No abstract

The banding characteristics of an extreme variant familial chromosome 22 short-arm enlargement are described. Ag-AS staining for nucleolar-organizer regions, identified two areas of rDNA actively coding for 18S and 28S rRNA, the one being a broad distal Ag-band and the other a narrower centromeric Ag-band. The DNA in the major portion of the enlarged short arm was highly methylated, as shown by the binding of antibodies to 5-methylcytidine after UV-denaturation of chromosomal DNA. Mean Ag-band size on the aberrant 22p+ correlated with the mean number of 22p+ associations. Association of 22p+ was no greater than that of other acrocentrics, in spite of a presumed excess number of rDNA gene copies. This case represents only the second such normal variant defined by these techniques.

SUMMARY Cytogenetic studies on a retarded girl showed a complex X; 15 translocation, karyotype 45,X,-1 5,+t(X;1 5). The translocation X chromosome was non-randomly partially inactivated, the inactivation being mainly confined to the X segment and in some cells only to the X long arm. Gene marker studies failed to show anomalous segregation of the hexosaminidase A gene or any other gene markers tested.

Two phenotypically abnormal, unrelated children with deletion of the distal segment of 7q (7q32→pter) are described. In one instance the mother was the carrier of a balanced translocation between chromosomes 6 and 7, and in the second case the deletion was a de novo event. Their phenotypes were compared to previously reported cases and found to have many non‐specific clinical features in common. Gene marker studies for some of the genes tentatively localized to chromosome 7 showed no anomalous segregation. The Hageman coagulation factor (Factor XII) activity in bath probands was normal, and hetero‐zygosity for alleles of the Kidd blood group in the first proband excludes assignment of the Kidd locus to the distal portion of chromosome 7q.