Clinical genetics and genetic counselling

Genetic disorders pose considerable health and economic problems because often there is no effective therapy. In any pregnancy the risk of a serious developmental abnormality is approximately 1 in 30 pregnancies; approximately 15% of paediatric inpatients have a multifactorial disorder with a predominantly genetic element.
People with a history of a congenital abnormality in a member of their family often seek advice as to why it happened and about the risks of producing further abnormal offspring. Interviews must be conducted with great sensitivity and psychological insight, as parents may feel a sense of guilt and blame themselves for the abnormality in their child.
The aims of genetic counselling should include:
ESTABLISHING AN ACCURATE DIAGNOSIS. Examination of the child may help in diagnosing a genetically abnormal child with characteristic features, e.g. trisomy 21, or whether a genetically normal fetus was damaged in utero.
A FULL AND CAREFUL HISTORY should be taken. The pregnancy history, drug, alcohol ingestion during pregnancy and maternal illnesses, e.g. diabetes, should be detailed.
DRAWING A FAMILY TREE is essential. Questions should be asked about abortions, stillbirths, deaths, marriages, consanguinity and medical history of family members. Diagnoses may need verification from other hospital reports.
ESTIMATION OF THE RISK OF A FUTURE PREGNANCY BEING AFFECTED OR CARRYING A DISORDER. Estimation of risk should be based on the pattern of inheritance. Mendelian disorders (see earlier) carry a high risk; chromosomal abnormalities a low risk. Empirical risks may be obtained from population or family studies.
INFORMATION on prognosis and management.
EXPLANATION of the implications for other siblings and family members.
GENETIC SCREENING, which includes prenatal diagnosis if requested, carrier detection and data storage in genetic registers.

Carrier detection

Carrier detection is offered in autosomal recessive disorders for conditions that are relatively common such as thalassaemias (Asian and Mediterranean populations), cystic fibrosis (Caucasian populations), sickle cell disease (African origin) and Tay-Sachs disease (Ashkenazi Jews). Families segregating the severe form of haemophilia A can now have more accurate genetic counselling to detect the inversion of the X chromosome (flip-tip inversion) using Southern blotting.

Prenatal diagnosis

For families at risk of genetic disease an intrauterine diagnosis is important either to reassure parents if the fetus is unaffected or to allow termination of the pregnancy if requested. Prenatal diagnosis might be carried out if there is a high genetic risk of a severe disorder and if no treatment is available for the particular disorder. High resolution ultrasonography has replaced amniocentesis in some centres for the diagnosis of neural tubedefects. However for diagnosing gene defects in which it  is necessary to study fetal DNA, amniocentesis (sampling of fluid from the amniotic sac) gives access to fetal cells. Chorionic villus sampling (transcervical or transabdominal under ultrasound guidance) can be performed at an earlier time (from 10 weeks’ gestation), which has the advantage of earlier and easier termination if requested. With embryos produced by in vitro fertilization, DNA analysis of one or two embryonic cells at 8-16 cell stage can be undertaken and if normal the embryo is implanted.
Maternal serum is widely used in the second trimester (16-22 weeks) for screening for neural tube defects and trisomy 21. High levels of maternal serum a-fetoprotein are associated with neural tube defects and some other fetal abnormalities. Altered levels of maternal serum afetoprotein, unconjugated oestriol and human chorionic gonadotrophin are associated with trisomy 21 affected pregnancies. First trimester maternal serum screening for trisomy 21 using pregnancy-associated plasma protein-A is under evaluation.
Genetic counselling should be non-directive, with the couple making their own decisions on the basis of an accurate presentation of the facts and risks in a way they can understand.

Methods available for prenatal diagnosis and screening.
Methods available for prenatal diagnosis and screening.

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