Antenatal and postnatal screening

• Allele


  Form of a gene coding for one version of a characteristic
• Amniocentesis


  Prenatal test to assess health of foetus using cells from amniotic fluid
• Anomaly scan


  Antenatal ultrasound scan that checks for physical abnormalities
• Antenatal screening


  Use of tests to identify risk of a disorder before birth
• Autosomal dominant


  Allele on chromosomes 1-22; always expressed in the phenotype
• Autosomal recessive


  Allele on chromosomes 1-22; expressed in the phenotype if the genotype is homozygous for the recessive allele
• Chorionic villus sampling (CVS)


  Prenatal test to assess health of foetus using cells from the placenta
• Embryo


  Stage of development up to 8 weeks; leads to the formation of a foetus
• Foetus


  Stage of a baby after 8 weeks of development
• Heterozygous


  Having two different alleles of the same gene
• Homozygous


  Having two identical alleles of the same gene
• Incomplete dominance


  When an allele is not completely masked by a dominant allele, thus affecting the phenotype
• Karyotype


  Display of matched chromosomes produced for medical purposes
• Pedigree chart


  Diagram showing the occurrence of phenotypes of a particular gene in a family tree
• Phenylketonuria (PKU)


  Metabolic disorder; tested for by postnatal screening
• Postnatal screening


  Diagnostic testing of new born babies
• Prenatal diagnosis


  Identification of the risk of disorders in unborn babies
• Sex-linked recessive


  Recessive allele carried on the X chromosome
• Ultrasound scanning


  Diagnostic procedure used for various prenatal checks (e.g. establishing stage of pregnancy and the due date)
• Antenatal screening


  1. Ultrasound imaging

  2. Biochemical tests

  3. Diagnostic testing (amniocentesis & chorionic villus sampling)
• Ultrasound imaging


  • Pregnant women are offered two ultrasound scans during pregnancy
  • Ultrasound scans involve high frequency sound waves, emitted by a probe, that pass through a gel placed over the female's abdomen
  • These waves are reflected off the foetus and are converted into an image on the screen
• Dating scan


  • Takes place between 8 and 14 weeks
  • Used with tests for marker chemicals, which vary normally during pregnancy
  • Determines stage of pregnancy and due date
• Anomaly scan


  • Carried out between 18 and 20 weeks
  • Identifies serious physical problems
• Measuring a substance at the wrong time during pregnancy


  Could lead to a false positive result
• Pregnancy tests make use of the changing levels of a hormone called HCG (human chorionic gonadotrophin) which increases in concentration following implantation and can be detected in the urine
• High levels of HCG

  Can indicate a risk of Down's syndrome in the foetus
• HCG levels remain high the first 3 months and all pregnancy
• Amniocentesis


  1. Carried out between 15 and 18 weeks of pregnancy

  2. Small amount of amniotic fluid withdrawn which contains foetal cells

  3. Cells cultured and stained so that a karyotype can be produced

  4. Poses a small risk of around 0.06% risk of miscarriage

  5. Results take around 2 weeks
• Chorionic villus sampling (CVS)

  1. Carried out between 11 and 14 weeks of pregnancy

  2. Cell sample, known as chorionic villus cells, removed from placenta

  3. 1-2% chance of miscarriage and a risk of infection

  4. Results take 2-3 days
• Advantages and disadvantages of diagnostic tests

  • AMNIOCENTESIS: Identification of chromosomal abnormalities, results take longer, not carried out until 16 weeks
  • CHORIONIC VILLUS SAMPLING: Can be carried out earlier than amniocentesis, has a higher risk of miscarriage at 1-2% compared to amniocentesis
• Pedigree chart


  A family tree which shows inheritance of a particular genetic characteristic
• Sex chromosome


  In humans, this is the X or Y chromosome. This determines the sex of the individual
• Autosome


  All chromosomes of the body except the sex chromosomes
• Sex-linked genes


  Genes carried on the X chromosome
• Carrier


  Someone who has the gene for a particular genetic condition but does not have the condition themselves (a heterozygous individual)
• Siblings


  Brothers or sisters
• Pedigree charts have standardised symbols which are shown in the diagram
• Sex chromosome


  In humans, the X or Y chromosome that determines the sex of the individual
• Pedigree charts have standardised symbols to represent different individuals
• Pedigree charts are constructed by genetic counsellors to analyse patterns of inheritance and calculate risk