By culturing heparinized whole blood, satisfactory chromosome preparations have been obtained easily and efficiently from blood samples as small as 0.05 ml. This technique eliminates any special treatment of the blood prior to initiating culture and minimizes the loss of leukocytes from the sample. It is particularly useful in those situations where separation of the leukocytes from other blood cells is difficult. Chromosomes from humans and deer mice have been studied using this technique.

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Cytogenetic studies of three subspecies of the deer mouse, Peromyscus maniculatus rubidus, P.m. gracilis and P.m. bairdii have demonstrated an intra- and intersubspecific chromosomal polymorphism in which the karyotypic patterns vary among individuals of the same and different subspecies while maintaining a diploid number of 48 and a constant karyotype in each animal. Present information suggests that pericentric inversion seems the best explanation for this polymorphism.

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