• D. E. Lea

      Articles written in Journal of Genetics

    • A radiation method for determining the number of genes in theX-chromosome ofDrosophila

      D. E. Lea

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      It is known experimentally that the efficiency of production of sex-linked lethal mutations inDrosophila melanogaster by radiation is, per ionization, less for densely ionizing radiations (neutrons) than for less densely ionizing radiations (X-rays). A theory is given correlating this effect with the target volume of the individual genes. The estimate of the target volume obtained by this method agrees with the estimate obtained by methods already known. It is further shown that on this theory the experimentally determined ratio of the mutation rates for X-rays and neutrons may be used to give an estimate of the number of genes in theX-chromosome ofDrosophila. Using the experimental figure of 1·6 for the ratio of the mutation rates, the number of genes in theX-chromosome is deduced to be 1860.

    • The effects of ionizing radiations on the chromosomes ofTradescantia bracteata. A comparison between neutrons and X-rays

      J. M. Thoday D. E. Lea

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      1. Neutrons produce qualitatively the same types of chromosome aberrations as X-rays, but more of all types of aberration are produced, per ionization, by neutrons.

      2. The ratios of the dose of X-rays to the dose of neutrons both measured in roentgens, required to produce equal numbers of aberrations are 2-4 for chromatid breaks, 3-6 for isochromatid rejoins, 4-5 for chromosome breaks, and from 10 to 5, at the doses used, for interchanges.

      3. Many of the interchanges produced by neutrons are caused by single ‘hits’, that is by single ionization paths.

      4. Neutrons and X-rays do not differ in any effects that they may have on the rejoining process.

      5. The results are attributed to the greater number of primary breaks, and their different distribution, when they are produced by the denser ionization paths of neutron radiation.

      6. It must be inferred that more than one ionization is needed to break aTradescantia chromosome thread.

    • The mechanism of the induction by radiation of chromosome aberrations inTradescantia

      D. E. Lea D. G. Catcheside

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    • The effect of ionization distribution on chromosome breakage by X-rays

      D. G. Catcheside D. E. Lea

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      1. The coefficients of chromatid breakage (compare Table 4) are highest with AgL-radiation (λ=4·1 A.) and fall off through CuK-radiation (λ=1·5 A.) to medium X-rays (λ=0·15 A.) and to AlK-radiation (λ=8·3 A.) where they are least.

      2. This is interpreted to mean that only the densely ionizing ‘tails’ of the electron tracks are effective in chromosome breakage, and that the ‘tails’ have a higher efficiency only where they traverse the chromatid; for the track of an AlK-electron which is shorter than a chromatid diameter is relatively inefficient.

      3. The probability of survival of chromatid breaks is the same in the pollen-tube nucleus as in the pollen-grain nucleus.

      4. The two chromatids of a chromosome are in contact 24 hr. before metaphase in the pollen-grain nucleus. They are slightly separated 15 hr. before metaphase in the pollen-tube nucleus.

      5. The probability of interchange in the pollen-tube nucleus is reduced by the different method of packing of the chromosomes in a long cylindrical nucleus as compared with the spherical one of the pollen grain.

    • The rate of induction of dominant lethals inDrosophila melanogaster sperm by X-rays

      D. E. Lea D. G. Catcheside

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      The rate of X-ray induction of dominant lethals in the sperm of the Oregon-R stock ofDrosophila melanogaster is, at low doses, 12% per 1000 r. for death in the embryo stage, and 20% per 1000 r. for death at any stage between the zygote and the adult. At higher doses of X-rays both rates increase. Over the whole dose range, therefore, the doseaction curve does not fit a single-hit type of action, though this is approximated at low doses.

      The rate of depression of the sex ratio of females to males, founded on our own and other data, is about 2·5% per 1000 r.

    • The relation between recessive lethals, dominant lethals, and chromosome aberrations inDrosophila

      D. G. Catcheside D. E. Lea

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      The suggestion is put forward that radiation-induced recessive lethals, or a large proportion of them, are due to chromosome breaks. About one-third of all the chromosome breaks primarily induced by the radiation are lethals. If the break restitutes, a lethal unaccompanied by chromosomal aberration (type A lethal) results. If the break takes part in chromosome interchange a type C lethal, which is associated with chromosomal structural change, results. Arguments are given against the alternative position-effect explanation of type C lethals.

      A quantitative theory of dominant lethals is developed on the basis that the dominant lethals are a mixture of single breaks which fail either to restitute or to interchange but instead undergo sister-union, and of non-viable chromosomal structural changes involving two or more breaks. The experimental curve of variation with dose of the yield of dominant lethals is successfully fitted, and also the curve of the yield of viable structural changes.

      It is shown that the recessive lethal and the dominant lethal theories are consistent in that they require the same postulated number of primarily produced chromosome breaks per unit dose (namely, 0·75 breaks per sperm per 1000 r.). Experiments on the distortion of the sex ratio in the progeny of irradiated males with ring-shaped or rod-shapedX-chromosomes are also shown to be consistent with the theory.

    • Dominant lethals and chromosome breaks in ringX-chromosomes ofDrosophila melanogaster

      D. G. Catcheside D. E. Lea

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    • The bearing of radiation experiments on the size of the gene

      D. E. Lea D. G. Catcheside

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      At various times it has been proposed that the size of the gene can be estimated from experiments on the yield of mutations obtained with known doses of X-rays. The assumptions involved in calculations of this sort are discussed, and some of the objections which have been raised against them answered. It is pointed out that similar assumptions, applied to experiments on the inactivation of enzymes and viruses, lead to estimates of the sizes of these bodies correct to a factor of two in diameter. The estimate arrived at for the size of the gene inDrosophila is 4–8 mμ diameter. The length of the euchromatic region of theX-chromosome thread in the sperm is deduced to be 10–20 μ.

    • Types of chromosome structural change induced by the irradiation ofTradescantia microspores

      D. G. Catcheside D. E. Lea J. M. Thoday

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    • The production of chromosome structural changes inTradescantia microspores in relation to dosage, intensity and temperature

      D. G. Catcheside D. E. Lea J. M. Thoday

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    • A mathematical theory of chromosomal rearrangements

      J. B. S. Haldane D. E. Lea

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      A theory is given of the process of chromosomal structural rearrangement following irradiation. The theory applies to nuclei in which the union of breakage ends is at random, and assumes that the number of breaks primarily produced is proportional to the dose.

      Formulae and tables are given enabling the proportion of nuclei undergoing eucentric and dyscentric types of rearrangement respectively to be calculated as a function of the dose, the cases of nuclei with 1, 2, 3, 4, 5, or many chromosome, arms in the set being separately considered. The five-arm calculation is compared with published experimental results of the irradiation of the spermatozoa ofDrosophila melanogaster.

    • The distribution of the numbers of mutants in bacterial populations

      D. E. Lea C. A. Coulson

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      Statistical calculations are made of the distribution numbers of mutants in a culture of bacteria in which the number of mutants increases on account both of new mutations and of division of old mutants. In this way the largely qualitative conclusions of Luria and Delbruck are extended and placed on a firm quantitative basis. The results of these calculations, which enable the mutation rate to be inferred from experiments with parallel cultures, are presented in the form of tables. Statistically efficient methods of using these tables are discussed.

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