8. New dottable A^d allele.

Of the seven original A^d derivatives from A^bEt/a et plants, six carried et (News Letter No. 23:54‑56. 1949). More extensive data now available support the conclusion that these dilutes arise by crossing over between genes comprising A^b. In testing these A^d cases for dottability, six were found to be negative in this regard. One, tentatively designated A^d‑2, has a high rate of dotting. Plentiful dotting occurs on endosperms of the constitution A^d‑2/a^dl/a^dl Dt Dt -. Moreover, A^d‑2/A^d‑2/A^d‑2 endosperms on selfed ears of homozygous plants are dotted. Tests indicate that the dotting is determined by the Dt gene as is the case with recessive a. There is no reported case of a dottable A^d gene among the large number of mutants obtained from A^b/A^b plants. Since the A^d‑2 mutant originated from an A^bEt/a et plant and carried et, it is tempting to consider that synapsis and crossing over in the mother cell were such that the A^d‑2 derivative now carries the proximal (A^d) component of A^b adjacent to and on the same strand with recessive a. By reason of the presence of the latter, A^d‑2 would now appear to dot. If this were the case it would be reasonable to expect that A^d‑2 should occasionally give rise to a dottable recessive a and that the occurrence of the latter would be associated with crossing over. From a number of A^d‑2/A^d‑2 plants crossed using a^dl a^dl Dt Dt pollen this past season two kernels with colorless, dotted endosperms were obtained. Although it is not known whether their occurrence is associated with crossing over, these mutants carry a dottable a derived from A^d‑2.

John R. Laughan