How can animals with different chromosome count have offspring?

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Zebras have between 32 and 46 chromosomes. Horses have 64, but they can interbreed.

Donkeys have 62 and they can interbreed with both zebras and horses.

What happens when the gametes fuse together?

In: Biology

To be fair, they cannot *always* interbreed. The differences between the genetic make up of these animals can, on occasion, cause a particular gene combination that does not allow for embryonic development to progress all the way to live birth (which is to say, does not allow for a viable hybrid).

To answer the core of your first question, however, viable hybrids of horses, zebras, and donkeys are possible because they are all species of the *Equus* genus, and therefore much of their genetic makeup is the same (or similar enough). The term for this in biology is ‘*homology*’. You may have heard the little factoid that about 99.9% of our human DNA is the same among all humans–it’s conceptually similar to that. These similarities and shared sequences among their DNA are the genes required for life, as well as the genes that make them, well, horselike. Because horses, zebras, and donkeys all have these important sequences, they can still make viable horselike hybrid offspring.

The process of gamete fusion is, as far as I know, no different from the process in any other animal, except that the mother and father contribute unequal numbers of chromosomes. Consider humans–the sperm, containing 23 chromosomes from the father, and the egg, with 23 from the mother, fuse to form a zygote containing 46 chromosomes. If a horse (64 chromosomes) and a plains zebra (44 chromosomes) mate, the horse’s gamete (32 chromosomes) and the zebra’s gamete (22 chromosomes) will fuse to produce a zygote containing 54 chromosomes. In fact, because of this, *the number of chromosomes a hybrid has is always halfway between those of its parents*.

However, the chromosomes are different in *structure*. Earlier I mentioned that because the *sequences* were similar enough, viable hybrids were possible, since gene sequence is the primary factor in gene function. But over the course of evolution, as horses, zebras, etc. grew more and more distinct as separate species, their chromosomal *structures* changed; horses, for example, have two large chromosomes that contain genes similar to those found on four zebra chromosomes (which explains why zebras have more chromosomes). Because of this difference in structure, the chromosomes can’t pair up perfectly during meiosis (the process of cell division required to form new gametes), which is why hybrids are effectively always sterile.