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I am trying to work out genetics for a race that has three types of healers. Having any healing ability is dependent on having the healer gene. What type of healer you are is determined by a secondary gene, currently indicated by R or M.

I'm using a 2-allele scenario here but would consider other options. The main point is that R and M appear roughly as frequently in the population while S is significantly rarer (think 10% S at most).

R and M have equal dominance. S is the result of double recessives (see Punnett square).

enter image description here

How could the phenotype for the five undetermined genotypes be established, presupposing that the following restrictions apply?

Restrictions:

  • There are only three possible phenotypes: R, M and S. There are roughly equal numbers of R and M phenotypes in the population, while S is a lot rarer.
  • R and M cannot be expressed simultaneously. Double dominants will override single dominants (so a person with RRMm will have phenotype R and a person with RrMM will have phenotype M.)
  • While RRMM might potentially result in non-viability, I don't really want that to be the explanation for all five, and making the black undetermined phenotypes equal S would throw out the balance of types.

(I will edit in any additional information needed as requested.)

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  • $\begingroup$ Why don't you want the non-viability of RRMM? Mother nature surely does not care; most non-viable embryos get naturally aborted very quickly, without anybody noticing anything unusual other than maybe a heavy period; in humans, about 40% of the products of conception don't even make it to implantation. (And more are lost after implantation. Only about 1 in 3 products of conception make it to full term.) $\endgroup$
    – AlexP
    Commented Apr 8, 2023 at 14:02
  • $\begingroup$ I'm okay with the non-viability of RRMM but not with the same answer for RrMm. $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 14:13
  • $\begingroup$ Since the current answers (and question) seem to rely on the (answers to?) another question, I have the idea this one ought to be closed until you have a clearer picture of the situation. Please let me know if I'm wrong about this, though. $\endgroup$
    – Joachim
    Commented Apr 9, 2023 at 14:29

2 Answers 2

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Epistasis in a 3 allele scenario:

While I'm apparently not as versed in genetic specifics as AlexP, I was already working on an answer involving something akin to Epistatis (not sure if that term is the best for my specific scenario, but it's close enough to get the point across) when AlexP posted his answer.

I'm also operating on your comment regarding "using a 2-allele scenario here but would consider other options".

Using a third allele that is epistatically linked to both of your original alleles produces something like this:

enter image description here

In this scenario, green (both light and dark) is one type of healer (A), blue (both light and dark) is the other type (B). Light blues and greens are simply dominant based on the primary A and B alleles. For the dark blues and greens, where the primary alleles A and B are equal, the third allele (C) acts as a tie breaker of sorts, through Epistatic-like relationships with the other 2 alleles. Dominant C provides dominance to the (B) allele, while recessive c causes dominance to the (A) allele. Purple is your non-healer group. Orange and red can be either non-healer or non-viable, depending on how you want to adjust the percentages.

I suspect that the orange areas would actually produce healer type B, in a realistic genetics scenario, but I leave that up to the discretion of the story teller, as assigning those to the B healer type would throw off the balance of the two types of healers in the population. It's unclear whether the difference would be too much to qualify for the "roughly equal numbers of ... phenotypes" restriction from the original question, which is why they are assigned a separate category in this answer.

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  • $\begingroup$ Roughly equal means they don't have to be exactly equal. A third gene involved sounds like a viable option. However, there are 3 healer types (2 being co-dominant) so I'm guessing in your scenario purple would be phenotype S? $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 15:15
  • $\begingroup$ @Solitaire Yes, in my example, purple equates to your phenotype S. If you assign red as non-viable, that leaves 62 viable combinations. If you also assign orange and both blues to phenotype B, you'll have 29 combinations that produce phenotype B, 25 produce phenotype A, and 8 produce phenotype C (your "S"), So you would have roughly 47% of the population as one healer type, 40% as the other healer type, and 13% non-healers. (Also, in true genetics, purple would likely also result in phenotype B, not a non-healer) $\endgroup$
    – Harthag
    Commented Apr 8, 2023 at 15:23
  • $\begingroup$ Ah, but that's where it gets interesting...since there's about another 8 (maybe more) genes involved in these people, and not everyone is a healer. $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 15:41
  • $\begingroup$ @Solitaire I look forward to those additional questions and complications. $\endgroup$
    – Harthag
    Commented Apr 8, 2023 at 15:43
  • $\begingroup$ worldbuilding.stackexchange.com/questions/245059/… $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 19:17
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  1. Incomplete dominance to the rescue.

    The relationship between to alleles R and r can vary from complete dominance (R completely masks r in the phenotype), to various degrees of incomplete dominance (R masks r anywhere from almost completely to barely noticeable), to full co-dominance (R and r blend in equal parts).

    In the specific case, let's say that R is almost but not quite completely dominant over r, and M is absolutely completely dominant over m. In this case, a RrMm individual will be mostly M phenotypically.

  2. Epistasis.

    The phenotypical effects of an allele can be very strongly dependent on what alleles are present at another locus (called the modifier). For example, in humans, a pair of genes (SLC24A4 and TYR) determine whether the eye color is blue or green, provided that the individual also has the allele of HERC2 which switches from black or brown eyes to blue or green eyes; if the individual has the normal allele of HERC2 they will have black or brown eyes no matter what alleles they have for SLC24A4 and TYR.

    In your example, it may be the case that the R phenotype can only appear if the individual in homozygous mm, that is, the M allele is epistatically dominant over R/r.

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  • $\begingroup$ I had a nasty feeling answering this question would rely on the answer to another question I haven't asked yet (since I can only ask a single question per post). Since R and M phenotypes appear roughly as often as each other, 1. is unlikely (unless R is significantly more likely to appear in the population, which makes S phenotype even rarer than intended). 2. requires a different question to be answered, so I'll see if I can run that through sandbox (be warned, this question is part of a MUCH more complicated system). $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 14:44
  • $\begingroup$ OK, if the R and M phenotypes are equally probable, there there must be a gene which chooses between R and M if the genotype is RrMm, just like in humans SLC24A4 and TYR choose between ble and green eyes if the HERC2 gene switches from brown to blue/green. Call it Ψ. Ψ and ψ are codominant, so that ΨΨ, Ψψ, ψψ are distinct; RrMm is R with ΨΨ or ψψ, and M with Ψψ. $\endgroup$
    – AlexP
    Commented Apr 8, 2023 at 14:54
  • $\begingroup$ I'm running the other question through sandbox now since this question is a sub-question of a much bigger question (but since I was told my questions were too open I was trying to trim it down). Short version: yes, there are multiple genes involved since healers aren't the only types around. Problem is I haven't determined how the other genes work for sure yet either. I might need to just throw numbers at it until it makes sense? (I know what HAS to be there but not totally clear on what MIGHT be there, if that makes sense.) $\endgroup$
    – Solitaire
    Commented Apr 8, 2023 at 15:06

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