The Gene: An Intimate History - Siddhartha Mukherjee Page 0,59

the other. How could one determine if any particular feature—height or intelligence, say—was the product of nature or nurture? How could one unbraid heredity and environment?

Galton proposed piggybacking on a natural experiment. Since twins share identical genetic material, he reasoned, any substantial similarities between them could be attributed to genes, while any differences were the consequence of environment. By studying twins, and comparing and contrasting similarities and differences, a geneticist could determine the precise contributions of nature versus nurture to important traits.

Galton was on the right track—except for a crucial flaw: he had not distinguished between identical twins, who are truly genetically identical, and fraternal twins, who are merely genetic siblings (identical twins are derived from the splitting of a single fertilized egg, thereby resulting in twins with identical genomes, while fraternal twins are derived from the simultaneous fertilization of two eggs by two sperm, thereby resulting in twins with nonidentical genomes). Early twin studies were thus confounded by this confusion, leading to inconclusive results. In 1924, Hermann Werner Siemens, the German eugenicist and Nazi sympathizer, proposed a twin study that advanced Galton’s proposal by meticulously separating identical twins from fraternal twins.III

A dermatologist by training, Siemens was a student of Ploetz’s and a vociferous early proponent of racial hygiene. Like Ploetz, Siemens realized that genetic cleansing could be justified only if scientists could first establish heredity: you could justify sterilizing a blind man only if you could establish that his blindness was inherited. For traits such as hemophilia, this was straightforward: one hardly needed twin studies to establish heredity. But for more complex traits, such as intelligence or mental illness, the establishment of heredity was vastly more complex. To deconvolute the effects of heredity and environment, Siemens suggested comparing fraternal twins to identical twins. The key test of heredity would be concordance. The term concordance refers to the fraction of twins who possess a trait in common. If twins share eye color 100 percent of the time, then the concordance is 1. If they share it 50 percent of the time, then the concordance is 0.5. Concordance is a convenient measure for whether genes influence a trait. If identical twins possess a strong concordance for schizophrenia, say, while fraternal twins—born and bred in an identical environment—show little concordance, then the roots of that illness can be firmly attributed to genetics.

For Nazi geneticists, these early studies provided the fuel for more drastic experiments. The most vigorous proponent of such experiments was Josef Mengele—the anthropologist-turned-physician-turned-SS-officer who, sheathed in a white coat, haunted the concentration camps at Auschwitz and Birkenau. Morbidly interested in genetics and medical research, Mengele rose to become physician in chief at Auschwitz, where he unleashed a series of monstrous experiments on twins. Between 1943 and 1945, more than a thousand twins were subjected to Mengele’s experiments.IV Egged on by his mentor, Otmar von Verschuer from Berlin, Mengele sought out twins for his studies by trawling through the ranks of incoming camp prisoners and shouting a phrase that would become etched into the memories of the camp dwellers: Zwillinge heraus (“Twins out”) or Zwillinge heraustreten (“Twins step out”).

Yanked off the ramps, the twins were marked by special tattoos, housed in separate blocks, and systematically victimized by Mengele and his assistants (ironically, as experimental subjects, twins were also more likely to survive the camp than nontwin children, who were more casually exterminated). Mengele obsessively measured their body parts to compare genetic influences on growth. “There isn’t a piece of body that wasn’t measured and compared,” one twin recalled. “We were always sitting together—always nude.” Other twins were murdered by gassing and their bodies dissected to compare the sizes of internal organs. Yet others were killed by the injection of chloroform into the heart. Some were subjected to unmatched blood transfusions, limb amputations, or operations without anesthesia. Twins were infected with typhus to determine genetic variations in the responses to bacterial infections. In a particularly horrific example, a pair of twins—one with a hunched back—were sewn together surgically to determine if a shared spine would correct the disability. The surgical site turned gangrenous, and both twins died shortly after.

Despite the ersatz patina of science, Mengele’s work was of the poorest scientific quality. Having subjected hundreds of victims to experiments, he produced no more than a scratched, poorly annotated notebook with no noteworthy results. One researcher, examining the disjointed notes at the Auschwitz museum, concluded, “No scientist could take [them] seriously.” Indeed, whatever early advances in twin studies were achieved in Germany, Mengele’s