Mutations that interfere with necessary vital functions are called lethal mutations. Their phenotypic effect is death. Dominant lethals are lost in one generations. Recessive lethals may linger a long-time in a population because they can be hidden in heterzygous condition. In Drosophila genes like curly wings (Cy), plum eyes (Pm) and stubble bristles (sb) influences the viability of the flies when present in homozygous condition. In such fly Cy cy h(heterozygous) will survive but Cy Cy (homozygous dominant) will not.