The nature of tri-allelic TPOX genotypes in African populations

Approximately 2.4% of indigenous South Africans have three rather than two TPOX alleles. Data collected during routine paternity testing revealed that the extra allele is almost always allele 10 and that it segregates independently of those at the main TPOX locus. Approximately twice as many females as males have tri-allelic genotypes which suggested that the extra allele is on an X chromosome. This is supported by the finding that males with three TPOX alleles invariably transmit two alleles to their daughters but only one to their sons. Tri-allelic genotypes were also found in paternity casework samples sent to the laboratory in South Africa from Namibia, Angola and Ghana. These occurrences suggest that the variant existed before the start of the Bantu expansion from central West Africa into Southern Africa.