For example: if one would combine a human egg with the sperm of a dog, the human egg will not be fertilized. But there are species who can cross, like mules?
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The sperm and the egg contain sets of chromosomes, which represent half of chromosomal pairs.
Each half, one from each parent, contain specific structures allowing them to align and fuse. Variations in the arrangement, length, and number of chromosomes prevent or degrade this fusion.
Genetically “adjacent” species can successfully interbreed, but almost always produce sterile offspring due to incomplete chromosomal matching.
Imagine that DNA is a book of instructions on how to assemble the body of that specific species, and how to maintain it afterwards. Each individual has a specific edition of the book, and they differ in just a few details here and there. So, when the body creates eggs or sperm, it’s like taking half of the book (let’s say, every other word), and when the cells merge, they create a new book. It will be pretty much understandable, and the few sections where it doesn’t make sense, it’s not that big of a deal (or it is, and we have things like disabilities).
The thing is, different species have books with different amount of chapters, pages, sentences, etc. So, when they try to merge, the resultant book is basically garbage. Or even cannot be made, imagine creating a book with pages of different sizes and trying to make it fit into a smaller cover.
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