one from each strain. If the resulting progeny have red eyes, the two strains are said to complement; if the progeny have white eyes, they do not. If the strains complement, we imagine that one strain must have a genotype aa BB and the other AA bb, which when crossed yield the genotype AaBb. In other words, each strain is homozygous for a different deficiency that produces the same phenotype. If the strains do not complement, they both must have genotypes 'aaBB', 'AAbb', or 'aabb'. In other words, they are both homozygous for the same deficiency, which obviously will produce the same phenotype. Complementation tests in fungi and bacteriophage Complementation tests can also be carried out with haploid eukaryotes such as fungi, with bacteria and with viruses such as bacteriophage. Research on the fungus Neurospora crassa led to the development of the one-gene-one-enzyme concept that provided the foundation for the subsequent development of molecular genetics. The complementation test was one of the main tools used in the early Neurospora work, because it was easy to do, and allowed the investigator to determine whether any two nutritional mutants were defective in the same, or different genes. The complementation test was also used in the early development of molecular genetics when bacteriophage T4 was one of the main objects of study. In this case the test depends on mixed infections of host bacterial cells with two different bacteriophage mutant types. Its use was key to defining most of the genes of the virus, and provided the foundation for the study of such fundamental processes as DNA replication and repair, and how molecular machines are constructed. Genetic complementation, heterosis and the evolution of sexual reproduction Heterosis is the tendency for hybrid individuals to exceed their pure bred parents in size and vigor. The phenomenon has long been known in animals and plants. Heterosis appears to be largely due to genetic complementation, that is the masking of deleterious recessive alleles in hybrid individuals. In general, the two fundamental aspects of sexual reproduction in eukaryotes are meiosis, and outcrossing. These two aspects have been proposed to have two natural selective advantages, respectively. Meiosis is proposed to be adaptive because it facilitates recombinational repair of DNA damages that are otherwise difficult to repair. Outcrossing is proposed to be adaptive because it facilitates complementation, that is the masking of deleterious recessive alleles (also see Heterosis). The benefit