, Volume 16, Issue 6, pp 668–680 | Cite as

Sex chromosomes of the chinchilla: Allocycly and duplication sequence in somatic cells and behavior in meiosis

  • Michael Galton
  • Kurt Benirschke
  • Susumu Ohno


The X-chromosome of the chinchilla is the largest member of the complement. Since its area equals approximately 9% of that of the haploid autosome set, it may represent a “duplicate-type X-chromosome”, in contrast to the “original-type” in which the ratio X:autosomes ranges from 5 to 6.5%. Unlike other known species with the “duplicate-type X-chromosome”, the chinchilla possesses a very small Y-chromosome.

It was inferred from the allocyclic behavior and asynchronous replication pattern of the sex chromosomes in somatic cells that one X-chromosome in the female and the male X-chromosome manifest a single genetically active region: the presumed active segment is inserted in the longer arm adjacent to the centromere. The remainder of these X-chromosomes, the entire second X-chromosome in the female and the Y-chromosome displayed positive heteropycnosis in mitotic prophase and completed DNA synthesis prior to mitosis later than any other elements: this cytologic behavior was held to reflect genetic inertness.

The existence of genetic inactivation of much of the “duplicate-type” X-chromosome mass of the chinchilla supports the hypothesis that a constant optimal ratio between the functional portion of the X-chromosomes and the autosomes is maintained in mammals despite a wide range of relative sizes of the X-chromosomes.

In female meiosis there is a free exchange of chiasmata between the X-chromosomes at first meiotic metaphase. In the male, however, the association between the large X- and small Y-chromosomes during meiosis is invariably end-to-end, denoting the absence of synapsis. This contrasts with the occurrence of side-by-side pairing of the XY-bivalent in other mammals possessing the “duplicate-type X-chromosome” but in which the Y-chromosome is correspondingly large.


Interphase Nucleus Late Replication Meiotic Metaphase Female Meiosis Genetic Inactivation 
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Copyright information

© Springer-Verlag 1965

Authors and Affiliations

  • Michael Galton
    • 1
    • 2
  • Kurt Benirschke
    • 1
    • 2
  • Susumu Ohno
    • 1
    • 2
  1. 1.Department of PathologyDartmouth Medical SchoolHanoverUSA
  2. 2.Department of BiologyCity of Hope Medical CenterDuarteUSA

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