MadSci Network: Genetics

Re: Is it possible to combine the DNA contained in two eggs to produce a fetus?

Date: Tue May 30 20:48:43 2000
Posted By: Irene Yan, Post-doc/Fellow, Vertebrate Embryology, Rockefeller University
Area of science: Genetics
ID: 955005802.Ge

I am sorry for the delay in the answer.  I had thought that I had sent it 
out a long while ago.  Here goes the answer to your question:
Simplistically, a diploid (2n) embryo can be formed by artificial joining 
of any two haploid (n) gamete nuclei of the same species. Therefore, if we 
followed the equation n+n=2n strictly, it shouldn’t matter if the embryo 
derives from the fusion of two female gametes. However, there are more 
variables to this equation.  There is a phenomenon called parental 
imprinting.  Imprinting is a mechanism by which some genes are
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.  For 
instance, consider a gene X, which is inactivated in the female gamete.  
An embryo consisting of a double copy of maternal genes would lack 
expression of gene X.  Conversely, a normal embryo with a copy of X from 
the mother (which is inactivated), can still express X from his paternal-
derived genes.
Your question, however, 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.  Some researchers interpret this 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 
	It is now known that imprinting plays a major role in various 
human diseases, for example, rhabdomyosarcoma, Huntington’s, and Wilm’s 
tumor, among others.  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. 

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