The revelations of the atrocities carried out by the Nazi regime during the Holocaust of the 1930s and 40s actually gave birth to the term genocide. After learning about the systematic slaughtering of 5 to 6 million Jews, with the Nazis' goal having been to exterminate all Jews everywhere, the also newly born United Nations (1945) propounded the Convention on Prevention of the Crime and of Genocide (CPPCG) in which was stated the very first legal definition of genocide: the act of killing, bodily harming, subjecting to conditions aimed at killing, preventing the births of, or "forcibly transferring children" of one group to another "with intent to destroy, in whole or in part, a national, ethnical, racial or religious group" (University of Hawaii, Rummel, "Genocide"; Convention on the Prevention and Punishment of the Crime of Genocide, Article II). Hence, based on that definition, we can certainly view as genocide the description of what the Nazis did to the Jews held in the concentration camp that is the center focus of John Boyne's young adult novel The Boy in the Striped Pajamas.
In Chapter Nineteen, Shmuel brings Bruno a matching pair of pajamas with stripes so that Bruno can blend in on the other side of the fence and help Shmuel find his missing father. By the end of the chapter, Bruno and Shmuel are herded together by soldiers with a hundred different people. Shmuel comments that "they make people go on marches" and speculates that that's what must be going on now. When asked by Bruno if the march lasts a long time, Shmuel makes the tell-tale comment, "I don't think so ... I never see the people after they've gone on a march. But I wouldn't imagine it does [last a long time]" (p. 110). From Shmuel's comment that he never again sees the people who have been taken on a "march" coupled with the description that Bruno, Shmuel, and the hundred other people have been herded and shut into a large, dark room, the reader knows that Bruno, Shmuel, and all the other people have just been herded into one of the gas chambers to be exterminated.
Since the legal definition of genocide refers to destroying an ethnic group "in whole or in part," we know that this isolated incident in the story certainly does fit the definition of genocide. Though the story describes only a small group of Jews being killed, a small group still fits the legal definition of genocide because it is a "part" of an ethnic group.