Margaret M. Loos
I.
Objectives:.
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1. To review methods of genetic screening
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2. To give realistic information about birth defects and flaws in genetic replication.
3. To allow students to form their own outlook in terms of this unit.
II.
Information:
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A. Of the 3,000,000 children to be born this year in the United
States it is estimated that 215,000 will have some genetically or teratogenically caused disorders.
Those that are genetically caused would be:
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1. Point mutations, producing either autosomal or sexual dominant disorders. These can be the result of defects in the sequence of bases in the codons (interchange of base pairs, insertion of a base pair or deletion of a base pair which does not have the opportunity to correct itself). It can even be the result of the loss of a gene.
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2. “Negative” recessive disorders. These are most difficult to deal with because of carrier guilt and need for recognition of the “masked” effect.
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3. Abnormalities in the number of chromosomes or makeup and arrangement of chromosomes, such as translocations, deletions and nondisjunctions.
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4. Combinations of these factors and environmental teratogens.
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B Teratogens:—A teratogen is a drug, chemical, virus, physical agent or deficiency state that, by acting during the embryonic or fetal period, alters morphology or function in post-natal period. The amount of damage during the 0-14 days of gestation is very low because either the ovum dies or regenerates completely. Until the 60th day (organogenetic period) the embryo is extremely sensitive and structural damage is most likely to occur from exposure to teratogenic agents. In the later part of pregnancy, the fetal period, venerability to transplacental insults is greater. A 50% increase in the number of birth defects, when an agent is present, would be considered an indication that the agent is teratogenic.
Materials with proven effects are certain drugs, alcohol, tobacco, some chemicals and large dosages of radiation. We must realize that 2,000 or so chemicals and drugs are added to our environment each year and this alone precludes adequate monitoring. Testing really can only be carried out on laboratory animals and the animal may not show the same results as in humans. For instance, thalidomide that caused reduction of arms and legs in the children of women, who were given that drug, wouldn’t affect rabbits or rats but would show up in monkeys and humans . . . . . . Testing done in rats leads to questions about aspirin, cortisone and vitamin deficiencies. Also, variation in dosages and individual susceptibility are factors. Medications that have done damage are usually prescribed for problems unrelated to pregnancy and the risk is often before pregnancy is diagnosed. Natural occurrences from the genetic factors also cause 2-3% of the infants to have birth defects.
III.
Assignment
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Some proven teratogens are:
alcohol
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DES
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cancer chemotherapy drugs
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hyperthermia (14-28 days)
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thalidomide
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tetracycline
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rubella
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Research one of these and their discovery and effects.
IV.
More Information
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Diagnosis of birth defect possibility is through:
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1. Medical monitoring in normal prenatal visits.
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2. Alpha fetoprotein measurements (for anencephaly and spinal bifid a.
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3. Amniocentesis
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4. Fetoscopy
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5. Screening of karyotypes
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6. Awareness of the parents
V.
Assignment
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As a sensitive informed person, write up a code for medical people who are involved in genetic screening and counseling.