1. Further studies on Ab.
The factor for shrunken endosperm (sh2) is reported by E.B. Mains to be in the third linkage group about one‑quarter of one unit from a1. Because of its favorable location it is being used as a marker gene in the current studies on the compound nature of Ab.
From the cross Ab Sh et/a sh Et x a sh twenty‑four crossovers in the a‑et segment were obtained and survived testing. Seven of these strands carried Ab and all seven were Ab sh Et. Seventeen carried a and of these fifteen were a Sh et and two were a bh Et. These data indicate that sh2 is distal to A1, that is, between A1 and Et.
In past experiments the occurrence of a large majority of Ad (pale) derivatives from Ab/a plants has been associated with crossing over in the a-et segment, the isolated pales being linked with the et allele carried on the a chromosome of the tested plants. It was argued that these crossovers occur within the Ab "locus" the Ad derivative representing a proximal component of Ab. Since sh2 is distal to A and so close to it, its use in these experiments should allow a localization of this crossover event.
From Ab Sh/a plants 27 Ad cases have been analyzed. Twenty‑three of these were Ad sh and the remaining 4 were Ad Sh; only 1 out of 400 Ad cases are expected to be linked with sh if there is no crossover basis for their origin. Hence this crossover event is localized within a segment less than one unit in length.
The four Ad Sh cases from Ab Sh/a sh plants suggest that Ad may arise without crossing over at the Ab locus. This is confirmed from studies of plants carrying Ab on one chromosome and a deficiency on the homologue and designated Ab/a‑XI the deficient piece is now known to include Sh2 also). The occurrence of Ad here could not result from crossing over involving homologues. Among 32,000 tested gametes from such plants one Ad case has been found. Analysis has ruled out illegitimate bases for its origin. It could have resulted following mutation of the residual component of Ab to a null form but it is not possible to rule out the alternative exploration of sister chromatid exchange which in this case would have be be unequal.