Abstract
Karyotypes were obtained from primary kidney cultures of nearly 250 individuals belonging to six nominate species of the avian genus Junco. Chromosome 2 was found to be dimorphic in four of the six species and chromosome 5 was dimorphic in five of the six species. The dimorphism appears to be due to the presence of a pericentric inversion in each of these autosomes. It is hypothesized that chromosomes 2 and 5 are ancestral and gave rise to chromosomes 2smand 5mrespectively. The sample of the slate-colored junco, Junco hyemalis, the largest studied, conforms to the Hardy-Weinberg equilibrium and suggests that a random sample was drawn from a randomly mating population.