DOWN-SYNDROME - CHARACTERIZATION OF A CASE WITH PARTIAL TRISOMY OF CHROMOSOME-21 OWING TO A PATERNAL BALANCED TRANSLOCATION (15-21)(Q26-Q22.1) BY FISH

A patient with a typical Down syndrome (DS) phenotype and a natural, k aryotype was studied by FISH. Using painting probes, Eve found that th e patient had partial trisomy of chromosome 21 owing to an unbalanced. translocation t(15;21)(qSB;q22.1) of paternal origin. To correlate ge notype with phenotype as accurately as possible, we localised the brea kpoint using a contig of YACs from the long arm of chromosome 21 as pr obes and performed FISH. We ended up with two YACs, the most telomeric giving signal on the der(15) in addition to signal ore the normal chr omosome 21 and the most centromeric giving signal only on both normal chromosomes 21. From these results we could conclude that the breakpoi nt must be located within the region encompassing YACs 280B1 and 814C1 , mast likely near one end of either YAC or between them, since neithe r YAC 814C1 nor 280B1 crossed the breakpoint (mast likely between mark er D21S304 and marker D21S302) on band 21q22.1. The same study was per formed on the chromosomes of the father and of a sister and a brother of the patient; all three carried a balanced translocation between chr omosomes 15 and 21 and had a normal phenotype. We also performed a pre natal study using FISH for the sister. The fetus was also a carrier of the balanced translocation.