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Capillary separation accurately sorts the DNA strands as each terminating base manifests a different colour so as to produce a cumulative strand and hence a small section of continuous nucleotide is revealed (Figure 3.3). Computer software then matches the sequence overlaps to establish a contiguous DNA sequence.
Limitations and Ethical issues
The determination of the entire DNA sequence contained in the human genome will not enable genetic specialists to look at a person's DNA sequence and predict everything about their appearance, behaviour and other characteristics.
An individual's genetic make-up makes a considerable contribution to a person's health, growth, development, appearance and behaviour, environment also plays a major role. This is not just the physical environment such as diet and climate but also education, housing and access to high quality health services.
- Who should have access to sequence information viz commercial organizations, nation states or individuals and on what basis.
- Clinical issues such as the education of health service providers and the general public
- Insurers to have genetic information about people who apply for health insurance
- Making and keeping records of genetic information raises serious problem of confidentiality
- Through the developing genetic technologies, many medical treatments made possible will initially be very expensive and there restricted availability will add to the problem as to who should get it first.
Future Developments and its Implications
- Knowledge about the structure of the genome and its importance
While the discovery of new genes and knowledge about their function is an obvious benefit, the organisation of the genes within the genome is also important. Questions such as: is it important for genes to be located on a particular chromosome, or in a particular order to function, will hopefully be answered.
- Development of new technologies
Polymerase chain reaction (PCR) has already enabled us to use very small fragments of DNA in clinical medicine for various diagnostic purposes. This is also very important in forensic science when only a small sample of tissue or blood may be available for study.
- Diagnosis and predictive testing for genetic conditions
Isolation of gene mutations will not only help in diagnosing disease during pregnancy but also later in life well before symptoms of disease start.
- New treatments for genetic conditions
The discovery of the gene and the protein which it codes for provides hope for the design of treatments based on correcting the malfunctioning gene or protein. Other treatments will involve the development of drugs guided by the genes and their products. This new field of pharmacogenetics has already yielded a number of drugs used in breast cancer treatment for example.
PRENATAL TESTING AND SCREENING FOR FETAL ABNORMALITIES
Prenatal testing is the use of special tests carried out during pregnancy. There are two kinds of tests.
Diagnostic tests: determine if the baby has, or will develop after birth, a particular problem. It is possible to detect conditions that are due to changes in chromosome number and structure as well as changes to the genetic information in particular genes. Some of the conditions that can be determined during pregnancy would be evident at birth or in the first year of life (birth defects) but others would not affect a person until later in childhood, adolescence or even as an adult.
Screening tests: determine if the baby has an increased risk of having a particular problem. They are not diagnostic tests and an increased risk result does not mean the baby will definitely have a problem.
There are a number of reasons that would suggest the consideration of prenatal diagnostic testing.
These include:
- having a family history of a person with a serious condition
- both parents are "carriers" of the same faulty gene
- where one of the partners in a couple has a serious condition which may be passed on to a baby
- having a previous child affected by a serious problem in growth, development and/or health
- exposure to some chemical or other environmental agent
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where the results of screening tests such as ultrasound or first and second trimester screening tests have determined that the woman is at increased risk for having a baby with a particular genetic condition in this pregnancy.
How is Prenatal Testing done?
Some are screening tests using ultrasound and/or an examination of chemicals in the mother's blood to determine whether the baby is at risk of certain problems. Other tests are diagnostic as they examine cells from the baby to determine if the baby actually has the condition in question.
There are a number of different tests and procedures that can be performed during pregnancy. Each has distinct advantages, disadvantages and limitations. Prenatal diagnostic testing, prenatal screening and ultrasound scanning are outlined below in the order in which they are offered.
8-10 weeks and 18 weeks
ULTRASOUND SCANNING
Almost every woman has an ultrasound scan at some time during pregnancy. Sound waves, which do not harm the baby, are passed through the amniotic fluid surrounding the baby to create a picture.
Using ultrasound, the depth of fluid in the space at the back of the baby's neck is measured. This measurement is used to calculate the risk or chance of the baby having certain problems with chromosomes, such as Down syndrome.
- A special blood test (Triple Marker Blood Test) measuring two proteins in the mother's blood taken at the same time as the ultrasound can give a more accurate risk result.
11-12 weeks
CHORIONIC VILLUS SAMPLING (CVS)
Diagnostic test
During a CVS cells from the developing placenta are taken and tested for certain conditions such as Down syndrome.
- Chromosomes or DNA from the placenta is examined in the laboratory.
- There is a small risk (less than 1%) of miscarriage due to this test (in addition to the background risk of miscarriage in the first 3 months of pregnancy).
15-19 weeks
AMNIOCENTESIS
Diagnostic test
Using a fine needle, a small amount of amniotic fluid is taken from around the baby and sent to the laboratory for analysis.
- The baby's cells are removed from the fluid and the chromosomes or DNA examined.
- The test will give a definite answer for problems such as Down syndrome.
- There is a slight risk of miscarriage (less than 1%) due to this test.
Relevant Data for Prenatal Testing
Thirteen percent (13%) of women 35 years of age will have an abnormal Triple Marker Blood test indicating an increased risk for trisomy 21, thus requiring an amniocentesis. However, only 71% of fetuses with trisomy 21 will be identified. Thus, for every 10 fetuses with Down syndrome, only 7 will be detected.
Ethical issues
Whether or not to have children is a major decision for any couple. It is even more difficult where one or both of the prospective parents knows, or just suspects, that they may carry a faulty gene associated with a health problem which could affect their children. When the decision is to proceed to pregnancy, there are a number of further decisions to be made with regards to the possible genetic testing of the embryo/fetus during the pregnancy. Detection of a genetic mutation or chromosomal abnormality may not provide all the information about the life of that fetus or person or the severity of a particular condition.
When a fetal abnormality is detected, parents need medical and social support once they decide to continue the pregnancy. Some may choose to terminate the pregnancy leading to moral, religious and cultural conflicts.
The genetic test results will certainly reveal the sex of the child. If no genetic abnormalities are found, what should be the view if the parents want to terminate the pregnancy simply as a way of choosing the sex of the children in their family?
Limitations
- Definite results may not be possible at all times.
- In some cases only an increased risk of the person being affected can be predicted.
- The finding of a change in a particular gene may provide little information about the persons disease and may rarely predict the severity of the condition or the age at which symptoms will appear.
- There may be lack of clear cut relationship between the genotype (genetic make up) and the phenotype (expression of the condition).The decision to act on this information can be very distressing.
Future Developments
Use of prenatal diagnostic techniques should only be encouraged if the presence of a genetic disease is certain or almost certain. It is recommended that government authorities should only encourage prenatal diagnosis and therefore support the cost of it if such conditions prevail. Such financial support aims to give equality of access to frequently very costly procedures.
In cases where diagnosis is reliable and the disease is frequent and particularly severe, it may be desirable to encourage a general extension of prenatal diagnosis methods. A public health programme for the collection of data on (sickle-cell anaemia and thalassemia) would be feasible as of now in areas where it is frequent, and will soon be a possibility for certain X-linked disorders. Further extension could be considered as soon as possible to the diagnosis of frequent, grave, and intractable genetic diseases (such as cystic fibrosis), taking into account the cost of testing.
Bibliography
- Biology Text Book (AS) pages (135 – 138)
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http://www.fetal.com/gen_triple.htm - (Relevant Data)