Re: What is the difference between horses, donkeys and zebras besides the skin?

Hello,
  I imagine everyone at some time or another has wondered about the
difference between horses, donkeys and zebras.  To grasp an understanding
of the difference it might be best to begin with a dictionary definition
of exactly what a horse is and comprises.
  The horse is a hoofed, herbivorous MAMMAL of the family Equidae,
the single living genus of which is Equus, which includes the domestic 
horse, the wild Przewalski's horse, the ASS, and the ZEBRA. All are swift, 
plains-dwelling herd animals with teeth adapted for grinding coarse
grass. The species can interbreed (see MULE), but offspring are usually 
sterile. The modern horse evolved in North America (where it later became 
extinct) and spread over the world. It was hunted by early humans
and domesticated by Asian nomads in the 3d millennium B.C. The fast, light 
southern breeds may have originated independently of the heavier northern 
draft breeds, and the small breeds called ponies may have evolved from a
wild European race. Until the mid-20th cent. horses were widely used for 
warfare, agriculture, and transportation.
   Now to really get a grip on the differences between these animals
it would be a good idea to do a chromosome analysis. 
A comparison of the DNA of different organisms shows how closely related 
organisms are to each other. Since for each species the DNA information is 
organized in a characteristic number of molecules called chromosomes,
and the information on these chromosomes is rarely reorganized (although it 
does sometimes happen), the number of chromosomes is a reasonable indicator 
of the relatedness of two simliar species. 
Sometimes the DNA information is reorganized. Chromosomes can sometimes 
fuse with each other or can exchange chromosome "arms". When this happens, 
DNA information is not always lost, but it is a bit mixed up. This
sort of rearrangement may not cause problems for the individual who 
carries the change --- as long as all the DNA is still present.  
So what does this mean for our friend the Mule??? Well, the Mule  is the 
product of two different species (a horse and a donkey) mating with each 
other. The fact that these two different types of animals can mate and 
produce viable offspring tells scientists that horses and donkeys are 
closely related. However, mules are always sterile. Why?
Horses and donkeys have different chromosome numbers. The fact that horses 
and donkeys have different chromosome numbers tells scientists that these 
two are different species.

For the mule, for the most part, having parents with different chromosome 
numbers isn't a problem. During mitotic cell division in the mule, each of 
the chromosomes copies itself and then distributes these two copies to the
two daughter cells. In contrast, when the mule is producing sperm or egg 
cells during meiosis, each pair of chromosomes (one from Mom and one from 
Dad) need to pair up with each other. Since the mule doesn't have an
even number of homologous pairs (his parents had different chromosome 
numbers), meiosis is disrupted and viable sperm and eggs are not formed.

So, there you have it, I hope you are satisfied with this answer.

June Wingert
Baylor College of Medicine
Center for Comparative Pathology
Houston, Texas   77030