Interessanterweise sind auch eineiige Zwillinge nicht vollkommen gleich.
Hier ein paar Punkte, die sie unterscheiden:
Aus einem Slateartikel:
That identical twins do not, in fact, have identical DNA has been known for some time. The most well-studied difference between monozygotic twins derives from a genetic phenomenon known as copy number variations. Certain, lengthy strands of nucleotides appear more than once in the genome, and the frequency of these repetitions can vary from one twin to another. By some estimates, copy number variations compose nearly 30 percent of a person’s genetic code.
These repeats matter. More than 40 percent of the known copy number variations involve genes that affect human development, and there are strong indications they explain observed differences between monozygotic twins. For example, it’s often the case that one identical twin will end up victimized by a genetically based disease like Parkinson’s while the other does not. This is probably the result of variations in the number of copies of a certain piece of DNA. Copy number variations are also thought to play a role in autism spectrum disorder, schizophrenia, and ADHD, all of which can appear in only one member of a monozygotic twin pair (PDF). If copy number variations can affect discrete and diagnosable disorders, then why shouldn’t they influence far more complex behaviors like your inclination to head to the polls on a Tuesday night in November?
That’s just the beginning of the genetic differences between monozygotic twins. As a result of mutations during development, about one in 10 human brain cells has more or less than the typical two copies of a chromosome. Identical twins also have different mitochondrial DNA, the genetic information stored in the cellar organelle responsible for processing glucose. Research suggests that mitochondrial DNA affects brain size among a host of other neurological traits.
Oder bei Steven Pinker, The Blank Slate:
Life is a pinball game in which we bounce and graze through a gantlet of chutes and bumpers. Perhaps our history of collisions and near misses explains what made us what we are. One twin was once beaten up by a bully, the other was home sick that day. One inhaled a virus, the other didn’t. One twin got the top bunk bed, the other got the bottom bunk bed. We still don’t know whether these unique experiences leave their fingerprints on our intellects and personalities. But an even earlier pinball game certainly could do so, the one that wires up our brain in the womb and early childhood. As I have mentioned, the human genome cannot possibly specify every last connection among neurons. But the “environment,” in the sense of information encoded by the sense organs, isn’t the only other option. Chance is another. One twin lies one way in the womb and stakes out her share of the placenta, the other has to squeeze around her. A cosmic ray mutates a stretch of DNA, a neurotransmitter zigs instead of zags, the growth cone of an axon goes left instead of right, and one identical twin’s brain might gel into a slightly different configuration from the other’s.67 We know this happens in the development of other organisms. Even genetically homogeneous strains of flies, mice, and worms, raised in monotonously controlled laboratories, can differ from one another. A fruit fly may have more or fewer bristles under one wing than its bottlemates. One mouse may have three times as many oocytes (cells destined to become eggs) as her genetically identical sister reared in the same lab. One roundworm may live three times as long as its virtual clone in the next dish.
Zu der Versorgung über die Nabelschnur und mögliche Auswirkungen auch noch eine andere Studie:
The second to fourth finger digit ratio (2D:4D ratio) is a sex-dimorphic characteristic in humans that may reflect relative levels of first trimester prenatal sex hormones. Low interdigital ratio has been associated with high levels of androgens. It has been reported in unrelated women that low 2D:4D ratio is associated with lesbian sexual orientation, but because of the nature of those samples, it was not possible to conclude whether lower ratio (and hypothetically, higher androgen levels) in lesbians are due to differences in genetics as opposed to differences in environment. To test the hypothesis that low 2D:4D in lesbians is due to differences in environment, interdigital ratio data were analyzed in a sample of female monozygotic (MZ) twins discordant for sexual orientation (1 twin was lesbian, the other was heterosexual; n = 7 pairs). A control group of female MZ twins concordant for sexual orientation (both twins were lesbian) was used as a comparison (n = 5 pairs). In the twins discordant for sexual orientation, the lesbian twins had significantly lower 2D:4D ratios on both the right and left hands than their heterosexual cotwins. There were no significant differences for either hand in the twins concordant for sexual orientation. Because MZ twins share virtually the same genes, differences in 2D:4D ratio suggest that low 2D:4D ratio is a result of differences in prenatal environment.
Das ist natürlich eine sehr kleine Studie, ich sehe sie insofern eher als ersten Ansatz, der aber zeigt, dass der Anteil der biologischen Faktoren bei der sexuellen Identität auch über die in anderen Studien ermittelten ca. 50% hinausgehen kann.