Good article explaining epigenetic imprinting

Scientific American has a nice article explaining epigenetic imprinting that should help people understand genetics a little better. Lots of people, even scientists, believe that the genes in a sperm and the genes in an egg are interchangable. As the article points out, Gregor Mendel's Laws of Inheritance assumed that it didn't matter which parent passed on which genes, and that incorrect assumption is strongly ingrained in the public's mind to this day. But now we are realizing that men and women have complementary genes.

When passing on DNA to their offspring, mothers silence certain genes, and fathers silence others. These imprinted genes usually result in a balanced, healthy brain, but when the process goes awry, neurological disorders can result.

When eggs and sperm are created by the process of meiosis in the gonads, two are created at a time by a stem cell that splits in two, dividing its pairs into two sets of single chromosomes randomly. Each gamete gets one chromosome from each of the 23 pairs, creating "haploid" cells from a "diploid" cell. Which single chromosome of each pair that each gamete gets, either from the mother or father, is random. In a man creating sperm cells, the original imprinting is stripped off of the genes that came from his mother, and new male imprinting is applied, so that it is ready to join with an egg. In a woman, the opposite happens, female imprinting is applied to the chromosomes she inherited from her father and she passes on female imprinted genes.

Another new discovery is that the DNA in a sperm is more tightly packed than in an egg, with a small portion of the gene left "open":

But sperm don't have elaborate cells, just a tightly packed nucleus and a tail for swimming to the egg. So when they form, the histones are stripped off and replaced with another molecule called protamine, which shapes the DNA into an even tighter bundle, where the genes cannot be read. The British researchers have found, however, that CTCF protects some histones in sperm from being replaced, leaving about 4 per cent of the genome in an open conformation, so that its instructions can be copied. Since the pattern of exposed areas is not random, they believe it must have a purpose, and the simplest explanation is that it is a key that influences the developing embryo even before the father's genetic contribution has been unpacked.

Both of these recent discoveries are setbacks for same-sex conception, in that they show that reversing a person's imprinting is much more complex than anyone understands. But more than just being setbacks, they really show that it would be unethical to attempt to create sperm for a woman or an egg for a man for procreation.