5. Utilization of translocations with endosperm markers in the study of economic traits. In studying the inheritance of any difficult trait, a simple test can be made for linkage with an endosperm character such as su or wx, especially if the multiple recessive combination occurs in one of the commercial inbred lines. For example, in studying resistance to bacterial wilt, a resistant line can be crossed with a susceptible sugary, and the F1 crossed to a susceptible sugary inbred. Comparison can then be made between the resistance of plants from starchy vs. sugary seeds of the backcross ear. This tests for resistance genes in the central portion of chromosome 4. If this test is negative then a similar test can be made involving translocation 1-4a. (Resistant × su T1-4a) × susceptible sugary inbred. A test of plants from Su vs. su seeds then becomes a test for resistance genes in the long arm of chromosome 1. From the standpoint of testing technique, it means that su can be used as a marker for any chromosome or part of a chromosome for which the proper translocation is available. And the same recessive sugary inbred line can be used for all backcrosses. The suppression of crossing-over in the neighborhood of the translocation aids in making the method more efficient in detecting linkages. If an appropriate series of translocations existed, it would be possible to cover the entire chromosome complement with the use of one endosperm gene such as su.
The series of translocations available at present is not sufficient to cover all chromosomes using only one marker gene. By using two series, one with su, the other with wx, it is possible to have at least one translocation for each chromosome. More translocations are being isolated and it is hoped that, year by year, the series available for this purpose viill be greatly improved and simplified.
Work on the inheritance of economic traits by using endosperm marked translocations is being taken up at several of the corn belt experiment stations. To facilitate these programs I have made the F1 crosses here at Pasadena with such translocations as are now available. These were:
su series - 1-4a, 2-4A, 2-4c, 4-5b, 4-5d, 4-6a, 4-8, 4-9a, 4-10b and a new 2-4 (a-29)
For sweet corn lines I was able to add 4-7a, a new 4-9 (F-22), 4-10 (B-45) and a multiple 1-3-4-5 (B-2)
wx series - 1-9a, 1-9c, 2-9b, 3-9a, 3-9b, 3-9c, 4-9b, 6-9a, 8-9a, 9-10b, and now 4-9 (F-22), and 6-9 (a-66)
pr series - 1-5a, 1-5c, 2-5b, 3-5b, 3-5c, 4-5c, and 4-5d
The above is too large a series for completion of tests, except for such traits as can easily be tested in the seedling stage. But the additional F1's may serve as a reserve for checking any indications of linkage.