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, 202 (1), 15-23

The Centenary of GENETICS: Bridges to the Future


The Centenary of GENETICS: Bridges to the Future

Barry Ganetzky et al. Genetics.


Figure 1
Figure 1
X chromosome nondisjunction leads to aberrant inheritance of sex-linked traits. (A) Proper chromosome segregation of X chromosomes in females results in wild-type XX females and vermilion-eyed XY males. (B) Nondisjunction of X chromosomes at the first meiotic division produces diplo-X and nullo-X eggs containing two and zero X chromosomes, respectively. Fertilization of these nondisjunctional eggs results in the production of exceptional red-eyed sterile X0 males and exceptional vermilion-eyed XXY females (adapted from Bridges 1916).
Figure 2
Figure 2
Models of secondary nondisjunction. (A) In Bridges’s model, an X chromosome has a choice of two pairing partners in meiosis, giving rise to different patterns of segregation: (1) the two X’s could pair and segregate, leaving the unpaired Y to segregate at random, generating X and XY eggs with equal frequency; or (2) an X could pair and segregate from the Y chromosome, leaving the unpaired X to segregate at random and producing X, XY, XX, and Y eggs with equal frequencies. (B) To explain secondary nondisjunction frequencies greater than 50%, Cooper proposed an alternative model wherein X chromosomes undergo exchange without regard for the presence of the Y chromosome. However, nonexchange X chromosomes will engage in trivalent association with the Y chromosome, which segregates to one pole and preferentially directs the two nonexchange X chromosomes to the other pole. (C) Xiang and Hawley confirmed the formation of a trivalent, but found that it precedes and does not inhibit recombination between the X’s. X chromosomes that do recombine disengage from the Y and segregate normally, leaving the unpaired Y to segregate at random. X chromosomes that fail to recombine remain associated with the Y and preferentially segregate away from it.

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Cited by 5 PubMed Central articles


    1. Baker B. S., 1989. Sex in flies: the splice of life. Nature 340: 521–524. - PubMed
    1. Baker B. S., Carpenter A. T., 1972. Genetic analysis of sex chromosomal meiotic mutants in Drosophilia melanogaster. Genetics 71: 255–286. - PMC - PubMed
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