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014 - Radiation and Mutation · Percheron horse

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  Speaker 1: Section thirteen of the Genetic Effects of radiation. This is
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  Speaker 1: a LibriVox recording. All LibriVox recordings are in the public domain.
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  Speaker 1: For more information or to volunteer, please visit LibriVox dot org.
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  Speaker 1: Read by T. R. Love of Pleasant Hill, California. The
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  Speaker 1: Genetic Effects of Radiation by Isaac Asimov and Theodosius Dobzhansky,
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  Speaker 1: Section thirteen Radiation and mutation. Where radiation is insufficient to
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  Speaker 1: render a cell incapable of division, it may still induce mutations,
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  Speaker 1: and it is in this fashion that skin cancer, leukemia,
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  Speaker 1: and other disorders may be brought about. Studies at the
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  Speaker 1: California Institute of Technology furnish information on the nature of
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  Speaker 1: radiation effects on genes. Mutations can be brought about in
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  Speaker 1: the sex cells too, of course, and when this happens,
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  Speaker 1: it is succeeding generations that are affected, and not merely
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  Speaker 1: the exposed individual. Indeed, where the sex cells are concerned,
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  Speaker 1: the relatively mild effect of mutation is more serious than
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  Speaker 1: the drastic one of nondivision. A fertilized ovum that cannot
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  Speaker 1: divide eventually dies and does no harm. One that can
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  Speaker 1: divide but is altered may give rise to an individual
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  Speaker 1: with one of the usual kinds of major or minor
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  Speaker 1: physical defects. The effect of high energy radiation on the
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  Speaker 1: genetic mechanism was first demonstrated experimentally in nineteen twenty seven
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  Speaker 1: by Mueller. Using Drosophila. He showed that after large doses
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  Speaker 1: of X rays, flies experienced many more lethal mutations per
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  Speaker 1: chromosome than did similar flies not exposed to radiation. The
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  Speaker 1: differences he observed prove the connection between radiation and mutation
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  Speaker 1: at once. Later experiments by Mueller and by others showed
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  Speaker 1: that the number of mutations was directly proportional to the
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  Speaker 1: quantity of radiation absorbed. Doubling the quantity of radiation absorbed
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  Speaker 1: doubled the number of mutations, tripling the one, tripled the other,
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  Speaker 1: and so on. This means that if the number of
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  Speaker 1: mutations is plotted against the amount of radiation absorbed, a
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  Speaker 1: straight line can be drawn. It is generally believed that
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  Speaker 1: the straight line continues all the way down without deviation
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  Speaker 1: to very low radiation absorptions. This means there is no
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  Speaker 1: threshold for the mutational effect of radiation, no matter how
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  Speaker 1: small a dosage of radiation. The gonads receive. This will
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  Speaker 1: be reflected in a proportionately increased like linlod of mutated
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  Speaker 1: sex cells, with effects that will show up in succeeding generations.
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  Speaker 1: In this respect, the genetic effect of radiation is quite
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  Speaker 1: different from the somatic effect. A small dose of radiation
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  Speaker 1: may affect growing tissues and prevent a small proportion of
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  Speaker 1: the cells of those tissues from dividing. The remaining unaffected
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  Speaker 1: cells take up the slack. However, and if the proportion
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  Speaker 1: of affected cells is small enough, symptoms are not visible
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  Speaker 1: and never become visible. There is thus a threshold effect.
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  Speaker 1: The radiation absorbed must be more than a certain amount
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  Speaker 1: before any somatic symptoms are manifest. Matters are quite different
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  Speaker 1: where the genetic effect is concerned. If a sex cell
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  Speaker 1: is damaged, and if that sex cell is one of
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  Speaker 1: the pair that goes into the production of a fertilized ovum,
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  Speaker 1: a damaged organism results. There is no margin for correction.
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  Speaker 1: There is no unaffected cell that can take over the
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  Speaker 1: work of the damaged sex cell once fertilization has taken place.
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  Speaker 1: Suppose only one sex cell out of a million is damaged.
