Lenin's ancestry was a state secret

Lenin considered himself and was always considered Russian, but he was not an ethnic Russian. Lenin's father was half Chuvash and half Kalmyk. The Kalmyks are a Mongol people, which is how Lenin got his slightly Mongoloid appearance, and the Chuvash are a Turkic people. 

This is interesting when one thinks that Trotsky and also, I think, though I cannot find it cited on the internet, Lenin himself referred to Stalin's 'Asiatic cruelty'. 

Lenin's mother's father was a Jew who converted to Christianity and married a woman of German and Swedish ancestry. This was something Lenin did not know. His sister discovered it after his death and wrote a letter to Stalin asking that this fact be publicised as it redounded to the credit of "this remarkable people" and would "undo and combat antisemitism". 

Instead, Stalin insisted that it be kept secret and one poor man went to the Gulag for ten years for repeating the story. In his verdict the judge at Moscow's Court-Martial Tribunal, which had been created to try “especially dangerous criminals”, said

The accused does not deny spreading these fabrications but does claim that he did not intend to commit counterrevolutionary acts... Kamenitzer has a university education and could not therefore fail to recognise the counterrevolutionary tenor of his fabrications.

In 1964 the well-connected and loquacious Communist novelist Marietta Shaginian, once described as “that twice-decorated and thrice mad woman”, researching a book on Lenin's ancestry, discovered that Lenin had a Jewish grandfather. She was summoned to attend the Party's chief ideologist, Mikhail Suslov, the third most powerful man in the country, who told her:

"Our common case is dear to you not less than to me, to the Party and Government, to the Soviet people. Therefore, there is no need to explain to you how damaging your book would be with statements from the secret archives documents. Neither the Soviet state nor the Soviet citizens of Jewish origin could easily recover from such a horrible blow. This is why I consider it necessary to ask your word of honour that such a book will never appear."

No book appeared, but Marietta Shaginian reported her discovery and her conversation with Suslov widely around Moscow. In high Communist circles in Moscow that autumn it was a subject of conversation that eclipsed even Brezhnev and Kosygin, who had just replaced Khrushchev in power.

It only became public knowledge in 1992.

Lenin was fiercely anti-racist and did not consider ethnicity important but he would have been pleased to learn about his grandfather. He once said

"An intelligent Russian is almost always a Jew or someone with Jewish blood in his veins."