Reproductive isolation from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not
What Is and What Should Never Be (Supernatural) What Is and What Should Never Be (Supernatural) "What Is and What Should Never Be" is the twentieth episode of the paranormal drama television series "Supernatural"s second season. It was first broadcast on May 3, 2007 on The CW. The narrative follows series protagonist Dean Winchester (Jensen Ackles) who finds himself in an alternate reality after a confrontation with a djinn . The creature appears to have fulfilled Dean's greatest wish: that his mother had not been killed when he was a child. Dean is happy in the new world until it becomes apparent that his previous work as a