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A mule has 32 horse chromosomes and 31 donkey chromosomes. A total of 63 chromosomes. Normally, an organism would have two copies of one chromosome, one from the mother and the other from the father. However, for mules, they will get two different sets of chromosomes since the mother is a horse, and the father is a donkey. These genes will not exactly be matched sets, but they are still relatively similar (enough to produce the mule offspring in the first place). To reproduce, a process called meiosis will happen, in order to get one copy of each chromosome into the sperm or egg. During this process, the chromosomes will need to “match up” to another. Unfortunately, the donkey and horse chromosome are not similar enough to pair. Additionally, the mule will also have an extra unmatched horse chromosome, so sperm or eggs are unable to be made.

In meiosis, there's "crossing over" in which chromosomes of the same type exchange information. The jist of it is that donkeys and horses are similar enough that their chromosomes can produce functional offspring in mixed pairs, but the exchange of information between those pairs during meiosis renders the sex cells of that offspring incompatible. It's a state of fragility. Additionally, horses have 64 chromosomes and donkeys have 62. With rare exception, mules have 63. 32 come from the mother, 31 come from the father. What's left is typically described as an "incomplete" set of genetic directions, which further impacts their ability to produce viable offspring.

So at what stage does the development stop? Is there still a fertilized zygote?

Recombination and uneven numbers of chromosomes as an impediment to viable offspring mostly makes sense in light of preventing chromosomal matching during the fertilization process. So yeah, fertilization occurs, but the resulting zygote is essentially informational garbage that fails to develop appropriately to thrive.

It doesn’t change the original question much, but there are actually quite a few well documented cases of fertile mules. This page lists some older examples going back to the 19th century; some more recent (peer-reviewed) cases are listed in

• A fertile mule and hinny in China
• Key Aspects of Donkey and Mule Reproduction
• Fertile mule in China and her unusual foal
• Fertile mules
• A foal from a mule in Morocco

With dozens of instances being documented in spite of farmers actively trying to prevent mules and hinnies from breeding, it's likely that a fairly significant percentage (though of course a minority) of them are fertile.

Mules can reproduce - it's just extremely uncommon (only 60 cases recorded in the last 500 years). But, that being said, the other responses re: the genetic component fill you in on why that's the case.

https://www.npr.org/2007/07/26/12260255/befuddling-birth-the-case-of-the-mules-foal

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Is it like male down syndrome people are sterile and females have difficulty combining with 'normals' contribution?

It should be noted that there are organisms where populations differ in their karyotype while still being compatible. This is quite common among rodents, where population of the same species can be easily identified with their karyotype, but still produce viable and fertile offspring.

So I don't know enough about this particular case to say where exactly is the problem, but just having a different number of chromosomes isn't such a barrier as people often make. Yet, it is still likely related because chromosomes represent organisational units. During meiosis, there are multiple steps where the material is divided. An unequal distribution might possibly be the cause, but again I don't know enough about this particular case and I would have to speculate.

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They have trouble making sperm or eggs because their chromosomes don't match up well. And, to a lesser extent, because of their chromosome number. A horse has 64 chromosomes and a donkey has 62. A mule inherits 32 horse chromosomes from mom and 31 donkey chromosomes from dad, for a total of 63 chromosomes.

are there any other examples of this in nature?

do ligers have the same issue in terms of reproducing?

is this why a chimpanzee and a human couldn't reproduce?

The term "hybrid" exists for this very reason. It's to specify the product of two species, which is imbued by nature with DNA compatibility issues that either partially or entirely prohibit the free diffusion of genes into a receiving population. In short, that example of offspring (mules, in this case) is not "viable" by definition. In this case, because mules are infertile, there's not a reproductive vector for a horse population to have donkey dna in it, or vice versa... with extremely rare exceptions. In other words, "close enough to an absolute that we consider them distinct."

Ligers, Zonkeys, Jaglions, pumapards, coydogs, wholphins... there's plenty of examples of hybrids in nature. Chimpanzees have 48 chromosomes, humans have 46, so yes there is a similar problem, not to mention a world of ethical constraints on ever testing that in a labratory setting.

Interestingly, human chromosome #2 appears to be the product of chromosome fusion in our genetic history, which apears to be the explanation for how we "lost" a pair of chromosomes with respect to our closest cousins.

Thank you this is fascinating! Do they still present as either male or female in some way, even though they can’t produce sperm or eggs? Or are all mules physically non-binary in some way?

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> The term "hybrid" exists for this very reason. It's to specify the product of two species, which is imbued by nature with DNA compatibility issues that either partially or entirely prohibit the free diffusion of genes into a receiving population. In short, that example of offspring (mules, in this case) is not "viable" by definition.

This is not correct. Hybrid doesn't imply in any way that the offspring are "not viable by definition".

See the etymological origin of the word: https://www.etymonline.com/word/hybrid

or scientifically its scientific use, as hybrid is used for any crossproduct, such as a hybrid breeds, plant varieties, and also species. Hybridization is very common in plants and doesn't automatically lead to non-viability of the F2 generation. In fact, up until modern times with industrial agricultural techniques, doing something like that would be crazy (unless the plants could be propagated in other way), but nowadays seedless fruit is considered something desirable.

It just happens that when we consider species hybrids among animals we get infertility. But again, there are other kinds of hybrids and taxonomy is absolutely arbitrary. There are circular species, species groups and other kinds of things, that can breed between themselves without problem.

They have different amount of chromosomes? But they seem to be such closely related animals.

