Most people think that identical twins have the same DNA but current research indicates they do not have the exact DNA and they do not have the same epigenetic markers (more on this later). This means that one twin may naturally be a heterosexual and the other a homosexual.
SAN FRANCISCO Identical twins may not be so identical after all. Even though identical twins supposedly share all of their DNA, they acquire hundreds of genetic changes early in development that could set them on different paths, according to new research.
Identical Twins Differ Genetically | LiveScience
But experience shows that identical twins are rarely completely the same. Until recently, any differences between twins had largely been attributed to environmental influences (otherwise known as "nurture"), but a recent study contradicts that belief.
Geneticist Carl Bruder of the University of Alabama at Birmingham, and his colleagues closely compared the genomes of 19 sets of adult identical twins. In some cases, one twin's DNA differed from the other's at various points on their genomes. At these sites of genetic divergence, one bore a different number of copies of the same gene, a genetic state called copy number variants.
Identical Twins' Genes Are Not Identical: Scientific American
Identical twins look pretty similar so unless the police get lucky like they did in Boston, video evidence cant usually be used to tell them apart. And identical twins share the same DNA so conventional DNA tests cant be used either. The police are in a real pickle.
Except that they dont have to be. As the police are aware, there are less conventional tests that can find the few differences between the DNA of identical twins.
None of our DNA is exactly the same as when we were a fertilized egg, floating towards our mothers womb. Our life experiences change our DNA in many different ways.
For example, there are chemical marks on our DNA that help to control which genes should be turned on where and to what level. These epigenetic marks are added and removed over our life time in response to our specific set of experiences. This is true for all of us including identical twins. In fact, scientists have actually looked at these epigenetic markers in identical twins and have found them to be different.
Genetic Sleuthing, Or How To Catch The Right Identical Twin Criminal | QUEST
ABOUT EPIGENETICS
The earliest research into the origins of homosexuality centered exclusively on DNA; however more recent studies have shown there are certain markers that can regulate genes by turning them on or off. Here is what has been discovered in the study of epigenetic factors.
Gayness may not be in our genes, but in the molecules that regulate them. New research suggests that epigenetic factors -- chemical "switches" attached to genes that turn them on or off -- are a more plausible heritable mechanism behind homosexuality than DNA itself.
Non-genetic changes to gene expression are called epi-marks, for epigenetics, the field of research dealing with the molecular on/off switches. Epi-marks are normally erased between generations, but there's recent evidence that they're sometimes passed from parent to child.
Researchers at the National Institute for Mathematical and Biological Synthesis (NIMBioS) looked at how epi-marks that influence testosterone sensitivity in the womb might contribute to homosexuality. Late in pregnancy, natural variations in testosterone levels can alter a fetus' sexual development. Sex-specific epi-marks protect female fetuses from masculinization in the presence of too much testosterone; boys are protected from feminization if too little testosterone is present.
According to computer modeling by the group, testosterone-buffering epi-marks passed from a parent to an opposite-sex offspring may result in the reverse effect: Girls who inherit sex-specific instructions from their fathers will be partially masculinized, while boys who get epi-marks from their mothers will be partially feminized. In this model, homosexuality occurs when stronger-than-average epi-marks influencing sexual preference from an opposite-sex parent escape erasure and are then paired with weaker-than-average sex-specific epi-marks produced in opposite-sex offspring.
The rest of the lengthy Article from Popular Science can be found at this link:
Could Scientists Have Found A Gay Switch? | Popular Science
These additional links are also instructive (there are over a million sites on the subject of the epigenetic origins of homosexuality):
Scientists uncover possible source of homosexuality - NY Daily News
Homosexuality ultimately a result of gene regulation, researchers find | Fox News
New Insight into the (Epi)Genetic Roots of Homosexuality | TIME.com