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008 - Genetic Load · Percheron horse

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  Speaker 1: Section seven 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 Ethan Montese, The Genetic Effects of Radiation, my
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  Speaker 1: Isaac Asimov and Theodosius Dobzinsky Genetic load. Some gene mutations
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  Speaker 1: produce characteristics so undesirable that it is difficult to imagine
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  Speaker 1: any reasonable change in environmental condition that make them beneficial.
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  Speaker 1: There are mutations that lead to the non development of
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  Speaker 1: hands and feet, to the production of blood that will
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  Speaker 1: not clot, to serious malformations of essential organs, and so on.
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  Speaker 1: Such mutations are unqualifiedly bad. The badness may be so
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  Speaker 1: severe that a fertilized ovum may be incapable of development,
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  Speaker 1: or if it develops, the fetus miscarries, or the child
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  Speaker 1: is stillborn, or the child is born alive it dies
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  Speaker 1: before mature, so that it can never have children of
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  Speaker 1: its own. Any mutation that brings about death before the
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  Speaker 1: gene producing it can be passed on to another generation
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  Speaker 1: is a lethal mutation. A gene governing a lethal characteristic
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  Speaker 1: may be dominant, will then kill, even though the corresponding
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  Speaker 1: gene on the other chromosome or the pair is normal.
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  Speaker 1: Under such conditions, the lethal gene is removed in the
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  Speaker 1: same generation in which it is formed. The lethal gene may,
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  Speaker 1: on the other hand, be recessive. Its effect is then
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  Speaker 1: not evident. If the gene is paired with as normal,
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  Speaker 1: the normal gene carries on for both. When this is
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  Speaker 1: the case, the lethal general men in existence and will
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  Speaker 1: every once in a while make itself evident. If two
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  Speaker 1: people each sharing as a carrier with such a gene,
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  Speaker 1: have children. A sperm cell carrying a lethal may fertilize
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  Speaker 1: an egg carrying the same type of lethal, with sad results.
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  Speaker 1: Every species, including man, includes the individuals who carry undesirable genes.
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  Speaker 1: These undesirable genes we passed along for generations, even if dominant,
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  Speaker 1: before a natural selection culps them out. The more seriously
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  Speaker 1: undesirable they are, the more quickly they are removed. But
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  Speaker 1: even outright, lethal genes will be included among the chromosomes
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  Speaker 1: from generation to generation, provided they are recessive. These deleterious
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  Speaker 1: genes make up the genetic load. The only way to
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  Speaker 1: avoid a genetic load is to have no mutations and
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  Speaker 1: therefore no gene pool. The gene pool is necessary for
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  Speaker 1: the flexibility that will allow species to survive and evolve
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  Speaker 1: over the eons, and the genetic load is the price
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  Speaker 1: that must be paid for that. Generally, the capacity for
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  Speaker 1: a species to reproduce itself is sufficiently high to make
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  Speaker 1: up quite easily the numbers launched through the combination of
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  Speaker 1: deleterious genes. The size of genetic load depends on two factors,
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  Speaker 1: the rate of which deleterious gene is produced through mutation
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  Speaker 1: and the rate of which it is moved by natural selection.
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  Speaker 1: When the rate of removal equals the rate of production,
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  Speaker 1: a condition of genetic equilibrium is reached, and the level
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  Speaker 1: of occurrence of that gene then remains stable over the generations,
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  Speaker 1: even though deleterious genes are removed relatively rapidly. If dominant
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  Speaker 1: and lethal genes are removed in the same generation at
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  Speaker 1: which they are formed, a new crop of deleterious genes
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  Speaker 1: will appear by mutation with every succeeding generation. The equilibrium
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  Speaker 1: level for such a dominant deleterious gene is relatively low. However,
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  Speaker 1: deleterious genes that are ascessive are removed much more slowly.
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  Speaker 1: Those persons are two. Such genes who alone show the
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  Speaker 1: bad effects are let the visible portion of iceberg and
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  Speaker 1: represent only a small part of the whole. The heterozygotes
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  Speaker 1: or carriers who possess a single gene of the sort
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  Speaker 1: and who live out normal lives, keep that gene in being.
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  Speaker 1: If people in a particular population marry randomly, and if
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  Speaker 1: one of a million is born homozygous for a certain
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  Speaker 1: deleterious recessive gene and dies of it, one out of
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  Speaker 1: five hundred is heterozygous for that same gene, shows no
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  Speaker 1: ill effect, and as the capable of passing it on.
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  Speaker 1: It may be that the heterozygote is not quite normal,
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  Speaker 1: but does show some ill effects, not enough to incommode
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  Speaker 1: him seriously, perhaps, but enough to lower his chances slightly
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  Speaker 1: for mating and bearing children. In that case, the equilibrium
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  Speaker 1: level for that gene will be lower than otherwise would be.
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  Speaker 1: It may also be that the heterozygote experiences an actual
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  Speaker 1: advantage over the normal individual under some conditions. There is
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  Speaker 1: a recessive gene, for instance, that produces a serious disease
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  Speaker 1: called sickle cell anemia. People possessing two such genes usually
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  Speaker 1: die young. A HETEROZYGOT possessing only one of these genes
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  Speaker 1: is not serious affected and has red blood cells that
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  Speaker 1: are apparently less appetizing to malaria parasites. The hetero as
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  Speaker 1: I go, therefore experiences a positive advantage if he lives
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  Speaker 1: in a region where the incidents of a certain kind
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  Speaker 1: of malaria is high. The equilibrium level of the sickle
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  Speaker 1: cell anemia gene can, in other words, be higher in
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  Speaker 1: malaria regions than elsewhere. Here is one subject area on
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  Speaker 1: which additional research is urgently needed. And maybe that the
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  Speaker 1: usefulness of a single deleitarious gene is great that we
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  Speaker 1: may suspect in many cases, and that there are greater
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  Speaker 1: advantages to heterozygosinths than we know. This may be the
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  Speaker 1: basis what is sometimes called hybrid vigor and a world
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  Speaker 1: in which human beings are more mobile than they ever
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  Speaker 1: have been in history, and in which intercultural marriages are
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  Speaker 1: increasingly common. Information on this point is particularly important. End
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  Speaker 1: of Section seven.