The Mathematical Formula for Designer Babies with Tiger Stripes or Leopard Spots

The leopard and the jaguar are renowned for their lovely spotted coats, and tigers for their stripes. Math and genetics show us how we can get the same things for our own offspring — along with why no striped animals have spotted tails.

As weird as it may seem, one model describes almost all mammal coloration patterns. All it takes to make an animal a certain color is the interaction of a couple of chemicals with the skin. One chemical stimulates melanin, causing darker coloration in the skin and fur of mammals, while another keeps melanin from being produced. These spread outwards through the body of the animal in the same way in every mammal. Clearly, though, every mammal doesn't end up having the same coloration.

The key to the differences in coloration is the fact that the chemicals spread outward in waves at different phases during the gestation period. Some start their move when the embryo is still tiny. Some start when it's nearly fully grown. If the animal is tiny, no pattern will form, which is why there aren't a lot of tiger-striped mice out there. If it's huge, the chemicals jumble outwards and back until they form a uniform color. This is why there aren't any tiger-striped elephants.

And tigers? Their chemicals move out at just the right time to form a series of peaks and valleys that lead to striped patterns on their fur. Leopards, though smaller than tigers, get hit with the waves at an embryonic stage at which they're a little bigger than the tigers. The chemicals interfere enough to form spots on their bodies. Giraffes get hit at a bigger stage and form the large brown patches that we see on them.

The Mathematical Formula for Designer Babies with Tiger Stripes or Leopard SpotsS

But it's not just the size of the embryo that determines the color, it's the size of the individual body part that the chemicals travel through. When moving from the body of a leopard or jaguar embryo to the tip of the tail, the waves of chemicals move from a very large surface to a small one. Since smaller surfaces develop a pattern that forms stripes, cheetahs, for example, have spotted bodies and striped tail tips. And unless someone could design a tiger with a tail larger than its body, no striped tiger could have a spotted tail.

But this equation gives us a clue as to how to play god (which always turns out well.) Human babies are a boring, uniform color. If their pigmentation could be started at just the right time, we might birth babies with tiger stripes, or leopard spots. That has to be worth an experiment or two!

Top Image: Colin Hines Photography

Second Image: Malene Thysson

Via Popmath.