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In my world, there are three species from the Homo genus that make together a clade named the aquatic humans: Homo maritimus, Homo corpulentus, and Homo gigas (the names respectively mean "marine human", "corpulent human", and "gigantic human"). They are often respectively named "merfolk", "ogres", and "giants".

All these species have blubber (vascularized subcutaneous fat, the type of body fat that is the main thermoregulation system of marine mammals and penguins).

Aquatic humans can interbreed with anatomically modern humans, and the resulting offspring is sub-fertile (hybrid females are more likely to get polycystic ovarian syndrome, and endometriosis, and hybrid males are more likely to get oligospermia, and teratospermia, but most hybrid offspring can still reproduce).

So, I wonder if there is a real-life gene that potentially could mutate to make humans have blubber. The following features of this gene are:

  1. It is autosomal (non-sexual chromosomes, there are seven groups: group A which includes chromosomes 1 to 3, group B which includes chromosomes 4 and 5, group C which includes chromosomes 6 to 12, group D which includes chromosomes 13 to 15, group E which includes chromosomes 16 to 18, group F which includes chromosomes 19 and 20, and group G which includes chromosomes 21 and 22).
  2. Mutations tend to have incomplete dominance (in my world, the allele responsible for blubber causes vascularized fat only in the abdomen, the torso, and the neck when heterozygous, and it causes vascularized fat in both the trunk, and the limbs when homozygous).

Note 1: My merfolk do have feet and hands with digits, but they are both webbed (I want to go a realistic way). However, adult merfolk who do have neither gigantism nor dwarfism, nor who are morbidly obese, nor gaunt, are as massive as beluga whales.

Note 2: my giants, when adults, are as massive as polar bears.

Note 3: my ogres, when adults, are as heavy as the average silverback gorilla, but they are as tall as the average anatomically modern human NBA player.

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    $\begingroup$ Here's a list of genes and what they regulate. Pick one you like. they all have homologs in humans, even if the functions are slightly different. academic.oup.com/conphys/article/8/1/coaa082/5898274 $\endgroup$
    – DWKraus
    Commented Sep 30, 2022 at 1:25
  • $\begingroup$ Do we know what gene in, for example, seals causes blubber? If not, then this might be an impossible-to-answer question. If so, why can't you use that to solve your problem? $\endgroup$
    – JBH
    Commented Sep 30, 2022 at 2:07
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    $\begingroup$ How is blubber fundamentally different from fat? $\endgroup$
    – Rhymehouse
    Commented Sep 30, 2022 at 5:04
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    $\begingroup$ @Rhymehouse This encyclopedia says "Although most other types of adipose tissue contain small to moderate amounts of collagen, blubber is distinct in being greatly enriched in collagen and elastic fibers." $\endgroup$
    – Daron
    Commented Sep 30, 2022 at 7:48
  • $\begingroup$ @DWKraus I expect many other of the mammifereviolet4694 "what gene controls this?" questions can be answered the same way by looking at studies of the relevant animals. $\endgroup$
    – Daron
    Commented Sep 30, 2022 at 7:50

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Frame chalenge: Pobably it would not be a single gene that could give humans blubber.

Gabriel Iglesias identifies six levels of fat.

  • Big
  • Healthy
  • Husky
  • Fluffy
  • Damn!
  • Oh! Hell no!

Your larger folk are going to be somwhere in the range of the largest two. The aquatic versions will have difficulty on land, say like a walrus.

But note that the life expectancy of a human even up to Fluffy is seriously reduced. Iglesias himself was advised by his doctor that his death was going to happen in months if he did not lose some weight.

There needs to be adjustments, at a minimum, to the locations and manner the fat is deposited. It needs to be disconnected from heart disease, diabetes, blood pressure inbalance, stroke, cancer, and a couple others I am forgetting just now. Normal humans now who are up to the Fluffy level have highly elevated rates of all of these conditions.

Combining that with the other adaptations seems to indicate that it will need to be tweaks (or more drastic changes) to many genes. The difference between a water-adapated humanoid and a land-adapted one is going to involve changes to many genes.

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