http://www.faqs.org/faqs/judaism/FAQ/07-Jews-As-Nation/section-9.html
"Recently, Kohen Madol Haplotype testing has been performed among the Lemba; these tests have proven the Lemba to have the highest concentration of the gene marker than any known halakhic Jewish group. This is reported in an article titled "Decoding the Priesthood" by Peter Hirshberg and Jane Logan, in Jerusalem Report (May 10, 1999 issue). According to this article, the Lemba have the same proportion of the gene as "Western" Jews and a remarkably high frequency among their Buba clan, a senior clan parallel to our Cohens. "
If Cohenim took African wives the Cohen gene (which must be some powerful gene) would be there but the descendants would not be considered Jews... by other Jews.