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009 - Mutation Rates · Percheron horse

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  Speaker 1: Section eight of the Genetic Effects of Radiation. This is
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  Speaker 1: a libriovox 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 Caveat The Genetic Effects of Radiation by Isaac
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  Speaker 1: Asimov and Theodosius Dobzansky. Mutation rates. It is easier to
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  Speaker 1: observe the removal of genes through death or through failure
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  Speaker 1: to reproduce than to observe their production through mutation. It
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  Speaker 1: is particularly difficult to study their production in human beings,
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  Speaker 1: since men have comparatively long lifetimes and few children, since
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  Speaker 1: their mating habits cannot be well controlled. For this reason,
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  Speaker 1: geneticists have experimented with species much simpler than man. Small
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  Speaker 1: organisms that are short lived, produce many offspring, and that
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  Speaker 1: can be penned up and allowed to mate only under
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  Speaker 1: fixed conditions. Such creatures may have fewer chromosomes than man does,
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  Speaker 1: and the sight mutation are more easily pinned down. An
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  Speaker 1: important assumption made in such experiments is that the machinery
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  Speaker 1: of inheritance and mutation is essentially the same in all creatures,
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  Speaker 1: and that therefore knowledge gained from very simple species, even
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  Speaker 1: from bacteria, is applicable to man. There is overwhelming evidence
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  Speaker 1: to indicate that this is true in general, although there
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  Speaker 1: are specific instances where it is not completely true and
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  Speaker 1: scientists must tread softly while drawing conclusions. The animals most
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  Speaker 1: commonly used in studies of genetics and mutations are certain
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  Speaker 1: species of fruit flies called Gersophala. The American geneticist Hermann J.
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  Speaker 1: Muller devised techniques whereby he could study the occurrence of
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  Speaker 1: lethal mutations anywhere along one of four pairs of chromosomes
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  Speaker 1: possessed by Gisophala. A lethal gene, he found might well
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  Speaker 1: be produced somewhere along the length of a particular chromosome
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  Speaker 1: once out of every two hundred times that chromosome underwent replication.
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  Speaker 1: This means that out of every two hundred sex cells
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  Speaker 1: produced by Gasuphla, one would contain a lethal gene somewhere
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  Speaker 1: along the length of that chromosome. That particular chromosome, however,
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  Speaker 1: contained at least five hundred genes capable of undergoing a
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  Speaker 1: lethal mutation. If each of those genes is equally likely
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  Speaker 1: to undergo such a mutation, then the chaz that any
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  Speaker 1: one particular gene is lethal is one out of two
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  Speaker 1: hundred times five hundred, or one out of every hundred thousand.
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  Speaker 1: This is a typical mutation rate for a gene in
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  Speaker 1: higher organisms, generally, as far as geneticists can tell, though
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  Speaker 1: the rates are lower among bacteria and viruses. Naturally, a
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  Speaker 1: Chasper mutation takes place every time a new individual is born.
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  Speaker 1: Fruit Flies have many more offspring per year than human beings,
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  Speaker 1: since their generations are shorter and they produce more young
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  Speaker 1: at a time. For that reason, though the mutation rate
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  Speaker 1: may be the same in fruit flies as in man,
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  Speaker 1: many more actual mutations are produced per unit time in
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  Speaker 1: fruitflies than in men. This does not mean that the
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  Speaker 1: situation may be ignored in the case of man. Suppose
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  Speaker 1: the rate of production of a particular deletarious gene in
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  Speaker 1: man is one out of one hundred thousand. It is
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  Speaker 1: estimated that a human being has at least ten thousand
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  Speaker 1: different genes, and therefore the chance that at least one
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  Speaker 1: of the genes is a sex cell is deleterious is
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  Speaker 1: ten thousand out of one hundred thousand, or one out
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  Speaker 1: of ten. Furthermore, it is estimated that the number of
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  Speaker 1: gene mutations that are weekly deleterious are four times as
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  Speaker 1: numerous as though that are strongly deliterious or lethal. The
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  Speaker 1: chances at least one gene in a sex cell is
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  Speaker 1: at least weekly deleterious then would be four plus one
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  Speaker 1: out of ten or one out of two. Naturally, these
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  Speaker 1: deliterious genes are not necessarily spread out evenly among human beings.
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  Speaker 1: With one to a sex cell, some sex cells would
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  Speaker 1: be carrying more than one, thus increasing the number that
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  Speaker 1: may be expected to carry none at all. And so
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  Speaker 1: it is supposed that very nearly half the sex cells
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  Speaker 1: produced by humanity carry at least one deliarious gene. Even
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  Speaker 1: though only half the sex cells are free of deligarious genes,
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  Speaker 1: it is still possible to produce a satisfactory new generation
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  Speaker 1: of men. Yet one can see that the genetic load
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  Speaker 1: is quite heavy that anything that would tend to increase
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  Speaker 1: it would certainly be undesirable and perhaps even dangerous. We
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  Speaker 1: tend to increase the genetic load by reducing the rate
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  Speaker 1: at which deliterarious genes are removed, that is, by taking
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  Speaker 1: care of the sick and retarded, and by trying to
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  Speaker 1: prevent discomfort and death at all levels. There is, however,
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  Speaker 1: no humane alternative to this. What's more, it is by
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  Speaker 1: and large only those with slightly deliterious genes who are
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  Speaker 1: preserved genetically. It is those persons with near sightedness, with diabetes,
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  Speaker 1: and so on, who, with the aid of glasses, insulin,
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  Speaker 1: or other props, can go on to live normal lives
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  Speaker 1: and have children in the usual numbers. Tho with strongly
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  Speaker 1: deliterious genes either die despite all that can be done
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  Speaker 1: for them even to day, or at least do not
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  Speaker 1: have a chance to have as many children. The danger
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  Speaker 1: of an increase in the genetic load rests more heavily
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  Speaker 1: than at the other end a measures that usually inadvertently
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  Speaker 1: or intentionally increase the rate of production of mutant genes.
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  Speaker 1: It is to this matter we will now turn end
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  Speaker 1: of Section eight