Hybrid dysgenesis manifests when crossing "P" strain males with "M" strain females and not when crossing "P" strain females (females with autonomous "P" elements) with "M" strain males. The eggs of "P" strain females contain high amounts of a repressor protein that prevents transcription of the transposase gene. The eggs of "M" strain mothers, which do not contain the repressor protein, allow for transposition of "P" elements from the sperm of fathers. In "P" strain females, the repressors are found in the cytoplasm. Hence, when "P" strain males fertilize "M" strain females (whose cytoplasm contain no repressor), the male contributes its genome with the "P" element but not the male cytoplasm leading to "P" strain progeny.