8. Mosaic plants in part heterozygous and in part homozygous for a chromosome 5 deficiency. - Breakage in the spindle fiber insertion region of chromosome 5 resulted in two chromosomes, one a deficient rod-shaped chromosome and the other its reciprocal, a ring-shaped chromosome, each with an insertion region, the two equivalent genetically to one chromosome 5 (McClintock, Proc. Nat. Acad. Sci., 1932). Two such cases were described. In one case, known as the large deficiency large ring, the ring involved approximately one-sixth of the length of the chromosome, including the locus of Bm1. In the other case, called the small deficiency small ring, the ring involved about one-twentieth of the length of the chromosome and also included the locus of Bm1.
It has been found that the small deficiency can function through the eggs without the small ring being present also. Pollen having the large deficiency plus the large ring-shaped chromosome (the full genomic complement for chromosome 5) can function as well as normal pollen with an intact chromosome 5. When two such gametes fuse, an individual having the small deficient chromosome, the large deficient chromosome and the large ring-shaped chromosome is produced. As stated in the above publication, loss of the ring-shaped chromosome occurs in some mitotic divisions. In the plants resulting from the described cross, the nuclei and thus cells which arise after such a loss of the ring chromosome will be homozygous deficient for the amount of chromosome represented by the length of the small deficiency. Such plants should be therefore, a mosaic of heterozygous and homozygous deficient tissue if cells whose nuclei have undergone the loss of the ring chromosome can continue to propagate themselves. It was known that the heterozygous deficient tissues do not vary noticeably from non-deficient tissues. If, in these plants, the homozygous deficient tissue is viable and if the homozygous deficiency alters the structure of the cell, streaks of altered tissue should be detectable. Streaks of altered tissue were very obvious in the leaves of such plants. A histological study of the nature of the alterations is being conducted by Mrs. Lucy Abbe. From the appearance of the homozygous deficient tissue it is probable that such tissue would be inviable if not surrounded by normal tissue. The original "double-deficient" plants were obtained by crossing plants having a normal chromosome 5 with bm1, a deficient chromosome 5 with no lucus for Bm1 and the ring chromosome carrying Bm1. The "double-deficient" plants were all Bm1 except one plant which was variegated for Bm1 and bm1. The introduction of the bm1 locus of the normal chromosome 5 into the deficient chromosome is believed to have occurred as the result of a non-homologous crossover between the normal and deficient chromosomes with a resulting shift in the position of the deficiency (as described by Stadler in the Amer. Nat., 1934).