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  • [♪ INTRO]

  • 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]

[♪ INTRO]

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Why Are Mules Sterile?

  • 18 2
    joey joey に公開 2021 年 06 月 29 日
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