Comparison of the Genetic Effects of Xrays and Ultraviolet Treatment. In 1939 and 194O an attempt was made to determine the relative frequency of mutation and other types of genetic alteration induced by comparable doses of Xrays and ultraviolet. Since there is no physical basis for equating doses of the two radiations, it is necessary to make the comparison on the basis of some biological equivalent, for example, to determine the effect upon mutation of two doses equal in inducing deficiencies or translocations. But since previous studies had shown that the deficiencies and translocations induced by ultraviolet are of types different from those produced by Xrays (or include various types in widely different proportions), the doses equivalent on the basis of one chromosomal effect would be widely different from those equivalent on another.

The doses used therefore were chosen arbitrarily at levels suited to the significant determination of mutation frequency, and their equivalence may be judged only by the frequencies of the various alterations detected. The Xray doses used are relatively low, so as to permit the survival of as many plants as possible and the production of well-filled ears, which is essential for the determination of mutation rates. The ultraviolet doses used are close to the tolerance limit for the wave lengths represented.

Since both types of radiation produce defective plants of various kinds, it is essential to reduce losses to a minimum and to consider the individuals lost as well as the survivors in the interpretation of the results. The populations used represent the entire seed population from the treated ears, and special precautions were taken to secure maximum germination and survival. Plants which died early or which failed for other reasons to yield a pollen specimen were classified as "+" (apparently normal plants, accidentally lost) and "-" (apparently defective plants). The treatments compared, populations used, frequency of endosperm deficiency (A, Pr, Su), and losses to pollen shedding are shown below: