Partial trisomies of chromosome 21 in man. Two new observations due to translocations 19;21 and 4;21.

Partial trisomy of the distal portion of the long arm of chromosome no. 21, resulting from a (familial) translocation between the chromosomes no. 19 and 21 in a female twin with Down's syndrome, supports the hypothesis that triplication of 21q22 is the cause of the physical signs of mongolism. Partial trisomy of the remaining segments of chromosome no. 21 due to a (familial) translocation between the chromosomes no. 4 and 21, however, may only cause mental deficiency.