Why can’t a mule have children?

Answer

This has to do with the chromosomes: the different pieces of DNA that contain all the information an animal needs to develop, grow and live.

Just like any other mammal, a mule (with a horse as mother) and a mule (with a donkey as mother) receive half of the chromosomes of the father via the sperm at fertilization, and the matching half of the father’s chromosomes via the egg. mother’s chromosomes.

Now a horse has 64 chromosomes and a donkey only 62. Mules and mules thus have 32+31 = 63 chromosomes in total. This is a big problem, because in order to reproduce, the sperm or egg cells of these animals must only contain half the normal number of chromosomes. Since 63 is an odd number and therefore not divisible by two, mules and mules cannot produce normal sperm or egg cells and thus remain childless.

Very exceptionally (this means a few dozen cases in the whole world) sometimes a fertile cross is found, often this is a female mule. There are unfortunately no fertile male mules or mules, so mules are still unable to reproduce.

It is very remarkable that when a fertile mule is crossed with a horse stallion, the foal is again 100% horse, and not half horse and half mule! In one way or another, the eggs of the mother (the mule) were able to separate the chromosomes of the own father and mother and decided to continue working only with the (half) set of the mother, in this case a horse mare. Also fertile mules only give the chromosomes of their own mother, so mule x donkey stallion = 100% donkey foal!