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  Speaker 1: If so, a damaged sex cell will on the average
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  Speaker 1: take part in one out of every million fertilizations, and
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  Speaker 1: when it is used, it will not matter that there
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  Speaker 1: are nine hundred ninety nine thousand, nine hundred and ninety
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  Speaker 1: nine perfectly good sex cells that might have been used.
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  Speaker 1: It was the damage cell that was used. This is
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  Speaker 1: why there is no threshold in the genetic effect of radiation,
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  Speaker 1: and why there is no safe amount of radiations. Insofar
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  Speaker 1: as genetic effects are concerned. However small the quantity of
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  Speaker 1: radiation absorbed, mankind must be prepared to pay the price
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  Speaker 1: in a corresponding increase of the genetic load. If the
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  Speaker 1: straight line obtained by plotting mutation rate against radiation dose
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  Speaker 1: is followed down to a radiation dose of zero, it
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  Speaker 1: is found that the line strikes the vertical axis slightly
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  Speaker 1: above the origin. The mutation rate is more than zero
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  Speaker 1: even when the radiation dose is zero. The reason for
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  Speaker 1: this is that it is the dose of man made
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  Speaker 1: radiation that is being considered. Even when man made radiation
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  Speaker 1: is completely absent, there still remains the natural background radiation.
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  Speaker 1: It is possible in this manner to determine that background
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  Speaker 1: radiation accounts for considerably less than one percent of the
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  Speaker 1: spontaneous mutations that take place. The other mutations must arise
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  Speaker 1: out of chemical misadventures, out of the random heat jiggling
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  Speaker 1: of molecules, and so on. These, it can be presumed,
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  Speaker 1: will remain constant when the radiation does is increased. This
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  Speaker 1: is a hopeful aspect of the situation, for it means
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  Speaker 1: that if the background radiation is doubled or tripled for
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  Speaker 1: mankind as a whole, only that small portion of the
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  Speaker 1: spontaneous mutation rate that is due to the background radiation
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  Speaker 1: will be doubled or tripled. Let us suppose, for instance,
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  Speaker 1: that fully one percent of the spontaneous mutations occurring in
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  Speaker 1: mankind is due to background radiation. In that case, the
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  Speaker 1: tripling of the background radiation produced in the United States
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  Speaker 1: by man made causes see table will triple that one percent.
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  Speaker 1: In place of ninety nine non radiational mutations plus one radiational,
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  Speaker 1: we would have ninety nine plus three. The total number
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  Speaker 1: of mutations would increase from one hundred to one hundred two,
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  Speaker 1: an increase of two percent, not an increase of two
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  Speaker 1: hundred percent that one would expect if all spontaneous mutations
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  Speaker 1: were caused by background radiation. Table. Radiation exposures in the
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  Speaker 1: United States. Open footnote Estimated average exposures to the gonans
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  Speaker 1: based on nineteen sixty three report of Federal Radiation Council.
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  Speaker 1: Close footnote. Natural sources millirems A external to the body
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  Speaker 1: one from cosmic radiation fifty point zero two from the earth,
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  Speaker 1: forty seven point zero three from building materials three point
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  Speaker 1: zero B inside the body one inhalation of air five
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  Speaker 1: point zero two. Elements found naturally in human tissues twenty
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  Speaker 1: one one point zero Total natural sources one hundred twenty
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  Speaker 1: six point zero millirems. Man made sources A medical procedures
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  Speaker 1: one diagnostic X rays fifty point zero two radiotherapy X
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  Speaker 1: ray radio isotopes ten point zero three, internal diagnosis therapy
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  Speaker 1: one point zero subtotal sixty one point zero millirems. B
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  Speaker 1: Atomic energy industry laboratories zero point two C luminous watch dials,
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  Speaker 1: television tubes, radioactive industrial wastes, et cetera. Two point zero
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  Speaker 1: D radioactive fallout four point zero subtotal six point two millirems.
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  Speaker 1: Total man made sources sixty seven point two millirem. Overall
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  Speaker 1: total one hundred ninety three point two millirems. End of
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  Speaker 1: section thirteen