Are there other closely related animals that have different number of chromosomes? Like between duck and geese or tiger and lynx, for example?

Does this thing also happened with plants and fungi?

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>are there any other examples of this in nature?

It's pretty common, but doesn't always happen.

>do ligers have the same issue in terms of reproducing?

Ligers are generally fertile. Note that it's not a hard cutoff, there's a range between hybrids that are almost always fertile and those that are almost always infertile. Plenty of species fall in the middle ground.

>is this why a chimpanzee and a human couldn't reproduce?

This has never really been thoroughly investigated, so we don't actually know if hybrids are possible. Hybrid incompatibility is about more than just differences in chromosome number, though. Specific adaptations can make egg and sperm incompatible (related species with overlapping ranges sometimes have adaptations like this to prevent hybrids) and sometimes the details of genetics can make hybrids nonviable.

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Fair, but not really. I used a wrong word. While I agree that your response is correct in pretty much every respect, it highlights the consequence of rudimentary practices in a de-centralized field.

To the first point, "the term hybrid is used for this very reason, as juxtaposed with species." Obviously, I wasn't clear enough in my meaning that you felt the need to point that out, so I'll just own that mistake.

As to the rest, I'm sure you know "species" denotes "a population capable of producing viable offspring." So why would we ever use a different word? Outdated understandings lead to outdated naming conventions which persist after the science has surpassed the action. A hybrid, by definition in the context of biological evolution, is "offspring produced by more than one species." That's an explicit indication regarding the presence of distinct populations or a statement about the capacity to produce viable offspring. One of these criteria is affected, otherwise they should just be called the same species, sub-species or maybe even same ring-species from the very get-go.

Like taxonomy as you mentioned above, asserting primacy to the factor of a population is more on the arbitrary side of things. It's a blurred line in many respects. Viability, however, is not.

Most people tend to think of species as a label. A is this, B is that, etc etc. Consequently, they gloss right over the correlating feature that A is NOT B, B is NOT C, and so on. The underlying question is "Are A and B the same species, or of different species?" Well, it's science, and we can test that, or at least keep our eyes peeled for a test performed by nature. Are they able to diffuse alleles into a receiving population without impediment? That's a closed-ended question. While the answers to that tend to be found somewhere along a spectrum, it's a tighter constraint on subjective opinion, and is consequently the preferred approach. Veering the other way by calling mixed-plant offspring "hybrids" is, for lack of a better description, kind of missing the point in so many ways. That's a practice rooted in history, but not so much in the core of science, which is testability/falsifiability. Since the discovery of DNA and the commercial availability of DNA tests or other advances in tech/science, there's hardly an impetus to use that more-arbitrary decision making process as a place-holding crutch until such a time as a verifiable answer becomes possible anymore. We can police that up, we just typically don't.

We tend not to go back and re-label things accordingly chiefly because it conflicts with naming rights, plus the fact that it's a rather large undertaking that would require updating an entire field. That's pretty much it.

With respect to the viability of f2 generation hybrids, eh... re-classifying the parent generations in the manner described above erases the issue you present, which is essentially what a debate surrounding the relationship between humans and Neanderthal is all about. That all boils down to motivation or a sense of importance to "get it right" rather than a question of if hybrids are viable or not, or if our definitions need to change at all.

So yeah, "hybrid" absolutely does imply that the offspring are not viable. We just incur some leftovers because the history of taxonomy is loaded with examples that represented the best effort possible at the time, which in turn leads to a lot of people still doing it now.

> As to the rest, I'm sure you know "species" denotes "a population capable of producing viable offspring." So why would we ever use a different word?

A different word for what?

The term "species" predates the theory of evolution. If you study taxonomy and phylogenetics, you will quickly find out how arbitrary it is and that there are many exceptions.

> A hybrid, by definition in the context of biological evolution, is "offspring produced by more than one species." That's an explicit indication regarding the presence of distinct populations or a statement about the capacity to produce viable offspring.

You were talking about the general definition of the term, not about its application in a particular context. And again, not all inter-species hybrids are infertile. See for example bovine hybrids: https://en.wikipedia.org/wiki/Bovid_hybrid

In particular, beefalo is a fertile hybrid breed resulting from crossing species of different genus. https://en.wikipedia.org/wiki/Beefalo

Similarly, many commercial plants are result of hybridization. The [brassica] https://en.wikipedia.org/wiki/Brassica genus is famous for this, see https://en.wikipedia.org/wiki/Triangle_of_U

> Like taxonomy as you mentioned above, asserting primacy to the factor of a population is more on the arbitrary side of things. It's a blurred line in many respects. Viability, however, is not.

Turns out it is. This is why we are usually talking in terms of populations.

> Most people tend to think of species as a label.

And they would be right. It a rather arbitrary label that predates the theory of evolution. What is species and what is sub-species is not well defined and depends whether one is "splitter" or "clumper".

> mixed-plant offspring "hybrids" is, for lack of a better description, kind of missing the point in so many ways.

You haven't demonstrated what point.

The rest is pseudoscientific nonsense anyway. You clearly lack the understanding of terms like "gene flow" and are instead trying to describe "diffusion of alleles between populations" in a paragraph of blabbering.

> So yeah, "hybrid" absolutely does imply that the offspring are not viable

I have clearly demonstrated that they are viable.

>I have clearly demonstrated that they are viable.

While simultaneously glossing right over the fact that all of it is attributable to a sense of vestigial practices.

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