The Effect of "Silenced" Genes on Pregnancy
The New York Times has an article, Silent Struggle: A New Theory of Pregnancy, describing the work of Dr. David Haig.
The article states:
This strategy takes advantage of the fact that most of the genes we carry come in pairs. We inherit one copy from our mother and one from our father. In most cases, these pairs of genes behave identically. But in the past 15 years, scientists have identified more than 70 pairs of genes in which the copy from one parent never makes a protein. In some cases, a parent's gene is silenced only in one organ.
Scientists do not fully understand this process, known as genomic imprinting. They suspect that it is made possible by chemical handles called methyl groups that are attached to units of DNA. Some handles may turn off genes in sperm and egg cells. The genes then remain shut off after a sperm fertilizes an egg.
The article goes on to say that the failure of proper imprinting can result in problems in fetal development and can lead to disorders discovered in the child later.
This imprinting or pattern of gene methylation is what I referred to here:
Reproductive cloning is not yet feasible. Gametes which come together during normal fertilization have nearly all their sets of genes turned on. This provides the embryo the potential to produce all the body's structures and functions. The exception to this is that one pattern of genes is turned off (methylated) in the egg and a different pattern of genes is methylated in those in the sperm. When these two gametes with complimentary patterns combine, the likelihood of having a healthy full term baby is increased.
When a skin cell, for example, has to be demethylated to return it to a poly-potential state for potential cloning, this indiscriminate process would remove the male and female methylation patterns as well, resulting in all kinds of potential difficulties in the developing fetus which cannot be ethically tolerated.
The same would apply to same sex procreation. No matter how one tried to produce two gametes, at least one would not have the male or female methylation pattern required for success.
In either case, producing an embryo with the same methylation patterns as occur in the natural fertilization process is way beyond our current understanding or abilities.
This is why I suggest that SSP is not going to happen any time soon.
Carl Zimmer, the author of this article, has more information and examples that would not fit into the article at his blog post.