When a series of lethals is to be tested inter se for allelism, the ideal test is one in which two lethals (A and B) are both known to be heterozygous in the parents that are crossed. If two or more ears per plant can be pollinated, there is no difficulty in making a test self on A and B plus a cross of A with B, repeating enough times to assure that the ideal test has been made [for a 3:1 trait, each paired test has 4/9 chance of success and 5/9 of failure to be an ideal test; thus since (5/9)6 = .03, 6 pairs consisting of 12 plants with 18 appropriate pollinations will have 97% certainty of including at least one ideal test, resolving the question of allelism].
Most often two ears per plant cannot be assumed and an optimal alternative is needed. In recent research here in which large numbers of such tests have been required, the procedure has been optimized as follows: Self a plant in source A and cross onto two plants in source B; test each of the two plants in B by crossing each onto one ear of a sib (set consists of 5 pollinations; failure of heterozygosity in source A is 1/3 and failure of showing heterozygosity in one or both plants of source B is 25/81; three such sets, totalling 15 plants, will have about 75% certainty of including an ideal test; 6 sets will have about 93% certainty).
E. H. Coe, Jr.
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