All species in the genus Peromyscus possess a diploid number of 48, but the number of total chromosome arms varies from 56 (e.g., P. crinitus) to 96 (e.g., P. eremicus). Data are presented, using these two extreme cases, to illustrate that all short arms of biarmed chromosomes are made of constitutive heterochromatin. G-banding preparations revealed that the long arms (euchromatin) of the two species are essentially the same. Since constitutive heterochromatin contains few, if any, structural genes, the two species, and presumably other species as well, do not differ drastically in their information content, but differ in the amount of chromatin material, including DNA. The results indicate that in studying karyotypic evolution, C-banding and G-banding (or Q-banding) are essential tools without which erroneous conclusions may be reached.
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