Basically, why arent’s there any people with colouration like tortoiseshell cats, Dalmatian dogs, or Holstein cows?
In: Biology
The short answer is, humans do have “spots.”
“Piebaldism”, which is the appearance of white spots, does happen in humans. But it’s a rare condition (due to a mutation in a particular gene) that we didn’t select for. (For one thing, because it increases the risk of skin cancer. And back in the day, probably made people think you’re a witch).
Why we don’t have human equivalents of tortoiseshell or calico (calico is actually tortoiseshell + piebald) is a lot more complicated.
Tortoiseshell coloring happens only in female cats (or male cats with a rare sex chromosome disorder), because in cats the gene alleles responsible for determining whether the cat has black or orange fur are on the X chromosome, and only females have 2 X chromosomes, allowing for the possibility of 2 different hair color alleles at the same time.
However, you get the big patches of color rather than alternating orange and black hair or a color that is a mix of black and orange (brown?) because very early on, when the XX embryo is just a ball of cells, one X chromosome is “inactivated” in each cell pretty much randomly, and stays inactivated in all the other cells that cell gave rise to. So it becomes all or nothing, either 100% orange or 100% black. (The technical term is mosaicism. Not to be confused with a genetic chimera, BTW.)
In humans, the gene which determines the amount of melanin in hair is on an autosome (non-sex chromosome, IIRC it’s #16), so both of the color alleles are active in every cell in males and females, and you just kind of get the average of the two.
And then it just gets more complicated, because dogs, for example, also have a piebald gene, but it’s different than the one (KIT, IIRC) in humans and cats. And to explain how some of it works, you have start digging into genetic concepts like variable penetrance and expressivity, gene dosage… It’s more BIO 200 level stuff than ELI5.
Also, other animals have coloration patterns like agouti, where individual hairs have alternating stripes of pigment. And in some others pigment expression depends on temperature…
I couldn’t confidently tell you the evolutionary reasons why humans ended up with such “boring” coloration, but perhaps the fact we’re mostly hairless and originally evolved in Africa means lighter-colored patches would have been a bad idea.
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