Genetic counselling

Possible stations

  • Discuss antenatal screening for genetic abnormalities
  • Explain to a newly pregnant mother about the tests for Down’s syndrome
  • Draw a pedigree chart for a family with an autosomal recessive condition (e.g. cystic fibrosis, sickle cell anaemia) or with an autosomal dominant condition (e.g. Huntington’s, myotonic dystrophy) and discuss the risk of the patient having an affected child
  • Explain genetic test results and implications, e.g. patient is a carrier for the cystic fibrosis gene

Basic inheritance patterns

Autosomal dominant

Only need one copy of abnormal gene (from either parent) to cause disease:

  • If a parent is affected, there is a 1 in 2 chance of the child being affected

Autosomal dominant inheritance

Autosomal recessive

Need two copies of abnormal gene (one from mother, one from father) to cause disease, and only one copy to be a carrier:

  • If one parent (only) is a carrier, there is a 1 in 2 chance of the child being a carrier
  • If one parent (only) is affected, the child will be a carrier
  • If one parent is affected and the other is a carrier, there is a 1 in 2 chance the child will be affected, and a 1 in 2 chance the child will be a carrier
  • If both parents are carriers, there is a 1 in 4 chance of the child being affected, and a 2 in 4 chance of the child being a carrier

Autosomal recessive inheritance

Conditions of interest for antenatal screening

  • Familial (inherited) genetic conditions
    • Autosomal dominant: Huntington’s disease, myotonic dystrophy
    • Autosomal recessive: cystic fibrosis, sickle cell anaemia, thalassaemia, haemochromatosis
    • X-linked recessive: haemophilia 
  • Developmental abnormalities (not genetic), e.g. neural tube defects (spina bifida), other structural developmental defects
  • Chromosomal abnormalities (caused by cell division error – ‘genetic’ but not usually inherited), e.g. Down’s syndrome – risk increases with age

Antenatal screening tests

Parental blood tests

  • Genetic testing of mother and father can be performed to determine exact risk of baby being affected by a familial (inherited) genetic condition
  • If there is a significant risk to the baby, invasive testing is offered

Screening for Down’s, Edwards’ and Patau’s syndromes

  • Combined test: scan + blood test (10-14 weeks) – initial screen in most cases (better than quadruple test)
    • Blood test (10-14 weeks): ↓pregnancy-associated plasma protein, ↑βHCG 
    • Nuchal translucency scan (11-14 weeks)
  • Quadruple blood test (14-20 weeks): ↓α-fetoprotein, ↓unconjugated estradiol, ↑βHCG, ↑inhibin A – initial screen if combined test missed or it wasn’t possible to obtain a nuchal translucency measurement
  • Non-invasive prenatal testing (≥10 weeks): a more accurate (97-99%) maternal blood test that looks at fetal DNA fragments in the maternal blood– offered on NHS to mothers with a high risk (>1 in 150) combined/quadruple test result, but also avaliable privately

Neural tube defect screening

  • Blood test ↑α-fetoprotein (14-20 weeks) – gives a risk value. Fetal blood from amniocentesis can also be used and is more accurate. 
  • Anomaly scan (20 weeks) – confirms

Invasive testing for genetic condition diagnosis

Invasive testing is offered if risk screening tests indicate high risk.

  • Amniocentesis (>15 weeks) – 1% miscarriage risk
  • Chorionic villus sampling (10-14 weeks) – 1-2% miscarriage risk
  • These tests give a definite answer to whether the child has a certain genetic condition. Results take 1-2 weeks but rapid tests for chromosome abnormalities can be done in 3 days.
  • They can be performed for: babies at high risk of Down’s syndrome, familial genetic conditions above 
  • Termination of pregnancy can be performed at any time if there is confirmed genetic abnormality, but is usually done at <24 weeks

Test your knowledge with some questions

What is the difference between a genotype and a phenotype?

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    A 24 year old female asks for advice because she has had a child with cystic fibrosis. Both her and her husband are unaffected and have no other children. She would like to become pregnant again but would like to know the risk of that child being affected and the risk they would be a carrier. She would also like to know about antenatal testing.

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      A 20 year old male asymptomatic patient has a parent with Huntington’s disease. He would like to know the chance he is affected.

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