A Walk Through the Maize Genome

This walk through the 10 chromosomes of maize was created using three maps. The cytogenetic map came from the physical appearance of maize chromosomes as seen under a microscope (Fig. 1).The linkage map came from observing how often mutants, such as those you will see in this field occurred together.

Figure 1. Anaphase of normal mitosis(left) and N2O-inducted abnormal anaphase (right). (Reprinted with permission of A. Kato).

Genes that are linked (ie. physically located near each other on the same chromosome) are observed together more often than chance. The more frequently they are seen together the closer they are to one another on the chromosome.

Linked genes do not follow the rule of independent assortment because they are physically associated with one another on the same chromosome. Emerson, Beadle, Fraser, Rhodes and McClintock used this type of information to make the first complete genetic maps of maize in 1935.

For 50 years the maps of maize progressed. More genes were added as new mutants were tested and linked together on the chromosomes by careful observation and statistical confirmation. Recently, a molecular map of maize was produced here at the University of Missouri.

Changes in the DNA sequence of a gene can alter the protein that is produced and result in altered phenotype which we often call mutants. Current estimates indicated there are about 55,000 genes in maize. Each chromosome in maize is subdivided into eight to twelve smaller sections called bins that provide a convenient way to organize information so that researchers can easily compare their data with each other.

Mutants in the "Walk Through the Maize Genome" are organized in order by bin. Each bin is represented by one or two visible mutants, though in reality there are hundreds of genes in each bin.

Some bins are not represented, if no visible adult mutant has been identified in that bin.

Figure 2. Representation of maize chromosomes with horizontal lines representing divisions between individual bins. (Courtesy MaizeGDB)

The stylized chromosomes in Fig. 2 represent the combined map of maize. In the 1930's numbers were assigned to the chromosomes of maize by their size and physical appearance with one being largest and 10 being smallest.

The narrowed area is the centromere, which divides a chromosome into two arms. The centromere helps each chromosome move to its proper location during meiosis. The horizontal lines indicate the division or each bin. Within each bin are the genes of maize.

The mutants you see today may only be part of a well characterized biochemical pathway or have undiscovered function. They are planted here to honor our past and inspire our future.