"Mules were the same way, I'd been told"
Public DomainMule - Credit: Dario u

A dhampir can only reproduce with a Moroi; likewise, a mule (the spawn of a male donkey and a female horse) is unable to breed with another one of its kind. It does happen that a female mule will produce offspring with a purebred horse or donkey, but this is an extremely rare occurrence, since the three species have a varying number of structurally different chromosomes that don't easily pair up to create successful embryos.

Rose's comparison of dhampirs with mules also happens to be appropriate in another way entirely, as both inherit the most advantageous traits of their composite species with few weaknesses. Mules are tough creatures that are calm, patient and known for their endurance, long lives and the fact that they are far less likely to stumble than a horse; they owe these characteristics to their donkey genes, while the speed, intelligence, strength, courage and vigour of a mule, as well as the animal's marked lack of obstinacy, can be attributed to its horse DNA.