In situ hybridization of five new and one previously described alpha-satellite 
sequences isolated from chromosome 21 libraries gave the following chromosomal 
distribution patterns: (a) two sequences (pTRA-1 and -4) hybridizing to 
chromosomes 13, 14, 15, 21, and 22 (also 19 and 20); (b) one sequence (pTRA-7) 
hybridizing to chromosome 14; and (c) three sequences (pTRA-2, -11 and -15) 
hybridizing to chromosomes 13, 14, and 21, with significant but weaker signals 
on 15 and 22. These results suggested the sharing of alphoid domains between 
different acrocentric chromosomes and the coexistence of multiple domains on 
each chromosome. Analysis of somatic cell hybrids carrying a single human 
acrocentric chromosome using pTRA-2 demonstrated a higher-order repeating 
structure common to chromosomes 13, 14, and 21, but not to 15 and 22, providing 
direct evidence for sequence homogenization in this domain among the former 
three chromosomes. We present a model of evolution and genetic exchange of alpha 
sequences on the acrocentric chromosomes which can satisfactorily explain these 
and previous observations of (a) two different alphoid subfamilies, one common 
to chromosomes 13 and 21 and the other common to chromosomes 14 and 22, (b) a 
different alphoid subfamily on chromosome 22, and (c) nonrandom participation of 
chromosomes 13 and 14, and 14 and 21 in Robertsonian translocations.