Unusual chromosome aberrations in 3 children with Down syndrome.

In 3 children with Down syndrome extremely rare chromosome aberrations were found. In the first patient, the karyotype showed 46 chromosomes with a de novo duplication of the q22-qter segment. This finding supports that the 21q22-qter band was responsible for the characteristic mongoloid features. In the second case, trisomy 21 was present and out of 78 investigated cells, 60 contained a small, supernumerary marker chromosome in addition to trisomy 21. The parents were cytogenetically and clinically normal. In the third case trisomy 21 with inv(10) (p13q22) occurred. The inversion was inherited from the mother with diminished fertility.