Crossbreeding Xenopus laevis, the African clawed frog, and Xenopus tropicalis, the Western clawed frog has asymmetric results. When a female African clawed frog is crossbred with a male Western clawed frog, the embryos are viable. However, the other way around, crossbreeding a male African clawed frog and a female Western clawed frog leads to embryos that die in the early stages of development. Researchers found the reason for this: the maternal molecular machinery of the Western clawed frog cannot fully recognize the paternal chromosomes of the African clawed frog. The separation of the paternal chromosomes during cell division is disrupted. As a consequence, the cells lack a large number of vital genes and quickly die.