Even without the limitations imposed by the expense and difficulty of simply creating transgenic organisms, organisms with altered copies of one or more genes, the process of genetically engineering organisms for specific phenotypes – physical characteristics rather than genetic ones – can be fraught with difficulties. To a student of traditional Mendelian genetics, however, this may not be self-evident. After all, if one can find the gene that governs a specific trait and isolate it, then it should behave the way in another organism of the same type.
But despite the optimism suggested by Mendelian genetics, in practice the process is incredibly difficult, even if you are only interested in copying genes from another member of the same species. This is because even two genetically identical individuals may not develop in an identical manner if grown in different environments – or even in the same one. For example, the first cloned cat was cloned from a calico, but did not share the distinct orange and black splotching characteristic of its “mother.” It was instead a tiger-striped, grey cat, with no orange on it at all despite the fact that the two cats were genetically identical. The difference in appearance was due to a phenomenon known as X-inactivation, a phenomenon that occurs in female mammals in which one of the two copies of the X-chromosome in each cell is randomly “switched off” during embryonic development. The embryo then develops as a sort of mosaic – some cells exhibit one form of the X-chromosome, and others the other. Since fur color is held on the X-chromosome, two female cats can look nothing like one another even if they are genetically identical.
Processes like X-chromosome inactivation mean that researchers must test transgenic organisms carefully to ensure that there are no unintended consequences from the creation of transgenic or cloned organisms. Additionally, they must test to ensure that, in different environments, the cloned genes act as intended. In the absence of such testing, millions of dollars and thousands of hours of researchers’ time could be lost in producing organisms that have little to no value relative to their intended purpose.
1. Which of the following can be correctly inferred from the passage?
A. Unintended consequences from creating transgenic organisms are only a concern when dealing with female mammals
B. X-inactivation is not described under Mendelian genetics.
C. Researchers can avoid unintended consequences by ensuring that they are dealing with similar organisms.
D. The monetary drawbacks of problems arising from transgenic organisms outweigh the benefits.
E. Cloned organisms usually develop differently than their “parents” due to X-inactivation
2. Which of the following experiments, if carried out, would best test one of the issues mentioned in paragraph 3?
A. Two cloned organisms are raised in different environments and monitored for differences in phenotype.
B. Two transgenic organisms with the same genotype are raised in the same environment.
C. An organism without a specific gene modification is compared to one that does have the modification.
D. Two organisms with different cloned genes are monitored for differences in their phenotypes
E. An organism with a cloned gene is monitored to see how efficiently the cloned gene performs.
3. Why does the author use quotes around the term "mother" in the second paragraph?
A. To highlight the fact that X-inactivation occurs in females.
B. To explain the relationship between the two cats in the example.
C. To signify the importance of the term "mother" to the reader.
D. To indicate that the term is more of a metaphor than as an exact relationship.
E. To emphasize that the two cats, despite their relationship, look completely different.
4. Which of the following can be inferred from the passage?
A. If two genetically-identical organisms exhibit different physical traits, those differences are a result of X-inactiviation.
B. Because of X-inactivation, no two organisms are ever truly genetically identical.
C. X-inactivation occurs more frequently in calico cats than in other mammals.
D. Not all pairs of genetically identical organisms are physically identical.
E. When identical twins exhibit different phenotypes, the differences more often result from having developed in different environments than from X-inactivation.
参考答案：
承接第二段)怎么避免X-inactivation