This is a study by E.G.Andersen in 1925 of the genotypic constitutions of broad echinus daughters of Drosophila Melanogaster, the fruitfly, where the crossing over of heterozygous genetic matter in the attached X chromosomes in primary non-disjunctional females, who gave exceptional offspring, was investigated. It was found that the crossing over occurred at the four-strand stage, only two strands crossing over at any one level, the crossing-over occurring freely between any two strands and random assortment of strands occurring at reduction division at the spindle fibre end with a disproportionate crossing over seen at times to join its complement (complementary crossing over).
The possibility of tracing the parents is small when offspring are exceptional but the chances of regular offspring arising when the detachment of the X chromosomes occurs and who may be crossovers or non crossovers is larger. When scute apricot crossveinless tan, forked fruit fly was crossed with a broad echinus cut garnet, one non disjuntional daughter (a broad echinus) who gave the exceptional offspring was the result; this female giving rise to broad echinus and bar males when mated with a bar male.
A.H.Sturtevant and G.W.Beadle studied chromosomal inversions in 1936, have discussed seven of them in the paper and concluded that homozygous inversions show the crossing over that is seen in homozygous individuals, the distribution of crossing over being less close to the spindle attachment and more when further from it, the crossing over being less in heterozygous inversions.
The selective elimination of crossover chromatids is expected in a meiotic process where most of the four products are eliminated as in the oogenesis of animals and megasporogenesis of most seed plants resulting in a decrease in the rate of production even though fertility is not reduced. However this is not seen in Drosophila as there is no crossing over in the male but inversions are seen more than translocations and fertility is decreased with single chromatid exchanges. At the first division of meiosis, the Y chromosome disjoints from the attached X chromosomes.