McCurntock, Barsara, Cornell University, Ithaca, N. Y.: The production
of maize plants mosaic for homozygous deficiencies: Simulation of the bm, pheno-
type through loss of the Bm locus. In two separate cases, two mitotically
functional chromosomes, a deficient rod-shaped chromosome and its reciprocal,
a small ring-shaped chromosome, were produced from a single chromosome-V
by means of X-ray treatment (RHOADES and McCLINTOCK, 1935). Beginning
at the spindle fiber attachment region, the section of the short arm composing
the ring and inversely the piece lost from the rod was 1/20 and 1/7 the length
of the normal chromosome respectively and included the locus of the Bm
gene (allele of by, brown mid-rib, producing a brown color in the lignified cell
walls). The larger ring-chromosome is characterized by frequent losses from
the nuclei during mitotic cycles and less frequently by changes in size resulting
in duplications and deficiencies. When two deficient rod-chromosomes, the
smaller and the larger, plus the larger ring-chromosome which covers both
deficiencies, are present in a plant, loss of the ring will give rise to tissues
homozygous deficient for the region represented by the smaller deficiency.
Changes in size of the ring should produce tissues homozygous deficient for
sections within the limits of the smaller deficiency. Five types of homozygous
deficient tissue have been distinguished. Through total loss of the ring: brown
cell walls, colorless plastids, poor growth capacity. Through changes in size
of the ring: (1) brown cells walls, green plastids, good growth; similar in detail
to tissue produced by the normal bm: gene. (2) Colorless cell walls, colorless
plastids, normal sized cells, good growth capacity. (3) Colorless cell walls,
colorless plastids, small cells, excessive proliferation at external surfaces. (4)
Same as (3) but with brown cell walls. .