In three inbred lines of Drosophila melanogaster w-mutations and several other markes had been isolated. Tests of primary non-disjunction were performed in the nine possible intra- and inter-strain crosses. Some combinations were studied further to elucidate the effects of maternal age on mating and brood pattern. It was shown that homozygosity per se had no influence on non-disjunction but that the frequencies of non-disjunction did differ in the three lines. The occurrence of non-disjunction is shown to be genetically determined by a complex dominant-recessive relationship. Among the inter-strain crosses two combinations, with a common mother, showed, in the first egg-laying period, a great excess of XO males. Similarities to the “hybrid dysgenesis” syndrome are discussed and it is concluded that the similarities are confined to chromosome loss. Contrary to most former studies, the majority of the present tests gave similar frequencies of exceptionals in both sexes. It is stressed that only XXY exceptionals are unequivocally indications of non-disjunction, while XO offspring may occur by other mechanisms of chromosome loss as well.
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