|MadSci Network: Genetics|
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 both. 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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