Absent IgA and deletions of chromosome 18.

Since extra chromosomal material in humans was first described by Lejeune, Turpin, and Gautier (1959) in association with Down's syndrome, patients with trisomic and partial monosomic con- ditions have been studied in an attempt to locate specific genetic loci upon specific chromosomes. An individual with a deletion of part of an autosome is hemizygous for genes on the homologue of the deleted fragment. As a result, recessive genes may be uncovered which, in the absence of a deletion, may have been masked by their dominant alleles. This type of 'deletion mapping' may be important in locating genes on autosomes. We here present studies on 4 patients with deletions involving chro- mosome 18 and discuss the possible relation be- tween this chromosomal aberration and the locus for immunoglobulin A (IgA).