Monosomics and nullisomics are extremely useful for determining on which chromosomes particular genetic factors are situated. The ordinary method of locating genes on the chromosomes is based on the determination of "linkage"—that is, the tendency of genes on the same chromosome to be inherited together. Where the number of chromosomes is large, as in polyploid plants, adequate determination of linkages requires that a large number of genes be available with which to work. Even if enough genes can be found, the linkage studies require that many crosses be made and tested.

With monosomics and nullisomics, comparatively few crosses are necessary to determine which chromosome carries a particular gene. In fact, dominant factors may be located without any crosses, simply by inspection of the nullisomic plants themselves, for in the complete absence of a gene, there is an absence of effect. Location of recessive genes usually requires that a strain carrying the dominant allele be crossed with each of the nullisomics or monosomics, but in wheat, for example, this means only 21 crosses.

In the second generation following these crosses, the recessive condition is shown by the normal proportion of plants (one out of four), except in one cross. In this one the frequency of recessives is much less than one in four, only nullisomic plants being affected; and the chromosome which was deficient in this particular cross is thereby identified as the one carrying the gene. Genes in wheat with effects hard to classify precisely (such as most of those for disease resistance) are located by crossing to the nullisomics, as before, and then backcrossing to the nullisomics for a few generations. Lines are eventually obtained, each of which has one known pair of chromosomes from the resistant variety, and the other 20 pairs from the standard variety. Tests of these lines show which chromosomes of the resistant variety carry the resistance factors.

A program for the use of nullisomics in cataloguing the genetic factors for disease resistance in wheat has been in progress since 1942 at the Missouri Agricultural Experiment Station and at Beltsville. In this program, in which H. A. Rodenhiser and I are collaborating, resistance to the widely destructive disease, black stem rust, has received most attention. The objective of the study is to find the major genes for rust resistance in wheat, and to learn on which chromosome each of these genes is located. The information obtained should enable breeders to put together new varieties with superior resistance to disease and to improve the resistance of existing varieties. In the building up of these new varieties the nullisomics and monosomics will again be useful, since they may be used to increase the ease and precision of transferring chromosomes and genes from one variety to another.

Genetics of Micro-Organisms

Finally, a few words about the recent active interest of geneticists in the micro-organisms, especially molds, yeasts, bacteria, and viruses. Some of the research being done is of the utmost significance to genetics, because of the information obtained on how genes act in the living cell. The results with bacteria and viruses are of considerable importance in the field of medicine. For example, a mutant strain of Penicillium mold has been obtained, following X-ray treatment, that is much more productive of penicillin than any previously known type. The findings of M. Demerec of the Carnegie Cold Spring Harbor Station in New York, on the way in which bacteria, through mutations, may adapt themselves to tolerance of high concentrations of penicillin, have far-reaching implications in medicine.

As concerns agriculture, some persons have supposed that new varieties of yeasts and fungi might be bred that would largely replace present soil grown plants as sources of food, but there is very little indication that this development will soon, if ever, take place.

The most important application to agriculture of the genetic work with micro-organisms will probably be the creation by geneticists of new strains and races that can profitably utilize various farm products, including waste materials. This will mean increased demand for the farmer's wares.

THE AUTHOR

E. R. Sears, a geneticist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, has been a member of the Department since 1936. Stationed at Columbia, Mr., Dr. Sears works with wheat and related grasses. His research on nullisomics in common wheat, each with a pair of chromosomes missing, provides a new approach to the genetics and breeding of wheat. With E. S. McFadden of the Department he has shown the way in which our common wheats probably arose in nature. Dr. Sears is a graduate of Oregon State College and Harvard University.