How do I tune the incidence of gene expression in my fictitious population?

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Premise

When designing new species, or even creating alternate reality scenarios, I often find myself desiring some trait(s) to be shared among some group, but with various degree of probability. The story may sets requirements about prevalence, sex and heritability. The constraints may be quantitative (e.g. probability $p$ of incidence) or qualitative (e.g. traits $T_1$ and $T_2$ are mutually exclusive).

Generally I resort to genetics for plausible explanations and try to arrange a fitting scenario, but being unfamiliar with the matter, this is a tedious process.

For illustration purposes, some type of constraints I find myself facing are:

• trait W has some probability $p$ to occur given that the mother has it with certain probability, but no information is available about the father and the general prevalence among males is $p_m$.
• trait X should only be observed in females, but may be passed on to the next generation silently by males
• trait Y is not systematic, but if one individual has it, then it must follow that their sibling have it too
• trait Z is rare, but if both parents have it, their children are very likely to have it too.
• etc.

Again, these are just illustrations to give a general idea of the constraints that stories might impose. These constraints might play important roles in explaining the hierarchical or cultural relationships between different members of the group, reliance on the trait as a survival strategy or consequences of reproduction with individuals from other groups.

Often the precise probability desired is not a hard constraint. In other words clipping probabilities to simple values like (0%, 25%, 50%, 75%, 100%) might be enough if that simplifies the design process.

Question

What methodology could one follow, once the general constraints have been laid out (quantitative and qualitative) to attribute the desired traits to the population's genotype?

Another way of phrasing the question (from my naive point of view): if you were presented with a list of experimental observation about some (set of) trait(s) and asked to provide a genetic model to fit the observation, what methodology would you follow?

Important note

To limit the scope of this question, I am only referring to intermediate timespan constraints: heredity may be part of the constraints but mutations and natural selection can be considered to happen on timescales irrelevant to the story. The question of whether the trait gives competitive advantage or not should therefore not be relevant.

posted almost 6 years ago

Alexis‭

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