Genomic Imprinting

Genomic imprinting is a molecular phenomena that occurs during gamete formation and prevents development of a viable fetus. Some genes are expressed differently depending on whether they are inherited from the mother or father. In cases such as parthenogenesis, there is no paternal genetic contribution, and these genes become fatal mutations.

Experiments have shown that inheriting all genetic material from only one parent can lead to the following problems:

Zygotes implanted with solely paternal genes will not develop a viable embryo. The placenta, however, will be functional.
Conversely, a zygote that is derived completely from maternal contributions can produce a complete embryo, but the placenta is underdeveloped and the embryo will not be able to survive on its own.

These experiments suggest that parthenogenesis would be ineffective in mammals. One possible way to avoid this defect is to manipulate the genes in a way that would allow maternal expression to be converted to paternal expression. This reprograming may be able to take place by nuclear transfer. [F]