mod(mdg4)^Z3-3298/mod(mdg4)^Z3-5578 males exhibit defects in chromosome segregation (i.e. random segregation of sex chromosomes) and conjunction during meiosis I; abnormal chromosome territories during S5, S6 and prometaphase I stages; an increased number of smaller DNA blobs (chromosome masses) during prometaphase I; prematurely separated bivalents into univalents during meiosis I; and a striking size variation among the nuclei present in spermatid cysts compared to controls.

mod(mdg4)^Z3-3298/Df(3R)GC14 males show a high frequency of 4th chromosome loss (22.7%) and chromosome 2 nondisjunction (45.0%) in meiosis.

mod(mdg4)^Z3-3298/Df(3R)GC14 females do not show increased X chromosome nondisjunction compared to controls and the frequency of recombination in large intervals on the X, second and third chromosomes in the mutant females is normal.

mod(mdg4)^Z3-3298 mutants show elevated frequencies of autosomal and sex chromosome nondisjunction. Analysis of mutant spermatocytes shows that nondisjunction is limited largely to meiosis I. Premeiotic and meiotic homolog pairing are normal. The earliest observable phenotype is that of "territory expansion" in late G2, with homologous chromosomes being at a greater distance from one another than in wild-type spermatocytes. mod(mdg4)^Z3-3298 spermatocytes show incomplete chromatin condensation and form up to eight large clumps of chromatin form by metaphase I, instead of the one central mass observed in wild type. The clumps represent bivalents and univalents, consistent with a defect in formation or maintenance of bivalents. In early anaphase I, chromosomes are distributed unevenly and asymmetrically along the spindle. In late anaphase I, there is a high frequency of laggards and polar chromatin masses of unequal size, indicating high frequency of nondisjunction.