http://www.sciencemag.org/news/2020/12/how-cats-get-their-stripes

A mutated gene gives this tabby blotchy spots instead of the typical stripes.

When Rudyard Kipling told how the leopard got his spots, he missed the mark. Leopards have “rosettes”; spots are for cheetahs, says Gregory Barsh, a geneticist at the HudsonAlpha Institute for Biotechnology. But whatever you call the markings, how wild cats and their domestic counterparts acquire them has long been a mystery. Now, Barsh and his colleagues have found an answer. In so doing, they have shown that a 70-year-old theory explaining patterns in nature holds true for fur color in cats, and likely other mammals as well.