Yuanyang allele pairs enable engineered obligate co-inheritance for population replacement and suppression

Mendels law of segregation, which dictates independent assortment of alleles, is a cornerstone of genetics. Here, we present the Yuanyang allele pair (YYAP), an engineered underdominance gene drive system that enforces obligate co-dependency between homologous alleles via synthetic toxin-antitoxin circuits. This design produces a striking deviation from classical Mendelian segregation: YYAP alleles cannot segregate independently, reshaping inheritance outcomes without altering the physical mechanics of meiosis. In Drosophila melanogaster, optimized YYAP strain K711 exhibits >99% hemizygous lethality when crossed with wild type, while maintaining stable transmission and high fitness in transheterozygotes. Cage experiments demonstrate efficient population replacement, positioning YYAP as a confined, resistance-proof alternative to CRISPR homing gene drives. Release of only males represents a self-limiting suppression strategy that was also successful in cage experiments. By imposing a near-complete postzygotic incompatibility, YYAP establishes a programmable framework that not only supports pest management, but also enables modeling of reproductive isolation and allelic co-dependency, thereby creating opportunities to explore speciation and broader synthetic inheritance systems in ecology and evolution.