Closely related species in the genus Equus can be bred to produce
offspring that are viable and robust, but almost always sterile.
A female horse (Equus caballus) bred with a male donkey (Equus
asinus) will
yield a Mule...
+ 
= 
A male horse (Equus caballus) bred with a female donkey (Equus
asinus) will
yield a Hinny...
+ 
= 
A donkey (Equus asinus) bred to a zebra (E. burchelli, E.
grevyi,
or E. zebra) yields a "Zeedonk"...
+ 
= 
Each species has a distinct chromosome number:
- Horse, Equus caballus 2n = 64
- Donkey, Equus asinus 2n = 62
- Grevy's Zebra, Equus grevyi 2n = 46
- Plains Zebra, Equus burchelli 2n = 44
- Montain Zebra, Equus zebra 2n = 32
The hybrids are viable because their genes--housed on chromosomes that appear to
have undergone major physical rearrangement (evident in the synteny of their chromosomes) during the adaptive radiation
of Equus species--are largely homologous. They have all the necessary genetic information encoding normal devlopment
and body function. This can be shown via chromosomal
hybridization in which chromosomes from different species are allowed
to pair as if during metaphase.
However, because the chromosomes have changed so much during Equus
evolution, the chromosomes cannot pair properly during meiosis to allow
crossing over and successful segretation of homologs into new daughter
cells. Hence, the hybrids are almost always sterile, as they cannot produce viable gametes.