Chromosome behaviour in the wing and abdominal cells is abnormal; an increased frequency of homozygous mwh- clones is seen in the wings of mit(1)131/Y;mwh-/+ males, and an increased frequency of y- mwh- twin spots in the abdomen of mit(1)131/Y;T(1;3)scJ4, y+ mwh-/+ males. There is an increase in the frequency of chromosome aberrations in larval neuroblasts (frequency of chromosomal aberrations is 1.68%).