Red coloration in the pericarp of corn is dominant to colorless pericarp. Hence the F, hybrid plant, which carries a gene from each parent, has red kernels. One-fourth of the F₂ plants carry two genes for red pericarp and one-half carry one gene for each color. Both types have red kernels, giving a ratio of three plants with red kernels and one plant with colorless pericarp.

At times, however, the characters are not inherited together, particularly when the locations of the genes on the chromosome are distant. A disruption of linkage results when an exchange occurs between the chromosomes in the region separating the genes. It is called crossing over.

When the homologous members of a pair of chromosomes begin to separate during reduction division, we mentioned that separation occurs first in the centromere region and subsequently the chromosomes uncoil. During the uncoiling process, numerous breaks may occur in the strands of chromonemata of the chromosomes, and the broken ends of chromonemata of homologous chromosomes may rejoin.

When such breaks occur in the chromosome region between two genes, a crossover occurs.

If the two genes lie at a considerable distance from each other, the frequency of gametes with crossovers may be as great as those containing the parental linkage, and from genetic evidence it will be difficult to determine if the genes actually are located on the same chromosome. The crossover mechanism provides a means of assortment of genes, even though they occur on the same chromosome.

IN CONCLUSION: Genes, which constitute the germ plasm and determine the hereditary traits of plants, are highly stable. Thought to be composed of complex organic molecules, they have the capacity to divide and reproduce themselves in kind.

The chromosome numbers of plants also are highly stable. Throughout the evolutionary process in plants, mechanisms that assure constancy of chromosome number through innumerable cell divisions of a growing plant and during the processes of sexual reproduction have developed.

During the processes of sexual reproduction, means are present, however, for assortment, segregation, and recombination of genetic factors. Tremendous genetic variability thereby is provided within a species. Occasional mutations of genes also contribute to the variability.

Genetic variability is especially significant in the evolution of plants. Plants that carry the proper combinations of factors are provided with the maximum opportunity to survive. Genetic variability also provides the basis for the potential improvement that man can make within a species to adapt it for his specific uses.

MARTIN G. WEISS is Associate Director, Crops Research Division, Agricultural Research Service. He received his doctor's degree from Iowa State University in genetics and plant breeding, and taught courses in Plant breeding at Iowa State University for 4 years. He has conducted research in the breeding of soybeans and forage crops since 1936.

JOHN E. SASS is a professor in the Department of Botany and Plant Pathology, Iowa State University. He received his doctor's degree from the University of Michigan in 1929. Since 1928 he has taught plant anatomy and related subjects at Iowa State University and has collaborated on the morphological phases of projects of the experiment station.