Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: chris on 19/12/2014 08:19:30
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Someone was asking me why mules and other similar interbred species are frequently not fertile. What is the genetic mechanism for this?
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One of the mechanisms is that Donkeys and Horses have different chromosomes.
In particular, the chromosome count:
Horses: 32 to 46
Donkey: 62
Horses: 64
See Wikipedia on Zebroids (http://en.wikipedia.org/wiki/Zebroid)
So, a Donkey/Horse cross (mule) would have 63 chromosomes, 32 horse chromosomes and 31 donkey chromosomes. Zebra crosses with donkeys or horses would be similar. In a sense, it is surprising that they are even viable, but they obtain a complete half compliment of horse chromosomes, and a complete half compliment of donkey chromosomes.
Mitosis (normal cell division) simply requires dividing all the chromosomes, and giving each new cell a complete set of chromosomes.
Meiosis (for reproduction) requires pairing up all the chromosomes and then dividing to give each cell half of the chromosomes. Assuming it actually occurs in a mule, it would be interesting to see how it happens. However, it would be difficult to get a complete haploid set of viable chromosomes. Cross-overs might even scramble the chromosomes even more.
It would seem logical that a protection mechanism would have evolved to prevent mismatched chromosomes from generating viable gametes, although I haven't seen the research.