware of... As even then 75% of the Heterozygous/Heterozygous offspring will have the phenotypically-dominant trait. $\endgroup$ – AMR Aug 20 '15 at 2:52
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In the Wikipedia article linked to in the question, one will note that there are several medical conditions associated with situs inversus, including congenital heart disease, primary ciliary dyskinesia, and Kartagener Syndrome. These can easily lead to reduced fitness for reproduction, especially PCD/Kartagener Syndrome, as one effect is male infertility due to lack of functional cilia on sperm.

Therefore, there likely is a negative pressure on the alleles responsible for this condition, explaining why it is so rare.

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    $\begingroup$ When I was a post-doc my mentor used experimental manipulation to create the equivalent of situs inversus in C. elegans embryos. The embryos hatched and grew into "left-handed" or "anti-worms." He published that as a single-author letter to Nature in 1990, and had a series of grad students working on isolating mutations in handedness. It turns out that the main way humans with situs inversus are identified is at fertility clinics. Sperm motility is cilia-based, and functional Fallopian tubes have cilia that move the just-released ovum down to the uterus. $\endgroup$ – mdperry Apr 2 '15 at 18:40
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If a polymorphism is neutral and has no effect on reproduction, it will not necessarily present itself to the population in a 1:1 ratio. For instance, for a single-gene polymorphism with two alleles in the absence of evolutionary forces like selection and gene flow, the trait will be at Hardy-Weinberg equilibrium, meaning the two different alleles will be present in some ratio (not necessarily 1:1) and will stay in that ratio from generation to generation. Of course, I'm not an expert on animal physiology and I don't know the genetic basis for situs inversus, but either way two equally fit traits may still be present in the population at very different ratios as there is no force pushing them to be present at equal ratios.

Traits undergoing density-dependent selection, on the other hand, such as sex, are under selective pressure to exist in a 1:1 ratio. There are unlikely to be any such forces acting on situs inversus vs situs solitus

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I am an eighty year old woman with Kartageners Syndrome. This means, I have bronchiectasis which was diagnosed in 1948. The disease was isolated in my left bottom lobe. I also have chronic sinusitis. I think cilia is lacking or if not lacking at least too short to be effective. For most of my childhood I was unwell but after age eighteen I seemed to be quite a bit better and I have lived a healthy life. It is only recently that I have had health problems relating to Kartageners and I tend to think my age is now contributing to lung infections and therefore the quality of my life is deteriorating.

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    $\begingroup$ @AliceD Sorry, but this answer is on topic. Kartageners Syndrome is a disease associated with situs inversus, and I believe that the purpose of the post was to refute the claim made by the OP that situs inversus is benign. $\endgroup$ – AMR Aug 20 '15 at 2:26
  • $\begingroup$ @AMR - you are right. Thanks. I flagged this answer to alert the mods that they should be dismissing my previous flag. Thanks again. $\endgroup$ – AliceD Aug 20 '15 at 2:31
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    $\begingroup$ @AliceD No worries. While I was familiar with situs inversus, I was not familiar Kartageners, but after looking it up, I got a clearer picture of the posters intention, and how often do we actually get to hear from someone who actually has complications from a 1 in 10,000 disorder (that is aware of it). It seems that the unfortunate thing for this poster is that it was partial and not total, as it seems that total reversal has the same phenotypic behavior as the majority of the population, except in rare occasions. It is only those partially affected that tend to have health related issues. $\endgroup$ – AMR Aug 20 '15 at 2:48

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