We have analysed the TaqI patterns obtained with an alphoid DNA probe specific for human chromosomes 13 and 21 in a number of unrelated individuals, as well as in the somatic hybrid WA 17 which carries chromosome 21 as a unique human chromosome. In certain individuals, two types of extra bands are superimposed over the relatively simple basic banding pattern exhibited by all individuals. Thus, three independent allele-specific DNA patterns are defined. The basic and normal organization of the alpha satellite in chromosome 21 consists of tandemly arranged arrays of repeats representing 11 times the 171-bp monomer of the alphoid DNA sequences. The supernumerary bands found in some individuals are organized in tandemly arranged subsets of repeats representing 18 times and 9.5 times the 171bp basic monomer, respectively. These less fragment alleles segregate in a Mendelian fashion. Linkage analyses suggest that they originate from chromosomes 13 and 21, respectively.