Frequency of Pollen Segregation in F₁. In populations so large as those required for the determination of mutation rates (particularly with low doses and control progenies), it is not feasible to determine the frequency of deficiencies and translocations by the direct cytological examination of every plant. Some indications regarding the frequency of chromosomal derangements may be obtained from the frequency and type of pollen segregation in F₁. Pollen segregation was recorded as to percentage and type of defective pollen, the types ranging from "a" (significant reduction in size but normal development of contents) to "e" (practically empty). In the table which follows, types a and b are listed as "subnormal", types c, d, and e as "aborted," and segregations of both classes in the same individual as "mixed."

The following facts determined from investigations in previous seasons are of help in the interpretation of the pollen records:

(1) "Directed segregation" in maize translocations is absent or extremely rare. The plants with segregating defective pollen therefore include all of those in which translocation has occurred as well as those with deficiencies.

(2) Gametophytic lethals at points of translocation are absent or very rare. If, as a result of "position-effect" or other causes, there were a tendency for mutational effects at the breakage points, it might be expressed by failure in development or functioning of the pollen carrying the translocation chromosomes. This does not occur. It is therefore possible to discriminate between segregating defective pollen due to translocation and that due to deficiency by transmission tests.

(3) F₁ plants with segregating defective pollen include many with cytologically detectable deficiencies not associated with translocation. Among pollen segregating plants from Xray treatment, these deficiencies include some which are obviously intercalary. Most of the cytologically detectable deficiencies are found in plants with "aborted" pollen, but in short intercalary deficiencies defective pollen is frequently of the "subnormal" class. The deficiencies from UV observed cytologically include none which is clearly intercalary. In all of the UV deficiencies so far observed cytologically the segregating pollen is of the "aborted" type.

(4) Among the plants with segregating defective pollen, the proportion due to translocation is much lower with ultraviolet than with Xrays. With high doses of ultraviolet, translocations unquestionably are induced, but the great majority of these are "deficiency-translocations"; that is, plants in which one or both of the chromosomes involved in the translocation has lost a segment. These deficiency translocations are usually very defective plants, and their frequency depends in large part upon the precautions taken to insure survival of the poorest plants of the progeny. Translocations of this type may not be detected by transmission tests; they may be identified only by direct cytological examination of the F₁.

The frequency of segregating defective pollen in these cultures is listed in the next table. The numbers and percentages given in parentheses represent the frequencies when each "high-sterile" is taken to represent two segregating factors for sterility.