Read: Why are mules and hinnies infertile

 Why are mules and hinnies infertile

mule infertility

Mules and hinnies have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.

This means Meiosis in mules does not work well, because the pairing of the chromosomes during the second division is imperfect. When the chromosomes don't segregate well, the sperm cells and the egg cells are either not produced, or produced in low numbers.

Low sperm count is probably the main cause of infertility in male mules. But it's also quite possible that there are regions in the mare's X chromosome that that interfere with other genes in the genome that are important for male fertility.

We do not know exactly which genes those are, but we know that this likely happened when Neanderthals and modern humans interbred, leading to male-only sterility . Although rare, female mules can sometimes produce an egg that survives and will produce live offspring with a donkey or horse. Basically female mules are drastically sub-fertile, not completely infertile.

So how do hybrid species even work? Not very well, not very efficiently, especially if the parent species have different chromosome numbers or have been separated by a long time (evolutionarily speaking).

Isolation of two species reproductively speaking occurs at various points:

1) the most drastic one where fertilization won't even occur, because the proteins on the surface of sperm and egg cells are too different for fertilization to occur (this occurs when the related species have been separated evolutionarily by a long time, in some cases millions of years).

2) an intermediate one where there is fertilization but the zygote fails to develop and there is a spontaneous abortion, in cases where the extra dosage of some genes is lethal to the embryo.

3) a full adult hybrid animal is born, but it is infertile or subfertile (the case of the mule). The mule has an extra chromosome which changes the dosage of some genes, but by the luck of the draw, the extra dosage of those genes does not kill the embryo and does not impede development.