Question

In many ways a mule is a superior animal to the horse or donkey. They are often stronger and can jump higher than either of their parents. Why then are they considered to be a "weak hybrid"?

They are sterile.
Their offspring are progressively weaker.
They do not live as long.
They are more prone to disease.

Answer 1

They are sterile.

Step-by-step explanation:

A mule is the offspring of a male donkey (jack) and a female horse (mare). Thus the number of chromosome of the donkey and the horse are quite different. A combination of the horse chromosome which is (64) and a that of the donkey chromosome which is (62) gives rise to 63 chromosomes in a Mule.

This major difference in number,forms and morphology typically prevents the chromosomes from pairing properly and producing successful embryos, as a result of this aftermath effect, it renders majority of the mules to be sterile.