The most prevalent type of hybrid that was grown in the United States in the 1930’s and 1940’s is known as a double-cross hybrid. As the name implies, producing a double-cross hybrid requires two stages of crossing involving two pairs of inbreds (see diagram below). In Step 1, two pairs of inbreds, A and B and Y and Z, are crossed to produce single-cross hybrids, AB and YZ. In Step 2, the two single-cross hybrids produced in Step 1 are crossed to produce the double-cross. Typically, A and B are closely related and Y and Z are also closely related, but neither A nor B is closely related to Y or Z. Unlike a single-cross hybrid, plants of a double-cross hybrid are not genetically uniform.
Compared to single-cross hybrid production, production of double-cross requires an extra step. During the early history of the hybrid seed industry in the United States, this extra step was necessary because the inbreds available at that time produced so little grain that making commercial quantities of seed of single-cross hybrids was difficult. Even though the inbreds of each pair of a double-cross hybrid were related, the resulting single-cross hybrids exhibited sufficient vigor to allow those single crosses to be used successfully as parents in mass production of commercial seed. In most environments, the best single-cross hybrid will have superior performance to the best double-cross hybrid. As breeders gradually improved the performance of inbreds through selection, it became possible to commercially produce the more desirable single-cross hybrids.
Double-cross hybrids may become important in the organic corn market. In production of organic hybrid seed corn, herbicides cannot be used. Therefore, seed producers desire parents that have good vigor and can compare successfully against weeds. The single-cross parents of double-cross hybrids have this desired vigor.