Why are mules born sterile?
That’s pretty complicated and to be quite honest we still don’t know the whole story. The basic problem is that after the ancestors of donkeys and horses split the chromosomes of BOTH species fused and then those fused chromosomes re-split. That has led to a situation where the information originally carried on a single chromosome in the ancestral species is now found on 2 different chromosomes in mules and 2 totally unrelated chromosomes in horses. Normally a developing foal embryo would receive two copies of each piece of genetic information: one from the mother and one from the father. In the case of a mule foal it receives a random number of copies. It will receive three copies of some pieces of information: 2 from the mule parent and one from the horse/donkey parent, or it will receive only a single copy from the horse/donkey parent. Because of that mismatch of genetic material the embryo has a hard time developing. Some traits it will lack sufficient information for since it has
