Re: Will it ever be possible to combine two human oocytes to form a zygote?

   Your question was addressed directly by Barton, Surani and Norris 
(Nature 1984 Sep 27-Oct 3; 311(5984): 374-6) who made mouse embryos that 
had double copy of maternal genes or of paternal genes.  Neither the 
paternal-only nor the maternal-only embryos developed normally.  The 
embryos that only had maternal genes were almost normal-sized, but the 
extraembryonic tissue tissue (structures that nourish the embryo, such as 
the placenta) was underdeveloped.  On the other hand, the embryos that 
only had paternal genes were puny, but the extraembryonic were almost 
normal.  This was one of the groundbreaking experiments that showed 
parental imprinting in mammalian genome. Imprinting is a mechanism by 
which some genes ar preferentially silenced in males and in females.  The 
genes that are silenced in male gametes are active in female gametes and 
vice-versa.  Thus, dividing the genes in the embryo as “mom’s” 
and “dad’s”.  This mechanism was first found in insects and now has been 
shown to happen in mammals as well, including humans.  As a consequence of 
imprinting, the embryo needs both maternal AND paternal genomes to develop 
properly. 
   Some researchers interpret parental imprinting as a difference in 
interest between the parents when it comes to the growth of their 
offspring. In species where males mate with multiple females (such as the 
mouse), the male would like to see the female invest as much as possible 
on his offspring, thus making the placenta larger. She, on the other hand, 
would like to produce additional offspring with other males, to ensure 
diversity.  Therefore, it would serve her interest to ration her 
resources.  The resulting embryo of a normal mating would be a balance of 
both. 
   How is imprinting generated and how is it maintained is still a subject 
of intense study.  Until we understand better the mechanisms that regulate 
imprinting, we will not be able to interfere with it.  Therefore, I can’t 
really answer your question regarding stripping of the imprinting signals 
to generate viable zygotes.  All I can say is, from what we know today, 
imprinting is a very complex mechanism that can’t be messed around with 
yet.  I hope this was helpful.
   If you are interested in knowing further about imprinting, I suggest 
that you read “Parental Imprinting of Genes” in the October 1990 issue of 
Scientific American pg 52-60.