There is a story on the wires at the moment about a woman whose mule has given birth (it mated with a donkey).
This is a very rare event, because:
In which case, how has it managed to this time? The article also mentions a couple of other mule-births this century, but they’re obviously extremely rare.
I would guess it came up with one less/more chromosome than normal. It happens to humans on occasion (usu. resulting in developmental disorders, though).
Is this similar to ginger female cats and tortoishell male cats being very rare, and nearly always infertile when they occur?
No.
Cats, no matter what color they are, have the same number of chromosomes. It has to do with the fact that those particular colors are sex-linked.
No, not quite:
With the cats the issue revolves around the two sex chromosomes. Orange ( ginger ) cats are orange because of an allele on the X chromosome. Since a female cat has two X chromosomes to the males one, the have to have the dominant allele for orange on both, while the male only needs it on the one to turn up orange. So orange cats are predominantly male. However female orange cats are much more common than male calicos ( I used to have one - passed away a couple of weeks ago 
) and generally are perfectly fertile.
To have a calico you need a dominant orange allele on one X chromosome and a dominant “non-orange” allele on the other - Obviously only females have that option. The exception is usually a result of Klinefelter’s Syndrome, which is a male cat with an extra sex-chromosome, i.e. XXY in stead of XY. This is quite rare and only about 1 in 3000 calicos is male. Klinefelter’s Syndrome generally results in sterility.
Now with fertile mules ( or hinnies ) we are talking even more rare. Here, as noted, it is caused by the mismatched number of total chromosomes, which results in a failure for the parental chromosomes to align properly during meiosis. However in a small number of female ( and only female ) mules the female seems to be able to pass on a complete set of her maternal chromosomes, without it getting muddied up with the paternal set like normal. So a female mule ( which resulted from a cross of a male donkey with a female horse, the reverse pattern is called a hinny ) successfully back-crossed with a stallion will produce a 100% horse foal. Freaky, eh ;)?
This doesn’t make sense to me. Isn’t there a 50-50 chance of each chromosome being either horse or donkey, and the fertile mules or hinnies just happen to pass on an even number of them during meiosis? This is the kind of thing that I REALLY need to draw a diagram for, but I’m guessing that , for example, chromosome 1-H and 1-D (H =Horse and D=Donkey) line up and 2-H and 2-D etc until you’re left with 32-H being all alone. Then the cell splits by meiosis, and, for example, 1-H and 2-D go into one cell, and 1-D and 2-H go into another, etc etc and you get 32-H into one or the other. Now if the one that DOESNT have 32-H is the egg that happens to be there when mating occurs, that doesn’t mean that its all horse, or all donkey.
Or is the deciding factor the DNA duplication that precedes meiosis…and something special happens at that point to segregate horse from donkey genes? Even so…couldn’t you still end up with a 50/50 mule if a mule/hinnie mated with a stallion?
You’d think so, wouldn’t you? I’ll be the first to admit I haven’t a clue as to what the mechanism involved here is ( maybe we need to invoke Edwino or someone similar ). But here’s one brief discussion:
*If the sire was a donkey, they are mules like their mother; if it was a horse, they are horses like their father. Fertility is even rarer among female hinnies than among mules, or perhaps records of hinny foals are scarcer be- cause there have always been fewer hinnies in existence. Served by a horse, a female hinny produces nothing. Served by a donkey, she bears a typical donkey. The instances of fertility among female hybrids of both crosses between the donkey and the horse are so few that it is impossible to be certain that they represent a general truth. If one accepts the risks of arguing from the particular to the general and the few facts are taken at their face value, how- ever, the reproductive pattern can be interpreted in terms of simple genetics, as follows.
Let the nonsex chromosomes of the horse be represented by H and those of the donkey by A. When the sex chro- mosomes X and Y are added, a genetic formula for male and female horses will respectively be HHXY and HHXX, and the formula for male and female don- keys will be AAXY and AAXX. The breeding of mules can then be written: AAXYX HHXX=AHXX (female) or AHXY (male). The breeding of hinnies would be written: AAXXX HHXY= AHXX (female) or AHXY (male).
The subsequent mating between a male horse and a female mule should therefore follow the pattern HHXYX AHXX, with the offspring either AHXX, AHXY,HHXX or HHXY, which is to say both sexes of mules and horses. The failure of such matings to produce either AHXX or AHXY, that is, either of the mule sexes, can be explained by assuming that true mule ova (AX) are not present in female mules. This leads to the conclusion that the ova of the mule carry only horse chromosomes. Similarly, the absence of horses from crosses between a donkey and a hinny is understandable if the ova of a hinny contain only donkey chromosomes.*
From here: http://www.owensvalleyhistory.com/bruce_morgan_49ers/the_mule.pdf
Here’s another comment:
From here: http://www.lovelongears.com/faq.html
I reiterate - freaky :D.
Wow. That’s pretty messed up! So what was this Medelian stuff they were teaching us at school if THIS kind of stuff can happen! Who needs cloning? Just induce this mechanism in humans, have them breed with one of their parents, and they can get their parent born again! DEFINITELY weird…
This is not a simple question. There are a number of genetic effects in play here – all the laws kind of go to pot when hybrids are made.
Odd numbers of chromosomes are not particularly damning to viability and even fertility. There are some human conditions with odd numbers of chromosomes and continued fertility – trisomy 21, Turner Syndrome, some Kilnefelter’s, and XYY come to mind.
We also have issues of imprinting – it matters which parents give you which genes, as was noted 3000 years ago between hinnies and mules.
I think it is very difficult to predict what the spindle looks like in the reduction division of meiosis I. It seems that you would need sheer luck to segregate all of the horse chromosomes one way and the donkey ones another, although there could be high genetic weirdness at stake. It is not unknown to have things like spindle defects or bizarre chromosome rearrangements or nonstandard chromosome numbers by nondisjunction.
Another interesting thing to note is the normal recombination which occurs in meiosis I between homologous chromosomes does not occur in mules or hinnies – recombination between paired homologous areas of chromsomes is inhibited as pairing causes chromosomes to break when they try to recombine. We actually use this fact every day in fruit fly genetics – we have engineered inversions on chromsomes which act to inhibit recombination.
It is also interesting to note that the equines started to diverge only an estimated 5-10 million years ago. Looking across the equines on the planet – zebras, donkeys, horses – we see 21 to 66 chromosomes and cytogenetically they all look totally different. This is completely removed from the fact that their actual genes are quite close. It just shows how fast things can change once true speciation has occured and two populations stop interbreeding.
Fertile male mules, while extremely rare, do happen. There was a famous one in China just a few years ago. The farmer that owned it went around from village to village making money on stud services.
(Given that species that diverged 5+m.y.a. and have different number of chromosomes can mate and produce the odd fertile offspring, I am surprised in the “Neaderthal gene” threads that cross breeding between Neanderthal’s and modern humans is considered unlikely. Yeah, right.)
It gets weirder. A male orange and white kitten can be born with black spots that form in the womb, sort of like moles on humans. Although breeders would call this an orange and white bicolor with mosaicism, if the spots were big enough it would look like a calico to most people.
Sometimes XX and XY cells can appear in the same cat. If the XX cells are in the skin and the XY cells are in the gonads, you can have a fertile male calico.
Resume mule-related thread.
This is all fascinating - thanks guys!
And very sorry to hear about your calico, Tamerlane. Hope he had a happy life.
True story: there is a mule in Wisconsin who sired two human children!
Haw HEe! 
Forget that–if we’re talking just 5 MYA, what about modern humans and chimps?
Now that I think about it, wasn’t there a Cecil column on that?