Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. Sterility in the mule comes from the same reason. A mare mule, called a "molly," has estrus cycles and can carry a fetus, as has occasionally happened naturally but also through embryo transfer. The difficulty is in getting the molly pregnant in the first place. Nature prefers the chromosome match-up to occur when the jack is the sire and the mare is the dam. It has been known for people to let a stallion run with a jenny for as long as six years before getting her pregnant.