A species is generally defined as a group of organisms that can interbreed and produce fertile offspring. While horses (Equus caballus) and donkeys (Equus asinus) can interbreed to produce a mule (or a hinny), their offspring are almost always sterile. This sterility arises because horses and donkeys have different numbers of chromosomes (horses have 64, donkeys have 62). The mule inherits 63 chromosomes, which cannot pair properly during meiosis, preventing the formation of viable gametes (sperm or eggs), thus leading to infertility.