• G. Pontecorvo

      Articles written in Journal of Genetics

    • The induction of chromosome losses inDrosophila sperm and their linear dependence on dosages of irradiation

      G. Pontecorvo

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      1. A method for detecting non-lethal losses ofX- andY-chromosomes produced by irradiation of mature sperm ofDrosophila is described in detail.

      2. Losses ofX andY are found to be produced by X-raying mature sperm, and those found by the method used are non-lethal to the zygote.

      3. Between 0 and 4000 r. the frequency of the above losses is, in first approximation, a linear function of the dosage, and it certainly varies less rapidly than the 1·5 power of the latter—the power found, for these dosages, in the case of gross rearrangements.

      4. The mechanism of origination of these losses is interpreted as being through the occurrence of single breaks followed by lateral fusion of sister chromatid fragments, leading to formation of a dicentric chromosome which is either lost in the first change or becomes subjected to repeated breakage at successive anaphases and is ultimately lost.

      5. It is suggested, as a result of comparisons of the frequency of the losses found with the amount of change of the sex ratio caused by the same dosage, that a major part of the losses are lethal to the zygote.

    • The problem of dominant lethals

      G. Pontecorvo

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    • Viability interactions between chromosomes ofDrosophila melanogaster andDrosophila simulans

      G. Pontecorvo

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    • Analysis of mitotic recombination inAspergillus nidulans

      G. Pontecorvo Elizabeth Tarr Gloor E. Forbes

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      A study of mitotic segregation in two strains ofAspergillus nidulans, made diploid and heterozygous for the same six markers in different coupling arrangements, shows that two processes are at work: (1) irregular distribution ofwhole chromosomes at mitosis leading to haploid nuclei recombining markers on different chromosomes; (2) mitotic crossing-over leading to diploid nuclei in which homozygosis for one marker goes with homozygosis for all the alleles linked to it in coupling and distal. Cross-overs involving more than one chromosome or doubles in the one chromosome seem to be rare, except possibly across the centromere. Pontecorvo’s (1952) deduction of a tendency for mitotic crossing-over to be concentrated in certain nuclei is wrong; it was based on failure to recognize process (1).

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