Genetic screening: ethical issues - Nuffield Council on Bioethics

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10 Chromosomal disorders 2.6 Chromosomal disorders fall into two broad categories: (i) (ii) where an entire chromosome is added or is missing. For example, in Down’s syndrome there is an extra (third) copy of chromosome 21 found in the cells of affected individuals (hence the technical term for it, Trisomy 21). In Turner’s syndrome, one of the X chromosomes in girls is missing. This type of disorder is not inherited but occurs during conception; rearrangement of chromosomal material. If this involves either net loss or gain of chromosomal material, harmful clinical effects are likely; on the other hand, if a simple exchange between chromosomes (translocation) or within them (inversion) has occurred, the chromosome make-up is ‘balanced’ and serious clinical effects are much less frequent. Types of genetic tests 2.7 All forms of genetic test aim to identify particular genetic characteristics but approach this in different ways. Chromosomal tests (cytogenetics) 2.8 Microscopic examination of chromosomes from cells in blood, amniotic fluid or fetal tissue may be used to detect the chromosomal changes mentioned above. Until recent years it was only possible to detect large alterations on a chromosome involving many genes, but new techniques are making it possible to detect much smaller defects, allowing disorders involving only a small amount of genetic material to be recognised. Tests for disorders involving a single gene 2.9 Genes cannot be seen using the microscope, so in the past tests for single gene disorders have been largely indirect, involving what the gene produces (protein), or another substance affected by it, rather than the gene itself (see paragraph 2.15). Since the protein is still unknown for the majority of genes, testing for single gene disorders has been very limited until recently.
11 Direct tests 2.10 A variety of techniques have now been developed for identifying important human genes directly. There are two main approaches:- (i) (ii) the gene may be isolated if the product (protein) it normally produces is known. This approach was used for the genes involved with the main blood cell protein haemoglobin (important for tests involving sickle cell disease and thalassaemias). The genes for some metabolic diseases, where a specific chemical defect involving an enzyme was already known, have also been isolated in this way; the gene may be isolated if its position on a chromosome is known (positional cloning). This approach is increasingly successful in allowing genes to be isolated even when we know nothing about their function or what protein they normally produce. One reason for this success is that detailed genetic maps of the different chromosomes are being produced. This approach not only pinpoints the chromosome region where the gene lies, but can provide genetic markers (identifiable pieces of DNA) which lie close to the gene, and can enable an accurate test for a genetic disorder to be made even before the gene itself is isolated. 2.11 Once the gene responsible for a disorder has been isolated it is possible to study the different changes (mutations) in it that can result in disease. These may range from complete absence of the gene to faults in a single chemical sub-unit of the gene. A single gene disorder may be caused by many different changes in the gene responsible. By careful study of particular populations of people it may be possible to determine which mutations for a disease are the commonest and most important, and to design a programme of testing accordingly. 2.12 Direct genetic testing by DNA techniques differs from most other forms of medical testing in several important respects. Any body tissue can be used since genes are present in almost all cells. Although blood is most commonly used, cells obtained by mouthwash are proving especially suitable for some <strong>screening</strong> programmes. Since genes do not usually change during life, a DNA test can be performed at any time from conception onwards. This is a practical advantage for tests in early pregnancy, as it can allow the detection of a serious genetic abnormality that would not show itself until after the child is born. However, this raises difficult <strong>ethical</strong> problems, especially in relation to diseases which do not appear until later childhood or adult life.
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10.20 We recommend that the Departm
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Genetic screening: ethical issues - Nuffield Council on Bioethics

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