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looking briefly at some of the related biochemistry. Without going into detail, different alleles of the same gene produce related proteins, which in turn produce the variations in expressed characteristics associated with that gene. Typically these proteins (or combinations of them) are powerful biological catalysts called enzymes,capable of modifying reaction rates by factors of 10,000 and more. For this reason, genes exercise extensive control over the ongoing reactions in a cellthe enzymes they produce modulate ongoing reactions so strongly that they are the major determinants of the cell's form. Moreover, the products of any given enzyme-controlled reaction may, and generally do, enter into several subsequent reactions. Thus the effects of changes in a single enzyme are often widespread, causing gross changes in cell form and function. The human hereditary disorder called phenylketonuria results from an (undesirable) allele of a single gene; the presence of this allele has pronounced effects upon a whole battery of characteristics ranging from hair color and head size through intelligence. It is equally true that several genes may jointly determine a given characteristic, e.g., eye color in humans. |
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All of this adds considerably to the complexity of the system, but the greatest complexities come about because the effects of different enzymes are not additivea phenomenon known as epistasis. For example, if a sequence of reactions depends upon several enzymes, for practical purposes the sequence does not proceed at all until all of the enzymes are present; subtraction of one enzyme stops the reaction completely. More complicated reactions involving positive and negative feedback are common, particularly those in which the output of a reaction sequence is a catalyst or inhibitor for some intermediate step of the reaction. The main point is that the effect of each allele depends strongly upon what other alleles are present and small changes can often produce large effects. The amalgam of observed characteristicsthe phenotypedependsstrongly upon these epistatic effects. |
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Because of epistasis there is no simple way to apportion credit to individual alleles for the performance of the resulting phenotype. What may be a good allele when coordinated with an appropriate set of alleles for other genes, can be disastrous in a different genetic context. Thus adaptation cannot be accomplished by selecting among the alleles for one gene independently of what alleles appear for other genes. The problem is like the problem of adjusting the "height," "vertical linearity," and "vertical hold" controls on a television set. A "best setting'' for "height," ignoring the settings of the other two controls, will be destroyed as soon as one attempts to better the setting of either of the other two controls. The problem is vexing enough when there are three interdependent controls, as anyone who |
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