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Why Can’t Mules Have Babies?


Hi, this is Emily. Animals of different species rarely get it
on, and when they do, they rarely produce offspring. But even rarer than the ligers, zonkeys, camas,
and beefalo of the world are hybrid animals capable of making babies of their own. In fact, the idea of a mule – the hybrid
progeny of a female horse and a male donkey – having its own baby is so implausible
that the ancient Romans used to employ the saying “when a mule foals” as a metaphor
for the impossible — the same way that Americans say “when pigs fly”, Turkish people say
“when fish climb trees” and Nigerians say “when the chicken has teeth.” Here’s why: in order to successfully reproduce,
animals need to create viable sex cells. Normal body cells have two copies of each
chromosome – one from the animal’s mom, and one from dad, sort of like two full decks
of cards – and both decks can simply be duplicated and then split off to form a new
cell. But sex cells are different: they’re produced
by taking each set of chromosomes and swapping a bunch of cards between decks before breaking
them up to form four cells, each with just one complete deck of cards. This card-swap works when the decks in each
set come from a mom and dad of the same species, because they have all the same cards in the
same order, so genes for eye-color get swapped with other genes for eye-color, and stripe
pattern genes get swapped with stripe-pattern genes. But in the cells of hybrid animals, the decks
aren’t identical because they came from parents of different species, so an eye-color card
might get swapped for a paw size card, or a bone-making gene for a kidney-making gene,
producing two really weird decks that give rise to totally non-functional sex cells. However, for reasons we really don’t understand,
it is sometimes possible for a hybrid animal to skip over the card-swapping step altogether
and create new sex cells that contain only the mom’s DNA. We’ve actually only seen this happen in
female mules: there are a handful of documented cases in which mules have made babies from
eggs containing only their mother’s DNA; bizarrely, this means that each female mule’s
offspring was genetically also her half-sibling. So, while – as far as we know – pigs never
fly, every once in a great while, a mule foals. This video was made possible by The Great
Courses Plus, a streaming video learning service with a library of over 7,000 lectures by leading
professors from some of the world’s top institutions. I’ve been perusing a course on genetics
and have learned that there actually used to be a company that cloned people’s pets,
which means that theoretically I could have a kitten version of my old, blind cat Yardley,
except would it be really Yardley?. To get a free one-month trial subscription
to The Great Courses Plus and show your support for MinuteEarth, go to TheGreatCoursesPlus.com/minuteearth,
or click the link in the description below. Thanks Great Courses Plus!

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