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:
- 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).
- 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.