Why are mules sterile?

Answer 1

First thing you have to realise is that all mules are NOT sterile. Most mules certainly are, but there have been numerous authenticated accounts of mules giving birth to perfectly healthy foals.

Next thing to do is to ignore those people saying that it is because of different or uneven chromosome numbers or because they are different species. Numerous species are perfectly interfertile despite being far more distantly related than horses and donkeys and having far more widely differing chromosomes numbers.

Horses and donkeys are simply different members of the same genus. Cattle and bison on the other hand produce perfectly fertile calves and are members of DIFFERENT genera. That makes them about as closely related as humans and chimps. They also differ by 3 chromosome pairs IIRC, as opposed to the single pair difference between horses and mules.

So why are mules 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 only half the required genetic information, while other traits will be overdeveloped because there are 3 or more genes where there should be only two. All mules will get pregnant if mated, but the embryo develops for just a few days and then dies because it lacks the correct genetic instruction to develop further.

As for why some mules, maybe one in a thousand, are fertile, nobody is quite sure.

Answer 2

Mules are hybrid. We breed a horse and a donkey and get a mule. The horse has 64 chromosomes and the donkey 62, which leaves the mule with an uneven number of chromosomes (63), which makes it sterile. Although, contrary to popular belief, they are not all female. You can have male mules, too.

Answer 3

The sterility is attributed to the different number of chromosomes the two species have: donkeys have 62 chromosomes, while horses have 64. Their offspring thus have 63 chromosomes which cannot evenly divide.

A female mule, called a "molly", has estrus cycles and can carry a fetus, as has occasionally happened naturally but also through embryo transfer. The difficulty is in getting the molly pregnant in the first place.

Answer 4

Because they are Hybrids. Mules are a cross between a horse and donkey. They have what is known as hybrid vigor. Mules have more strenth than a donkey and more endurance than a horse.

Hybrid crops are controversial to some people. Hybrid corn and grains do not produce seeds and some people think that makes poor farmers dependant on seed producers.

Hybrid chickens like the Cornish cross are used because they
are good layers, but the eggs will not hatch.

Answer 5

Yes, they are a cross between a donkey and a horse and are always born sterile female.

Mule trains were always led by a male horse. The female mules instinctively follow the male.

Answer 6

Mules are a cross bewtween two animals, I think a horse and donkey. Since these two animals are not the same species, the result is sterile.

Answer 7

Horse + Donkey = Mule

The mule is sterile because it cannot reproduce.

Answer 8

Becuase you are mixing two different species of animals (a donkey and a horse) the chromosomes won't match to create a fertile offspring. Therefore the mule won't have the reproductive system to have children.

Answer 9

the definition of a species is: when two animals come together to produce a fertile offspring, they are the same species.
a mule is able to be born because the 2 species (hores and donkeys) are very closely related with the same number of genes, but the offspring will not be fertile.
Tigers and lions can do the same thing, with the same result.

Answer 10

mules are a cross between a horse and a donkey and because they are different species they produce offspring that cannot mateor reproduce aka sterile.