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Horse plus donkey: it seems like an unlikely combination. There’s the height difference, for a start.

But there’s also the problem of donkeys and horses being totally different species. And yet, this strange combination can result in even stranger offspring:. The mule.

The mule is a cross between a horse and a donkey, but mule parents don’t go on and have mule offspring. With only a few exceptions, mules can’t even become parents at all. Because mules are exclusively the offspring of a female horse and a male donkey.

When a male horse and a female donkey get together, you end up with the lesser-known sibling, the hinny. Either way, you get a hybrid that has donkey-like ears and a horse-like build. But it's not the hybrid appearance that stops mules (or hinnies) from breeding with each other or even with donkeys and horses.

It's genetics. Different species have a different number of chromosomes or structures within cells that contain our DNA. When living organisms reproduce, the offspring gets those chromosomes in pairs, one from each parent.

Two members of the same species will have the same number of chromosomes, so the offspring gets exactly half of their chromosomes from each parent. Horses have 64 chromosomes, while donkeys have 62 chromosomes. In the mule’s case, that’s 32 pairs of chromosomes from the horse and 31 pairs from the donkey.

So, mules have one chromosome that is left sort of hanging out in limbo after the other 31 pairs come together. But this limbo chromosome can be a major hurdle that stands between mules and parenthood. Usually, non-hybrid species can make sperm and egg cells during a process called meiosis.

Where cells divide to produce sex cells that contain only half of the parent cell's genetic information. During meiosis, chromosomes pair up and exchange genetic information, so there's a different mix of genes from each parent in the resulting gametes. In mules, meiosis doesn't happen, partly because the horse and donkey parents’ chromosomes aren’t homologous.

Homologous chromosomes are a matched pair of chromosomes, similar in size and structure and carrying different versions of the same genes. Non-homologous chromosomes, like those of horses and donkeys, are too different in size and structure, so they can't be easily paired up. So the odd chromosome from the mule remains unmatched.

All this means that mule chromosomes just have a hard time pairing up, disrupting meiosis so no eggs or sperm are created, meaning no mule offspring for mule parents. But it wouldn't be fair to end this story without confirming that there are very rare documented cases of female mules giving birth to viable offspring. But in order for this to happen, a lot of unlikely things have to come together.

The mule has to sort of accidentally produce an egg that contains an even number of chromosomes. Then, that egg has to be compatible with the sperm cell it eventually pairs up with. Both variables are so unlikely that there have been only a handful of recorded cases of mules giving birth.

Even then, it's actually more likely that this mule parent was just a donkeyish-looking horse or a horsish-looking donkey, which is something that scientists try to rule out whenever they’re studying those rare reports of mule offspring. So although mules continue to be a preferred animal for things like packing and trekking into the Grand Canyon, there’s no danger of them taking over the wild west or the world any time soon! Thanks for watching this episode of SciShow!

If you liked learning about mules but would like to know more about how a world run by animal hybrids would look,. I bet you’ll love our podcast, SciShow Tangents! In it, some of the fun people involved in SciShow get together for a lightly competitive knowledge showcase.

Every episode, they rack up points for teaching the others, and everyone listening at home, the most mind-blowing science facts related to the week’s theme. If you love science, laughing, and lighthearted, nerdy competitions, you should check it out! You can find SciShow Tangents anywhere you get your podcasts. [♪ OUTRO].