GAMETIC LETHALS ON THE FOURTH CHROMOSOME OF MAIZE
LANTS of maize, heterozygous for su, have been observed that pro-P duced no su seeds when selfed or pollinated by su pollen, but whose pollen when applied to su ears produced in excess of 96 percent su seeds (figure 2, H, I). This anomalous condition is caused by two gametic lethals, small pollen-I, ( s p ) , and lethal ovule-I (lo).l Preliminary reports on the inheritance of these two factors have been published, (MANGELS-DORF 1931 , 1932 , SINGLETON 1932 . Both sp and lo are closely linked
... h su(sp su = 5 percent recombinations; su lo= 2.2 percent) and give distorted ratios of Su:su because of this close linkage and the fact that one gene is lethal to the male, the other to the female gametes. Pollen grains of the composition sp are smaller than normal (figure 5 , C, D, and E) and in most * The cost of the accompanying half-tone illustrations has been borne by the Galton and Factor symbols not accompanied by numerals represent either the first or the only gene with Mendel Memorial Fund. that literal symbol; that is, s p = s p~, etc. GENETICS 35: 366 July 1940 Downloaded from https://academic.oup.com/genetics/article/25/4/366/5937116 by guest on 22 April 2021 LETHALS IN MAIZE 367 since lo was linked with su when first observed while sp was linked with Su. The most probable explanation of the origin of lo is that its occurrence in a stock segregating for sp was purely a coincidence although there is a possibility of a causal relationship. The first lo/ + ear had I I . I percent of su seeds. The two preceding generations had 53 and 64 percents respectively, showing presence of the s p factor but absence of the lo factor. All studies of the lo condition were made by the senior author in Connecticut. The inheritance of this factor was complicated a t first because the stock was also segregating sp and gave the unusual results described above. However the two factors have been separated and each studied independently and in combination. INHERITANCE OF S p AND 10 Disturbance of su ratios Both the factors s p and lo were discovered because of their close linkage to su and the inheritance of each has been studied largely through its disturbance of the su ratio in selfed and backcrossed ears. The sp condition was originally termed "high sugary" since it was in the repulsion phase, sp+/+su, and gave a high percentage of su kernels (figure I), The other gametic lethal, lo, was originally called "low sugary" because the coupling phase, lo su/ + +, the one first observed, gave a very low percentage of su kernels. The factor sp, produces extremely low sugary ratios when in the coupling phase sp SUI + +, while the original "low sugary," lo, produces very high sugary ratios in the repulsion phase Zo+/+su (figure 2 ) . The disturbance of the su ratio in stocks heterozygous for lo or sp can peihaps best be understood by presenting in tabular form the functional male and female gametes. This is presented in table I .