I remember singing it in the playground in the late 1970s: “Hitler has only got one ball/ The other is in the Albert Hall/ Himmler is very similar/ But poor old Goebbels has no balls at all.” Of course, even at the age of eight, we all suspected that the idea that Hitler was one nut short of a lunchbox was probably not true. How on earth could anybody possibly know?

However, it now appears that not only might the ditty have been true, but Hitler had a hidden genetic disorder that would likely have hindered the development of his sexual organs. A gift for any wartime writer of bawdy ballads – he had a one in 10 chance of having a micropenis.

The answer to how we may know such intimate knowledge of the biological design of the Führer lies in a patch of dried blood on the sofa on which Hitler blew his brains out in April 1945. The stain was found by Colonel Roswell P Rosengren of the US Army, one of General Eisenhower’s press officers, who cut some fabric off the couch and took it home. Eight decades later, that blood has been analysed by scientists, who have managed to sequence Hitler’s DNA, and the picture his genome reveals is extraordinary.

The results are being shown tonight in a Channel 4 documentary called Hitler’s DNA: Blueprint of a Dictator. The big revelation is that Hitler’s genetic markers show he had Kallmann syndrome, a genetic disorder that delays and hinders puberty, often resulting in a very sm