why are mules infertile? If a horse and a donkey have close enough genetics to have offspring, why is that offspring left with genetics that render them unable to have their own offspring?
In: 8368
So is the NUMBER of chromosomes of a species the actual determining factor of breeding?!?
Like if we could eliminate all… physical barriers of breeding. What is the process that prevents the hybridization of two different species?
Horses and donkeys have a different number of chromosomes. Specifically, horses have 64 chromosomes and donkeys have 62. During sexual reproduction the chromosomes are split in half and the offspring gets one half from each parent to make the full set. So, the horse donates 32 chromosomes and the donkey donates 31.
That means the mule has 63.You may notice that 63 is an odd number. The genes in the chromosomes are similar *enough* that as they get activated they can still create a functional animal. When the mule tries to reproduce, its reproductive cells try to split the chromosomes in half and fail to do it evenly. When the reproductive cells combine, the chromosomes *can’t* match up correctly because of the odd numbered split. Without matching correctly, they can’t be activated correctly and the offspring will fail to develop.
EDIT: Fun side fact, this is how seedless watermelon work, with some extra steps. One watermelon is treated with a chemical that causes all of the chromosomes to double. Then, pollen from that watermelon is used to pollinate a normal watermelon. So a normal watermelon would have ZZ chromosomes, and reproductive cells with Z; the treated watermelon has ZZZZ, and reproductive cells with ZZ. When you combine the reproductive cells of both, you get a watermelon with ZZZ – an odd number, and incapable of reproducing. The seeds that this watermelon makes are a hodgepodge of chromosomes and do not fully develop.
Fun side fact #2: camels and llamas have the same number of chromosomes. Despite being different species that evolved apart for millions of years on different continents, they are still similar enough to have hybrid offspring like horses and donkeys. However, since camels and llamas have the same number of chromosomes, their offspring can themselves have viable offspring.
Fun side fact #3: hybrids are normally named with the first letter of the father animal and the rest of the name of the mother animal. Thus, a *liger* is a hybrid between a male lion and female tiger. Mules don’t follow this rule, AFAIK because humans have been breeding mules for about as long as we’ve been domesticating horses and donkeys. A mule is the offspring of a male donkey and female horse (which could also be called a “dorse” but that sounds dumb). Offspring from a male horse and female donkey is called a “hinny” (a female donkey is called a Jenny) and they are apparently pretty different from mules. Mules are a lot easier to breed.
Edit: **Why does the odd number make reproduction (usually) impossible?**
Imagine DNA as an instruction book with pages (genes) organized into chapters (chromosomes). Every book has two copies of every chapter. They’re *mostly* identical, but with very small changes. Maybe different size screws or more screws or whatever. To reproduce, the chapters are separated, and then pages are swapped between two different books so that now you get a chapter that says to use fewer screws, but also bigger screws. Build the thing and see if it works better.
For that to work, the two books have to be very close. Like, you could use instructions for two kinds of tables, but not a table and a bookshelf.
If they’re *almost* close enough, like a table and a computer desk, you can mix the instructions and they’ll still work and you get something that’s kind of like both. But if you take *that* book and try to split up the chapters you get an odd number of pages. When you try to mix them back together it throws off the page numbers. The new books you try to make end up with important pages missing, or extra pages that add, like, an extra leg that has nowhere to go.
The builder is pretty simple and follows the basic knowledge that all instructions have duplicate pages, so they have to use one out of every two. They don’t have to always skip the odd number or even number, but they can only use one or the other for every two pages. Throw in an entire chapter of extra pages, though, and it completely throws off which pages the builder can use. Since it becomes very obvious that it isn’t going to work pretty early on, the builder will give up pretty quickly.
Edit: **What about chromosome disorders in humans, like Down’s Syndrome?**
Yes, Down’s Syndrome, caused by trisomy 21 (three chromosomes at #21) causes fertility problems, with women being fertile about 15-30% of the time. Their offspring have a high chance (~50%) of also having trisomy 21.
Men with Down’s Syndrome appear to be far less fertile, but it’s been documented a couple times AFAIK. Some research suggests that the apparent low fertility may come from ignorance of sex itself, rather than necessarily the inability to produce viable sperm. That is, they just don’t have sex often so they don’t father children very often.
I haven’t looked up stats for other chromosome disorders, but most of them can’t develop enough to be born or aren’t capable of life after birth so it’s mostly moot. When it does happen, I’m sure there are fertility issues there.
Keep in mind that humans reproducing with chromosomal disorders are still both humans so reproduction is already the expected result. It’s already genetically much easier. Horses and donkeys (and other hybrid species) are related but *not* the same species so reproduction is already *not* genetically guaranteed.
If a mule had a genetic deformity that gave it an extra chromosome, would it then be able to reproduce as it now has an even number?
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Basically, it’s because of chromosomes.
Horses have 64 chromosomes
Donkeys have 62 chromosomes
When you make a baby, half the chromosomes come from mom, half come from dad.
If 2 horses have a baby, 32 chromosomes come from each parent. So the baby gets 32+32=64 chromosomes. Perfect.
If 2 donkeys have a baby, 31 chromosomes come from each parent. So the baby gets 31+31=62 chromosomes. Perfect.
But if a horse and a donkey have a baby… Well, horse parent gives 32, and donkey parent gives 31… That baby is left with 63 chromosomes.
If mules have 63 chromosomes, that is a problem, because you can’t split that in half. If a mule tries to have a kid, the chromosomes won’t line up correctly, and the fetus won’t start to develop
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Sheep and Goats can breed too, the offspring is called a Geep:
https://en.wikipedia.org/wiki/Sheep%E2%80%93goat_hybrid
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