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Chromosoma

, 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
Article

Summary

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.

Keywords

Interphase Nucleus Late Replication Meiotic Metaphase Female Meiosis Genetic Inactivation 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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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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