Actually that’s not so. From Wikipedia :
Genocide is “the deliberate and systematic destruction of, in whole or in part, of an ethnic, racial, religious, or national group”, though what constitutes enough of a “part” to qualify as genocide has been subject to much debate by legal scholars.While a precise definition varies among genocide scholars, a legal definition is found in the 1948 United Nations Convention on the Prevention and Punishment of the Crime of Genocide (CPPCG). Article 2 of this convention defines genocide as “any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group.”
Note the last two especially.
However, a subgroup vanishing just because the people involved willingly have children with other groups isn’t genocide. It’s just something that happens; groups come, they go.