Introduction to Genetic Screening
Genetic screening is a process by which a population is tested to determine which individuals possess the gene for a certain genetic disease. Many developed countries, with varying degrees of success, have screened for sickle cell anemia, cystic fibrosis, Tay-Sachs disease, thalassemia, and the XYY genotype.
The XYY Story
Nondisjunction, a failure in the distribution of chromosomes during meiosis, sometimes causes the formation of a sperm cell with two Y chromosomes. When this abnormal sperm cell fertilizes an egg, it leads to the development of an otherwise normal boy with two Y chromosomes, instead of the usual one. The first patient to be discovered to have this genotype (this occurred in the 1960's) was known to be slightly aggressive - a fact that excited geneticists. "Could aggressive behavior in men be traced to 'XYY syndrome'?" they wondered.
Patricia Jacobs, a British geneticist, with four other colleagues, did a study of 197 men in a high-security mental hospital in Scotland. The study, published in the British journal Nature in 1965, showed that 3.5 percent of the men studied had the XYY genotype. This is about 20 times higher than the normal occurrence of the defect. It was also noted that men in possession of the XYY genotype tended to be taller than ordinary men. The authors concluded their Nature article by stating it was not clear whether or not the XYY genotype "is related to aggressive behavior".
Many geneticists of the time ignored this uncertain ending to Jacobs' study. They thought the Y chromosome, in addition to influencing male physical characteristics, also exerted a degree of control over mental characteristics like aggressiveness. More studies followed the original one by Jacobs, and most confirmed the high rates of XYY genotypes found in mental hospitals and prisons. These studies received a great deal of media coverage, sensationalizing the genotype's "aggressive influences".
XYY Screening
Countries such as the U.S., Canada, Denmark, and England began allowing maternity hospitals to test for XYY genotypes in babies. A study carried out by Stanley Walzer and Park Gerald tracked the development of XYY boys, keeping a watch for aggression or mental disturbance. Eventually, this study's morality and validity came under criticism from scientists and citizens. It was revealed that XYY studies consistently omitted a control group of normal XYY males, tended to downplay environmental factors that influence aggression and mental health, and simplistically defined complex behaviors such as aggression. Despite all the studies done on XYY men, there is no evidence that the genotype affects behavior in any way. This is just one example of misguided genetic screening that only leads to paranoia and panic.
The Sickle-Cell Screening Fiasco
Another example of misguided screening with disastrous results is the sickle-cell anemia screening carried out in the U.S. in the 1970's. Since screening regulations were left to be defined by the individual states, the program was inconsistent and sometimes discriminatory. Some states made the screening mandatory only for black people. Others, such as Massachusetts, declared that carriers of the sickle-cell trait were themselves diseased, leading to job discrimination and confusion over marriage and health insurance.
PKU: A Screening Success Story
PKU, or phenylketonuria, is a disease in which victims cannot digest the amino acid phenylalanine. The acid builds up in their brains and often results in mental retardation. It can be treated by putting affected children on a phenylalanine-free diet to the age of six, when the brain is developed enough to be undamaged. Also, women with PKU must stick to the diet while they are pregnant to avoid damage to the developing fetus. Maternity hospitals now regularly perform a simple test for the disease on all babies, so that affected children can be treated immediately.
Lessons Learned From Screening
Many valuable lessons have been learned from various screening programs carried out in many different countries. The best screening programs are well-funded and well-organized. Good information is provided to participants, emphasizing the difference between carrying a certain disease and actually having it. For those discovered to be carriers, counseling is provided that explains the disease and its effects, defines options for treatment of victims, and explores methods of prenatal diagnosis if necessary. If these basic rules of good screening are followed, then confusion and discrimination is likely to be avoided, and people can make informed, intelligent lifestyle and reproductive choices.
Screening: Ethical Principles
- Screening should only be carried out for a recognized genetic disease, such as cystic fibrosis. Except in approved research studies, it should not be employed to identify people who possess abnormalities that have not been linked, or have been only loosely linked, to actual symptoms.
- Emphasis should be placed on good organization and information dissemination during a screening program. These programs should follow the rules of good screening stated above.
- Positive results should be kept absolutely confidential, and should not affect job security, health insurance, or marriage.
Created by Kate Stafford and Michael Mannor for ThinkQuest.
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