In nature, whether it is animals or plants, due to natural factors, it is rare to make a combination of different organisms and reproduce, which is both a natural law and limited by genetic thinking. There are exceptions to everything, and there are also animal hybridizations in nature, but this is extremely rare.
At the beginning of the progress of human technology, the phenomenon of artificial interference hybridization using the different characteristics of various animals and plants has long been commonplace, just like mules, which are the descendants of "horses and donkeys" hybridization, and mules must die if they are pregnant.
The word hybrid is not unfamiliar in daily life, for example, the rice we eat Chinese three meals a day is "hybrid rice". Originally, the harvest was very poor traditional rice, and after Mr. Yuan Longping studied hybrid rice, he solved the problem of food and clothing for the people in China.
The hybrid rice is selected to use two genetic differences, but their advantages can complement each other and hybridize with conventional rice. Hybridization between plants is also called grafting, and peach grafting plums is also a very routine operation, which can make the plums larger and less sour, and even make the plums have a peach fragrance.
The cross-biological combination between animals is called hybridization, such as zebras are the offspring of hybridization, as well as the "tiger lion beast" born after the combination of male tiger and female lion, the "cetacean dolphin" born of the combination of whale and dolphin, the "mountain sheep" born of the combination of male goat and female sheep, etc., these are the offspring of hybridization. When it comes to interbiological hybridization of human intervention, the longest-running hybrid is the hybridization of horses and donkeys, the descendants of which are mules.
Similarly, the purpose of artificial hybridization is to take the respective strengths of the two organisms and avoid the shortcomings of the two organisms. The horse's strength is large but the endurance is poor and the temper is relatively dry, while the donkey's strength is generally good, and the temper is docile, and the offspring mule produced by the combination of the two inherits the advantages of both sides, with great strength and endurance, and the most important thing is to have a good temper of hard work and resentment.
Of course, not all the offspring of horses and donkeys are mules. After the male horse and the female donkey are crossed, a mule is born, which is similar in size and strength to the horse; while after the male donkey and the mare are crossed, the mule is born, and the size and strength are more like donkeys. It is not difficult to see that the difference between mules and mules is still quite large. The offspring of horses and donkeys are more mules, not only because the mules have great strength and good endurance to become servitudes, but also because the fertility rate of horse mules is only about 30%, while the birth rate of mules is more than 70%.
There is a well-known saying in "The Severed Queen": All the gifts given by fate have long been secretly marked with prices!
The mule born of the combination of horse and donkey, combined with all the advantages of both sides, can be said to benefit mankind and help people to do more work without complaint. But everything comes at a cost, that is, mules are highly sterile, and this "infertility" is a genetic attribute that is difficult to reverse. In the final analysis, the inability of mules to procreate is the cause of "horses and donkeys".
The offspring between higher animals are mostly conceived by fertilized eggs, which is formed in a way called "meiosis". First of all, take us humans as an example, both men and women have 32 chromosomes, and the chromosomes in germ cells are half of the somatic cells, that is, after the parents are combined, the offspring will have half of the chromosomes of both parents, form a pair, and recombine into 32 chromosomes in the human body.
The number of chromosomes varies between different organisms, but many offspring of cross-biological hybridization rarely appear "highly sterile", mainly because the chromosomes in the germ cells of both parents can be just formed to form pairs, while the offspring produced by horses and donkeys are called infertility, the main reason is that the chromosomes between the two cannot form a complete pair.
Horses have 64 chromosomes, 32 in germ cells, 62 chromosomes in donkeys, and 31 chromosomes in germ cells. When the horse and the donkey are combined, the mule's chromosome is 32+31, that is, there will be one chromosome that cannot be paired, which is why the mule is highly sterile.
After the birth of the mule, it can be said that it has become the strongest laborer, and it is generally used to carry heavy objects to take the most difficult road, such as driving things up the mountain. Mules, whether they are driving people or carrying heavy loads, and complete remote transportation, are able to complete tasks so well that mules are constantly "made" artificially. Because mules, although male and female, are highly sterile, the combination between this species of organism cannot naturally produce offspring, and mules mostly artificially allow horses and donkeys to complete breeding.
Again, there are exceptions to everything, although the probability of mule pregnancy, theoretically only about one in four billion, but it also proves that the existence of mule pregnancy is a fact, and our country has such an example. It was in 2017 that the mother mule in a farmer's family in Weichang Mongol Autonomous County, Hebei Province, China, successfully gave birth to a mule cub, and all the vital signs and actions of the cubs were normal. Such successful cases are often prone to cause sensations in the surrounding areas, and the media reported on them at that time.
In fact, the successful birth and birth of cubs by mules has been recorded in ancient China. In the records of relevant materials, in ancient China, the descendants born of "stallion with female mule" were called "foals"; and the offspring born of male donkeys with female mules were called "horses". It can be proved that since ancient times, the low probability of mule fertility is determined by genes, but it is an indisputable fact that mules can give birth smoothly, and "pregnancy must die" is not an absolute event.
The "high sterility" of mules is determined by genes, and the phenomenon of mule childbirth is rare for decades or even hundreds of years, but it is impossible to determine that mules are "absolutely sterile". Mules are originally the product of cross-biological combinations, and their biological properties are like humans in a certain field, even if modern medical technology is developed, genes and heredity are difficult to be fully interpreted or controlled.
Fertility is a natural thing, but under the hybridization to form a mule's high infertility, some people say that the sadness of the mule, but in fact can not be so arbitrary, whether for the natural world, or for the field of human technology, biological diversity is better development. In the end, there are two sides to everything, whether it is natural hybridization or artificial intervention, mules do have reproductive shortcomings, but if there is no cross-biological combination of horses and donkeys, then there will be no mules in the world.