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  The more that is discovered about the intricate organization of the nervous system, the more it seems remarkable that genes can successfully specify the development of that system. Human genes contain too little information even to specify which hemisphere of the brain each of a human's 10" neurons should occupy, let alone the hundreds of connections that each neuron makes. For such reasons, we can assume that there must be an important random factor in neural development, and in particular, that errors must and do occur in the development of all normal brains.

  The most vivid expression of such errors occurs in genetically identical (isogenic) organisms. Even when reared under the same conditions, isogenic organisms are rarely exact copies of one another, and their differences have revealed much about the random variations that result from an organism's limited supply of genetic information. In isogenic Daphniae, for example, even though the position, size, and branching pattern of each optic neuron are remarkably constant, there is some vari-ability in connectivity, and the number of synapses varies greatly. This variability is probably the result of random scatter beyond the resolution of genetic control and is best termed "imprecision", since its converse, the degree of clustering about a mean, is conventionally, called "precision".

  Imprecision should be distinguished from developmental mistakes: wrongly migrated neurons, incorrect connections, and the like. To use a computer analogy, minor, rounding-off errors occur universally and are analogous to imprecision, but occasionally a binary digit is incorrectly transmitted, perhaps ruining a calculation, and this incorrect transmission is analogous to a developmental mistake. Thus, imprecision is a form of inaccuracy inherent within the limits of design, but mistakes are forms of gross fallibility.

  Both impression and gross fallibility can plausibly be blamed on the insufficiency of genetic information, since either could be reduced by adding more information. It is universally accepted among information theorists that codes and languages can be made mistake-resistant by incorporating redundancy. However, since the amount of space available in any information system is limited, increased redundancy results in decreased precision. For example, || when written incorrectly in English, "three point one four two," can be understood correctly even though a typographical error has occurred. More precision could be gained, however, if those 24 spaces were filled with Arabic numerals; then || could be expressed to 23 significant digits, although any error would significantly change the meaning. There exists a trade-off, the more precisely a system is specified, using a given limited amount of information, the greater the danger of gross mistakes. The overall scheme by which genetic information is rationed out in organisms, therefore, must involve a compromise between two conflicting priorties: precision and the avoidance of gross mistakes.

  1. Which of the following best expresses the main idea of the passage?

  (A) Although studies of isogenic organisms have shown that all organisms are subject to developmental variations, there is still scientific debate over the exact causes of these variations.

  (B) Because of limitations on the amount of information contained in the genes of organisms, developing nervous systems are subject to two basic kinds of error, the likelihood of one of which is reduced only when the likelihood of the other is increased.

  (C) The complexity of an organism's genetic information means that much of the unusual variation that occurs among organisms can best be explained as the result of developmental mistakes.

  (D) New findings about the nature of the genetic control of neural development support the work of some scientists who argue that the computer is an extremely useful model for understanding the nervous system.

  (E) The major discovery made by scientists studying the genetic control of neural development is that both imprecision and gross developmental error can be traced to specific types of mutations in specific genes.

  2. According to the passage, one of the reasons it has been assumed that there is an important random element in human neural development is that

  (A) genes cannot specify certain types of developmental processes as wen as they can others

  (B) the intricacy of the nervous system allows small developmental errors to occur without harmful effects.

  (C) the amount of information contained in the genes is less than the amount necessary to specify the location of the eurons

  (D) the number of neurons in the human brain varies greatly from individual to individual

  (E) it is theoretically impossible for an organism to protect itself completely from gross developmental mistakes

  3. The author suggests which of the following about the findings of information theorists?

  (A) Their findings provocatively challenge the standard explanation of redundancy in genes.

  (B) Their findings provide useful insights into understanding the rationing of genetic information.

  (C) Their findings help to explain why imprecision can occur in neural development but not why gross mistakes can occur.

  (D) Their findings suggest that genes may be able to specify neural development more accurately than had previously been thought.

  (E) Their findings support the work of those who use computer operations as models for understanding genetic control.

  4. According to the passage, of the following aspectsof the optic neurons of isogenic Daphniae, which varies the most?

  (A) Size

  (B) Connectivity

  (C) Position

  (D) Branching pattern

  (E) Number of synapses

  5. Which of the following best describes the organization of the first paragraph?

  (A) A specific case is presented, its details are analyzed, and a conclusion is drawn from it.

  (B) A discovery is announced, its most significant application is discussed, and possibilities for the future are suggested.

  (C) A generalization is made, specific situations in which it is applicable are noted, and problems with it are suggested.

  (D) An observation is made, specifics are provided to support it, and a generalization is derived.

  (E) A hypothesis is presented, its implications are clarified, and applications of it are discussed.

  6. The author uses all of the following to clarify the distinction between imprecision and gross mistakein neural development EXCEPT

  (A) classification of borderline phenomena

  (B) a description of the relationship between the phenomena denoted by each term

  (C) specific examples of the phenomena denoted by each term

  (D) an explanation of at least one of the key terms involved

  (E) analogies to other types of phenomena

  7. Which of the following can be inferred from the passage about the genetic information of Daphniae?

  I. There is probably some degree of redundancy in the information controlling neural development

  II. Most of the information for neural development stored in the genes is used to specify the positions of the optic neurons.

  III. There is sufficient information to preclude the occurrence of gross mistakes during neural development.

  (A) I only

  (B) II only

  (C) III only

  (D) I and II only

  (E) II and III only

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