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)41/Y;mwh-/+ males, and an increased frequency of y- mwh- twin spots in the abdomen of mit(1)41/Y;T(1;3)scJ4, y+ mwh-/+ males. Chromosome condensation is abnormal in third instar larval ganglia at 29oC, but normal at 18oC. Both euchromatic and heterochromatic portions of the genome are affected at 29oC.