【B】Scientists have long known that specific genes are associated with a number of seriousdiseases and birth defects. Scientists have used this knowledge to develop tests to identify defectivegenes, which are the result of    21    , a natural process that alters the genetic material.Researchers have identified a large number of genes that are responsible for life-threateningconditions, such as cystic fibrosis, a disease that shortens people’s lives by attacking their lungs,and Huntington’s disease, a fatal brain condition. Once these genes are    22    , genetic tests formany such diseases become available. These tests can indicate if a person has a specific defectivegene. By 2011, researchers had developed more than 2,000 genetic tests, which allow doctors toinform patients if they have    23    these genes and if they risk passing them to their children.
        The tests are a significant    24    in genetic research, because they provide people who havegenetic defects with important information. However, the tests also introduce complex ethicalissues. If patients find out that they have a dangerous genetic defect, they may not know what todo. Their decision will depend on several factors. In some cases, identification of the gene onlysuggests the    25    that the patients will develop the disease associated with the gene. Forexample, women who have inherited the harmful BRCA gene mutation have a much higher chanceof developing breast cancer than other women do.
【題組】21.
(A) mutation
(B) addition
(C) variation
(D) complication