What makes that a female of one species cannot get pregnant from the semen of a different species?

Question

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?

In: 1

Answer 1

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.