Well, one usual claim is that individuals from every human population are fully interfertile, completely able to interbreed... Of course, like so many sweeping statements, no one has carefully tested this for all possible combinations.It is true that research has not answered this question, but a recent study tell us that some combinations seem as fertile as same-race couplings. I can't access the full text, but here's some of the abstract:
This study uses 2000 - 2005 American Community Survey data on married (n =272,336) and cohabiting (n = 48,769) couples to compare the fertility of endogamous and exogamous couples. Interracial and interethnic partnering do not affect fertility for cohabiting, Black-White, Mexican-White, and Puerto Rican-White intermarried couples, but it does reduce fertility in Chinese-White and Asian Indian-White intermarriages (Fu, Vincent Kang. Journal of Marriage and Family, Volume 70, Number 3, August 2008 , pp. 783-795(13)).