FM7/X females heterozygous for Df(3L)66C-T2-10 show 30.9% X and 19.7% fourth chromosome nondisjunction.
The primary cytological defect in heterozygous oocytes is malorientation of the nonexchange fourth chromosomes. The majority of spindles appear structurally normal.
Heterozygosity for Df(3L)66C-T2-10 results in 39.2% X chromosome nondisjunction and 27.7% fourth chromosome nondisjunction in In(1)FM7/X ; svspa-pol females. The level of X chromosome nondisjunction in heterozygous Df(3L)66C-T2-10 females carrying two normal sequence X chromosomes is only 4.0%, while the frequency of fourth chromosome nondisjunction in these females is 42.1%. Achiasmate X and fourth chromosomes are often improperly placed on metaphase I spindles of oocytes in In(1)FM7/X ; Df(3L)66C-T2-10/+ females; achiasmate X chromosomes are found on the same half-spindle 21% of the time and fourth chromosomes are found on the same half-spindle 31% of the time.