Controlling elements and the gene.

In a recent brief review (McClintock, 1956), a description was given of types of elements carried in t,he maize chromosomes that serve to control gene action and to induce, at the site of the gene, heritable modifications affecting this action. These elements were initially discovered because t,hey do not remain at one position in t,he chromosome complement. They can appear at new locat,ions and disappear from previously determined locations. The presence of one such element at or near the locus of a known gene may affect t,he action of this gene. In so doing, it need not alter the action potentials of the genie substances at the locus. Therefore, these elements were called controlling elements. It was also shown that controlling element,s fall int’o groups, the members of each operating as an integrated system in the control of gene action. In this report, some aspects of controlling elements mill be considered that could not be discussed in the above-mentioned review. This will necessitate mention of some of the well known gene loci in maize, and symbols for them will be used in the discussion. So that a readv reference may be available, the pertinent i&formation about each of these loci is given in the following list. Chromosome 1: P, pericarp and cob, color. A large number of alleles are known? including P”“, which gives variegated pericarp and cob color. Chromosome 3 : Al , anthocyanin pigment produced in kernel and plant; al, standard recessive allele, no anthocyanin in kernel or plant. Shs , normal development of endosperm tissues; sht , recessive allele, shrunken endosperm. Very closely linked with Al ; less than a quarter of a crossover unit distal to it. Chromosome 5: Pr, purple aleurone color; pr, recessive allele, red aleurone color. Located in long arm of chromosome 5. AZ , anthocyanin pigment developed in kernel and plant’; at, standard recessive allele, no anthocyanin developed in kernel or plant. Located in short arm of chromosome 5. Gives approximately 28 per cent recombination with Pr. Chromosome G: Y, yellow starch in endosperm; v, recessive allele, white starch. Chromosome 9: I, dominant inhibitor of aleurone color in kernel. C, allele of 1, pigment produced in aleurone layer of kernel; c, recessive allele, no pigment in aleurone layer. Shl , normal development of endosperm of kernel ; shl , recessive allele, shrunken endosperm. Located approximately four crossover units proximal to I. Bz, purple anthocyanin pigment in plant and aleurone layer of kernel; “62, standard recessive allele, bronze color in both plant and kernel. Located approximately two crossover units proximal to Shl . WZ, amylose starch produced in pollen and endosperm, stains blue with solutions of I-KI; wx, recessive allele, starch is amylopectin, stains red-brown with I-K1 solutions. Located approximately 15 crossover units proximal to Bz.