system
Given the following post, respond as if you're commenting on Reddit:
human
What makes a Zebra, A Zebra, and not just a differently colored horse? How far apart are they genetically?
gpt
They have different numbers of chromosomes. A donkey has 62 chromosomes, a horse has 64 and the zebra has 32 (or 46 depending on species). Even within zebras there are different species, so keep this in mind. Viable hybrids are possible (think mules or hinnies for horseXdonkey crosses). These hybrids are called zebroids; zedonks are well-known. 
A hybrid has a number of chromosomes somewhere in between the sire and dam. This chromosome difference makes female hybrids poorly fertile and male hybrids generally sterile due to chromosome segregation problems during meiosis (a phenomenon called Haldane's Rule). 
As an example of this difference in chromosome number, but similar overall gene content, zebras have two longer chromosomes that contain similar gene content to six horse chromosomes. Overall, most of their genes are the same, which is why they can interbreed. But because they're packaged differently into chromosomes, such that a bunch of horse genes on chromosome 5 may be split acros two smaller chromosomes in zebras, they produce infertile offspring.