In 1937 Dr. Josef Mengele joined the Nazi party and the SS. Three years later he was drafted to be a medical officer. By 1940 he had been sent back to Germany after being wounded and not long after he began working at the Kaiser Wilhelm Institute (KWI) for Anthropology, Human Genetics, and Eugenics. Eugenics is the science of improving a human population by controlled breeding to increase the occurrence of desirable heritable characteristics. Which is why it's not very much of a surprise that shortly after this when he was promoted to an SS Captain and was transferred to begin working at Auschwitz.
Experiments On Twins
He began his career at Auschwitz by working in the "Gypsy Camp" where most prisoners were gassed or became his test subjects. His other interests that continued to develop with his power over prisoners. He was fascinated by the connections between twins, this is what consumed most of his studies. According to BBC news, he had an "unlimited supply of twins" and subsequently wouldn't be help responsible if they all died due to working on the "Final Solution". Professor Paul Weindling of Oxford Brookes University, who is the author of Victims and Survivors of Nazi Human Experiments, said, "I found a record of a prisoner doctor and bacteriologist who was forced to work for Mengele that there were 732 pairs of twins," he says, and suggests the doctor was interested in genetics. "I think Mengele might have been interested in the inheritance of the propensity to having twins." Mengele even performed experiments in which he sewed twins together in such a precise way that even their veins were attached.
Investigating the limits of human endurance and existence at extremely high altitudes, he placed the victims in the low-pressure chamber and thereafter the simulated altitude was raised. Many victims died because of these experiments and others suffered injury, torture, and ill-treatment.