What indicates mixed chimerism in STR PCR analysis?

Mixed chimerism in short tandem repeat (STR) polymerase chain reaction (PCR) analysis is indicated by the presence of both recipient and donor alleles in the genetic material being analyzed. This scenario typically arises in situations where an individual has received a stem cell or organ transplant from a donor. In such cases, the recipient's original cellular DNA patterns can coexist with those from the donor.

When both sets of alleles are detected through STR analysis, it demonstrates that the recipient's immune system has accepted the donor cells to some extent, resulting in a situation where both genetic contributions are present. This coexistence is critical in assessing the extent of chimerism and can have implications for monitoring transplant acceptance, potential rejection, and overall patient health.

The other options do not support this definition: the presence of only recipient or only donor alleles would indicate that one population has entirely replaced or dominated the other, thus not reflecting a mixed chimeric state. No alleles present would suggest a failure to obtain or detect any genetic material, which is not indicative of mixed chimerism either.