Partial monosomy or trisomy resulting from crossing over within a rearranged chromosome 1

Two sibs, both monosomic for the same portion of 1q (q25q32), had similar severe mental and physical retardation. An older sib was found to be trisomic for the same portion of 1q and less severely affected. These partially monosomic and partially trisomic No. 1 chromosomes resulted from meiotic crossover in the mother, who is a balanced heterozygote of the type 46,XX, ins (1) (p32q25q32).