Among a number of crosses between rice varieties, lethality of F₁ plants was found in two crosses. It was shown that this hybrid inviability was due to a set of “complementary dominant lethals”.
It is generally found that when distantly related varieties of rice are crossed, a few F₂ plants show a poor growth. In a cross showing this phenomenon typically, the underlying genes were analyzed. It was found that a set of duplicate genes, A₁-a₁ and a₂-A₂, are concerned, and that possessing two A's, i.e. A₁A₁, A₂A₂ or A₁A₂, is necessary for normal growth. These genes may be said as “complementary recessive lethals”. since the recessive alleles, if combined, produce a lethal effect. It was discussed that this type of duplicate genes may be widely spread among rice varieties, and may work as an isolating barrier between distantly related varieties.