Acrocentric bivalent associations were studied in 232 human male germ cells at 
pachytene in order to understand better the preferential involvement of 
chromosomes 13, 14, and 21 in Robertsonian translocations. The tendency of each 
acrocentric bivalent to associate with another was not correlated with NOR 
activity, as measured by silver staining. Good agreement was noticed between 
their ability to associate and the amount of satellite DNA in human acrocentric 
chromosomes. The distribution of two-by-two acrocentric bivalent associations 
was random. In order to reconcile this result with the nonrandom distribution of 
Robertsonian translocations, a molecular hypothesis is proposed. The model is 
based on homology of recombinational sites, interspersed at regular interval in 
satellite DNA, which could increase the probability of accidental unequal 
crossing-over between two specific acrocentric chromosomes.