If a daughter who has a grandparent with Huntington's tests positive for that form of the gene, it's obvious that her parent on that side of the family has it too. Unless the daughter hides the test result, the parent will know his or her own status. On the other hand, someone may refuse to seek out and share genetic information with a parent, brother, or sister who may be at risk. A woman with breast cancer, say, may not want to go through testing to find out whether an inherited gene helped lead to the disease. Without knowing which form of the gene had added to her risk, her relatives wouldn't be able to find out whether they had the same susceptibility.
Testing raises other family issues as well.
Many people, including most geneticists and genetic counselors, say they feel strongly that children generally should not be tested for disorders that appear only later in life, such as Huntington's, Alzheimer's disease, or certain kinds of cancer. Nevertheless, surveys have shown that these tests are often performed anyway. Familial adenomatous polyposis, which affects about 50,000 people in the United States, is a good example of a disease that many geneticists feel warrants early testing. We all react unpredictably to genetic test results, whatever the outcome. Some people who get the result they feared become very upset - and some feel happy.