* Different Species: Miniature horses are horses (Equus caballus), while jacks are male donkeys (Equus asinus). They are different species, even though they are closely related.
* Chromosomal Differences: Horses have 64 chromosomes, while donkeys have 62. This difference in chromosome number makes it difficult for their offspring to develop properly.
* Infertility: While a female horse and a male donkey can produce a hybrid offspring called a mule, the opposite combination (male horse and female donkey) is very unlikely to result in a viable pregnancy.
In summary: Even though miniature horses and donkeys are related, they are not compatible for breeding due to their different species and chromosomal structures.