ture of knowledge on homoeology and on the genetic organization of polyploid wheat.

Not surprisingly, because of his discovery of methodologies for determining the chromosomal locations of genes, Sears used this procedure to help others with practical problems. These included identification of the genetic status of disease resistance genes—for example, in 1957 with W. A. Loegering and H. A. Rodenhiser. In addition, using chromosome substitutions, the stem rust resistance genes in Hope, Thatcher, Red Egyptian, and Timstein were positioned on nine different chromosomes. Telocentric mapping subsequently enabled Sr9 and Sr16, respectively, from Red Egyptian and Thatcher to be placed about forty cross-over units apart along the long arm of chromosome 2B. Subsequently, bunt and powdery mildew resistance genes were chromosomally located.

Aneuploids also enabled Sears to contribute to the understanding of gene action in wheat. In particular, he was the discoverer of the hemizygous ineffective condition in which, when a recessive allele is carried on a monosomic chromosome, so no dominant allele is present, the recessive phenotype is not expressed. Sears explained that two doses of such recessives are necessary for gene products to pass the threshold at which the recessive phenotype appears.

Ernie Sears' astonishing discoveries show that many homoeologous genes in hexaploid wheat continue to perform essentially the same function as in the diploid ancestors from which they came. Nowadays, RFLP mapping, by and large, shows similar gene orders on homoeologous chromosomes and that, although isoenzymes may show some differences, homoeo-alleles generally still produce essen