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2006-07-10 05:48:44 · 21 answers · asked by Anonymous in Science & Mathematics Zoology

I know they are a cross between two species - horse and donkey. However, I am not sure why they come into being when other cross breeds can't (human/chimp say), and subsequently are born sterile.

2006-07-10 05:54:56 · update #1

21 answers

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 they are hybrids or because horses and donkeys 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.

Thirdly ignore those people saying that mules are clones. Mules are the product of sexual reporduction. If they were clones then by definition they would be either entirely horses or entirely donkeys.

And finally mules have perfectly functional reproductive organs so that isn't the reason.

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.

2006-07-10 14:17:20 · answer #1 · answered by Anonymous · 0 1

although donkeys and mules are part of the same family and therefore look similar and have similar external appendages that allow interbreeding, at a molecular level there is an incompatability. All animals have a set number of chromosomes, which are long molecules that are composed of DNA. Horses have 64 and donkeys have 62. To be able to produce an embryo which has the same number of chromosomes as its parent, sperm and egg cells halve their chromosome number. So for example, the horse sperm will have 32, and the egg 32 chromosomes. When two horses mate, the 32 from the male and the 32 from the female fuse in the fallopian tube to form a 64 embryo. The same is true of donkeys and all other organisms. Thus when a donkey sperm/egg (31) and a horse sperm/egg (32) fuse, it produces a 63 chromosome embryo of a mule.

The reason a mule is sterile is that it cannot divide its chromosomes neatly in half to produce sperm or egg that can fuse with another mules sperm or egg.

A mating between two mules will not usually result in a viable pregnancy. If occasionally, thru chance, the division of the germ cells that produce egg and sperm offer up a chromosomes that can "zip" together, pregnancy will be viable.

2006-07-11 06:37:01 · answer #2 · answered by Allasse 5 · 0 0

Horses and donkeys have a different number of chromosomes.

As a result mules have an odd number of chromosomes, that is, odd as in, not an even number.

To generate viable gametes (sperm or eggs), cells undergo a specialized form of division that reshuffles the genes and halves the number of chromosomes, this can only happen if the chromosomes are even in number or have some built in compensatory mechanism, like with the recombining portions of the Y chromosome. Mules don't have one unless there's been some kind of duplication mutation. The odd number of chromosomes prevents the formation of viable sperm or eggs.

There have been cases of mules breeding successfully, but these are exceptions caused by polyploid mutation of the gamete cells. Polyploidy is a doubling of the chromosomes which would allow for the creation of viable gametes. Many plant species came into being in this way, but without deliberate human help the odds of polyploid mules establishing themselves as a new species is unlikely.

2006-07-10 18:38:16 · answer #3 · answered by corvis_9 5 · 0 0

Mules are almost always sterile cases), as almost all hinnies are. . 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.

2006-07-10 12:58:46 · answer #4 · answered by Sqdr 3 · 0 0

A mule is the offspring of one donkey and one horse. Donkeys have 62 chromosomes while horses have 64. As a result, mules have 63 chromosomes. In order to produce offspring, a creature must have an even number of chromosomes (because each parent gives half their chromosomes to the offspring, and so the number of chromosomes they have must be divisible by 2).

2006-07-10 12:57:29 · answer #5 · answered by HM 2 · 0 0

mules are sterile because one parent is a horse and the other a donkey... genetically these animals have a different number of chromosomes so that the mule ends up with an odd number of chromosomes. Therefore a mules chromosomes can not be equally divided and so causes sterlility.

2006-07-10 12:56:41 · answer #6 · answered by diablo_rebel_trucker 1 · 0 0

as u say, they r a cross between a donkey and horse, but when that cross occurs something happens with the DNA or the "makeup" or the mule, meaning its reproductive system does not work properly. I don't quite know what it is though! sorry

2006-07-14 08:40:28 · answer #7 · answered by nervous 1 · 0 0

The horse and donkey have different chromosome numbers and the resulting mule has an uneven number so functional gametes cannot be produced.

2006-07-11 15:27:32 · answer #8 · answered by kano7_1985 4 · 0 0

Read the document at the link for an interesting article about hinnies (the reverse cross to mules)

2006-07-15 02:12:56 · answer #9 · answered by myrtguy 5 · 0 0

They are hybrid anmimals, crosses between horses and donkeys. All cross breed animals are sterile.

2006-07-10 12:52:24 · answer #10 · answered by Anonymous · 0 0

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