system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we breed with other species?

Why cant we breed with species like gorillas, when close species, like tigers and lions or horses and mules, can?
gpt
The general rule of thumb for interspecies breeding is that it comes down to chromosomes, which are strands of DNA. The rest of the Great Apes (Gorillas, Chimps, Bonobos, Orangutans) have 48 chromosomes, but 2 of these chromosome have fused in Humans, leaving us with 46 chromosomes. The greater the difference in the number of chromosomes, the harder it is for species to breed. Basically, the sex cells (sperm/egg) have problems aligning and combining when the chromosome count is off.

This hasn't stopped animals with different number of chromosomes from interbreeding. Mules are the best example. Horses have 64 chromosome and donkeys have 62, so this is similar to the difference between Humans and the rest of the apes. So we can see that a different number of chromosomes is a problem, but not an impossible barrier, for making hybrids.

The other main biological problem comes down to the immune system. Your body automatically recognizes when things are "not-self." Any outside matter that enters into your system is immediately attacked by your immune system. For pregnancy, this concept is most important when it comes down to blood types. The "positive/negative" you hear about human blood types refers to a specific blood type. Basically, if a mother is "negative" and the fetus is "positive" than the mother's immune system will attack the blood of the fetus, which can lead to a miscarriage. Chimps and humans have equivalent systems that they share. So, a humanzee fetus would need to have a mother and father with matching blood subtypes.

The biggest problem to breeding humans with other apes is not biological. The simple fact is that, while it is biologically possible (if not likely), making a human-ape hybrid is a bit too mad science-y for most people. No reputable institution would fund the research, no ape center would allow their animals to take part, and the outcry from the public and scientific community would shut down any private project pretty quickly.