Bred as work animals for at least 3,000 years because of their ability to withstand harsh conditions, mules are the offspring of a male donkey and a female horse. The less common cross of a female donkey and a male horse is called a hinny. Hinnies are more horselike in appearance than mules, but are often considered inferior. Like mules, they are sterile, since their uneven number of chromosomes results in an incomplete reproductive system. Why are hinnies rarer than mules? Discuss
Source: The Free Dictionary