Why can't mules have children?

Mules are hybrids of horses and donkeys, which can be divided into male and female. Male mules and most female mules are infertile because the chromosomes are not paired (63), and germ cells cannot undergo normal division (that is, meiosis). The female mule has sexual function, and the uterus can conceive embryos, but the most difficult thing is to make the female mule pregnant. Strictly speaking, the offspring of male donkeys and mares are called horse mules, and the offspring of male and female donkeys are called donkey mules.

The number of chromosomes of both mules and mules is abnormal because donkey chromosome 2 is homologous to horse chromosome 1, donkey 13 and horse 11, donkey 14 and horse 13. Donkey 16 and donkey 25 add up to horse 5, and horses have 32 pairs of chromosomes, while donkeys have 31 pairs. Although horses and donkeys are closely related, their gametes can not only combine with each other, but also develop into complete individuals, but the number of chromosomes in their offspring is 32-31, no matter how one chromosome is lonely.

But the mule is still lucky, because if this is the number of chromosomes in a horse's cell (or monomer 2n-1 in the case of a horse), it basically cannot survive, and for diploid species that reproduce sexually, pure haploids are better able to survive. On the other hand, if it is the composition of chromosomes in a donkey cell, it becomes trisomy, and the extra chromosome must have three. 32-31 mules are not really 31-to-1 relationships. In other words, during meiosis, it does not simply separate 31 pairs of chromosomes from each other, and then randomly assign the extra one. This is the trouble encountered by mules and even other hybrid animals, whose chromosomes tend to be randomly separated and combined freely in the process of separation. Because for the surviving trisomy, although its fertility is lower than normal, it is also quite high, with the exception of that extra chromosome, the rest can be paired normally and separated from each other.

For an unfortunate mule (in the case of a female mule), what are the chances of producing a normal egg, for example, in an extreme case where the egg happens to contain 31 chromosomes from a donkey or 32 chromosomes from a horse. Obviously, this probability is quite low (interested comrades can calculate it), of course, the actual situation does not need to be so extreme, after all, the mule itself can survive, that is to say, it can be a harmonious combination of horse and donkey chromosomes in the egg.

In the ancient books of our country, the offspring of male horses and female mules are called "Ji", and the offspring of male donkeys and female mules are called "". The typical example of domestic female mules giving birth to foals is the report that Zong Enze of Lanzhou Animal Husbandry Research Institute of Chinese Academy of Agricultural Sciences has carried out systematic research for many years, totaling about 13.

There are also many reports abroad that mules give birth to foals. In 1903, a mule in Estanz, Africa, gave birth to a foal. In 1913, a female mule mated with a male donkey on the island of Sabrus and gave birth to a male colt. (still using a male donkey), and gave birth to a foal. In 1923, a female reel was mated with a multiplicative Berber horse in Morocco, which was similar in appearance to her father, the Berber horse. In 1927, a female mule was mated by a male donkey in Nebraska to give birth to a colt, and in 1939 in Arizona, a female mule was mated by a male donkey to give birth to a colt, exactly like her mother, a mule.