Nurture vs. Nature

Twin studies can be an invaluable tool for scientists who are interested in studying whether aspects of human behavior are influenced by genes or environment (ie learned). Generally, this means tracking down adult identical twins who happen to have been separated at birth, getting their consent, and enrolling them in a study. Psychiatrist Peter Neubauer took things to a new level when he initiated a program that split up adopted twins at birth and then followed the differences and similarities that emerged during their lifetimes. Apparently the results of the study will be sealed in a library at Yale until 2066, however one pair of these twins has since found out about the study and they speak about the experience in the video below. Neubauer discussed the general topic of genetics and behavior in a 1996 book, Nature's Thumbprint: The New Genetics of Personality, which I haven't read but you can find here.

Tetragametic chimerism

I have a riddle for you: "The patient, who we will call Jane, needed a kidney transplant, and so her family underwent blood tests to see if any of them would make a suitable donor. When the results
came back, Jane was hoping for good news. Instead she received a hammer blow. The letter told her outright that two of her three sons could not be hers."
Well if you've paid attention to the title of this post you probably already know the answer. Jane is a chimera. She is a mix of cells from two twin sisters, conceived from two pair of gametes, that somehow got mingled up into one person. There are 30 or so reported cases of true tetragametic chimerism in the literature, often because they lead to hermaphroditism when the chimera is of a male and a female twin. However many more lurk around us without knowing. It may be just a few cells in the blood, or a patch in one organ, and you would probably never know...except from when you get blood type or genotype discrepancies. Such a case actually popped up recently when Tyler Hamilton was charged with blood doping after competing in a cycling race. He had two blood types in his veins, but it wasn't because of blood transfusion, he is a chimera. And the prevalence is mind boggling: 20-30% of the pregnancies that start out as twins end up as a single baby, and nearly 70% of all people may be chimeras. Although not everyone agrees with these statistics, it may explain, weird auto-immune disorders. And perhaps some transsexual people really are a girl trapped in a man's body, literally. here.

Science question of the day

Ok suppose two identical twins have sex with a woman only hours apart, and she becomes pregnant, is there a way to trace the father? Obviously the standard DNA tests will not work, but are there any epigenetic alternatives? One of the problems is that methylation of the genome is reset at an early embryonic stage. While there may be imprinting, is it possible to match it to the father's "consensus" sperm imprinting, or is it sperm cell specific? Any ideas?