Maize

As a major crop plant, maize was already the subject of study by plant breeders at the time of the rediscovery of Mendel's laws of inheritance at the beginning of the twentieth century. The inheritance patterns of readily observed traits were uncovered through controlled crosses and the examination of progeny. In many respects, maize was an ideal model system for this early period in the study of genetics. Male and female flowers are borne separately and are easily manipulated for controlled crosses. Large amounts of pollen are produced in the tassels (male inflorescence) over a period of days, and one ear (female inflorescence) contains many seeds (kernels). Large progeny arrays could be produced in one season.

Mutations caused differences in these ears of corn stored in the Maize Genetics Cooperation Stock Center, a repository for mutated maize located at the University of Illinois, Urbana-Champaign.

The high genetic diversity of maize provided many interesting mutant phenotypes to study, many of which were recessive. These could be maintained in a heterozygous state by the outcrossed breeding system (most fertilizations are the result of pollen transfer among plants) and easily uncovered by selfing (fertilizations that result from a plant's own pollen). There was also ample scope for selection of extreme phenotypes in continuous (quantitative) traits. A drawback for maize, compared to short-lived fruit flies, is that it only produces one or two crops per year, depending on location. However, many early maize geneticists knew that kernel phenotypes, which were discernable at harvest time, often predicted phenotypes in the adult plants, and could be used to set up the following season's crosses.

One of the earliest breakthroughs in crop breeding was the detection of hybrid vigor in maize by George Harrison Shull in 1908. He found that the progeny of two inbred lines were more productive than their wind-pollinated progenitors. This discovery provided the stimulus for the commercial propagation of maize and made it one of the most productive food plants worldwide.