Steps of Clinical Genetics:

1. History Taking
2. Pedigree charting (family tree)
   

   Family Tree : An effective tool of medical genetics

3. Clincal examination: urinogenital examination
4. Dermatoglyphics:
   1. print of both hands and finger tips recorded with ink
   2. analysis of total ridge count and atd angle
   3. normal atd angle is 38 – 40 (increased in Down’s Syndrome and decreased in Klinefelter’s Syndrome)
   4. Buccal smear: Buccal mucosa smear examined for presence of barr body to detect genetic sex of person
5. Karyotyping
6. Biochemical assays
7. Other investigations: X rays, biopsy, biochemical investigations
8. Counselling

Karyotyping:

• Karyotyping is a method of study of chromosomes in which a set of chromosome arranged in a standard classification from photomicrograph of metaphase is studied.
• From peripheral blood lymphocytes, bone marrow, skin, amniotic fluid cells

Indications for chromosome analysis:

• Multiple congenital abnormalities.
• Unexplained mental retardation.
• Sexual ambiguity.
• Infertility.
• Recurrent miscarriage.
• Still birth.
• Malignancy & chromosome breakage syndrome

Method: routine blood cultures

1. Blood planted in culture vials containing tissue culture medium supplemented by phytohemagglutinin (PHA)
2. Commonly used media are TC199, HAM F10, RPM1, etc. having nutrients and human AB serum
3. Heparin used as anticoagulant
4. Antibiotic added to prevent contamination
5. PHA in culture vials stimulate cell division
6. Vials kept for incubation at 37 c
7. At 69^th hour, colchicine is added for cell division arrest
8. After 1 – 3 hours, centrifugation, supernatant discarded and cells treated with hypotonic KCl solution allowing cells to swell
9. Centrifugation again, supernatant discarded and fixative (methanol and acetic acid) added
10. Slides prepared and observed microscopically
11. Atleast 30 chromosome spreads analysed

Classification of Chromosome:

Chromosome consists of centromere and 2 arms labeled as p and q

Classification By Position of Centromere:

1. Metacentric
2. Submetacentric
3. Acrocentric: Except Y chromosomes, all have satellites attached by secondary constricitions
4. Telocentric

Denver Classification: arranged in decreasing size

1. A: 1, 2, 3
2. B: 4, 5
3. C: 6 to 12, X
4. D: 13, 14, 15
5. E: 16, 17, 18
6. F: 19, 20
7. G: 21, 22, Y

1, 3, 16, 19, 20 (metacentric)

13, 14, 15 (large acrocentric with satellites)

21, 22 (small acrocentric with satellites)

Y (acrocentric without satellites)

Remaining submetacentric

Genetic Counselling

Steps of Genetic Counselling:

1. Accurate diagnosis of disorder
2. Advising the patient/ family members
3. Management of disorder either curative/supportive

Prenatal Diagnosis:

Indications :

1. Advanced maternal age
2. One of the parents is a balanced translocation carrier.
3. If autosomal or X-linked recessive metabolic disorder.
4. One child with neural tube defect.

Procedure:

1. Amniocentesis
2. Chorionic villi sampling
3. Ultrasonography
4. Fetoscopy
5. Fetal blood sampling
6. Maternal blood sampling
7. Preimplantation diagnosis

Amniocentesis:

• Amniocentesis involves aspiration of amniotic fluid for biochemical and/or genetic investigation.
• Optimum time- 16 to 18 weeks,2^nd trimester.
• Cells obtained cultured for 1-3 weeks.
• Prepare the karyotype.
• Banding to determine the structural arrangement

Chorionic Villus Sampling:

• 9-12 weeks, during first trimester.
• USG evaluation of pregnancy is performed beforehand.
• Rather easy and less traumatic.
• Direct analysis of chromosome / 24 hrs. after culture

Ultrasonography:

• Done routinely for the verification of viability of embryo, determination of gestational age, diagnosis of multiple gestations, diagnosis of various congenital anomalies and well as guide for the passage of instruments for invasive procedures.

Fetoscopy:

• Visualization of fetus using a fiber optic self-illuminated instrument.
• Inserted in the amniotic cavity under local anesthesia.
• 18-22 weeks of gestation.
• Detect the limb malformation, facial defect, cleft lip, cleft palate, ear deformities.
• 3-5% abortion but useful for skin biopsy and fetal blood sampling.

Fetal and Maternal Blood Analysis:

• An attractive method because of its non- invasiveness nature.
• Fetal cells get available in the maternal blood after15 weeks of gestation.
• Difficulties in extraction of fetal cell.
• 20 ml sample may contain 0-20 fetal cells.
• 1fetal cell/ 1 million maternal cells.
• Need to be improvised to make it a standard procedure.
• Maternal blood can be used for assessment of neural-tube defect.
• Raise in the alpha-fetoprotein level in the serum of maternal blood.

Preimplantation Diagnosis:

• Suit for IVF procedure
• Fertilize ovum get developed into 8 celled stage.
• One of the cells taken out to assess genetic defect.
• If not found any disorders then get implanted into mother’s womb for pregnancy.
• Useful for those who can’t terminate the pregnancy due to religious background.

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Last updated: June 15, 2011

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