Antenatal and postnatal screening

• During antenatal care, which two techniques can be used to obtain cells for production of a karyotype?

  Chorionic villus sampling (CVS) and amniocentesis
• Which of the following antenatal tests can produce cells for culturing to produce a karyotype for the diagnosis of a range of conditions?

  Amniocentesis
• A sex-linked gene carried on the X-chromosome of a man will be transmitted to

  100% of his female children
• Colour blindness is a sex-linked recessive trait. A woman would have a 50% chance of being colour blind if


  her father is colour blind and her mother is a carrier
• Two alleles of a gene code for different proteins. Both proteins are present in the heterozygote. This is an example of

  incomplete dominance
• Phenylketonuria is caused by a single autosomal gene. A man and a woman, who are unaffected, have an affected child. What is the

  probability that their next child will be affected?

  25%
• All babies are tested for PKU immediately after birth. State the term used to describe this type of diagnostic testing.

  Postnatal (screening).
• Describe how brain damage can be prevented in babies diagnosed with PKU.

  Restricted diet / low phenylalanine diet.
• Describe a pattern of inheritance, shown by a family history, which would indicate that the condition is

  1. recessive and b. sex-linked.

  Recessive – Disease skips generation / does not appear in every generation

  Sex-linked – More males will be affected than females
• indicate the area from which cells are removed during CVS.

  fetal tissue
• Describe the process by which a karyotype is produced from

  cells removed during CVS.

  Cells are cultured (to obtain sufficient cells).

  (Karyotypes then show) the chromosomes (from the cells).
• Suggest an advantage of using CVS rather than amniocentesis

  during antenatal screening.

  CVS can be carried out earlier (in pregnancy than amniocentesis).
• Name the type of antenatal screening tests which are

  routinely carried out to monitor the concentration of certain

  substances, such as protein, in a pregnant woman’s blood.

  Biochemical.
• The diagram shows cells in the testes dividing to produce sperm cells. State how many of the sperm cells shown in the diagram contain

  1. an X chromosome

  2. autosomes.

  X chromosome – half of them

  autosomes – all of them
• Metabolic pathway 2 operates only when metabolic pathway 1 is

  blocked. Explain how such a blockage might occur.

  A mutation occurs in the gene which produces the enzyme that controls metabolic pathway 1. 1

  Phenylalanine is not broken down into tyrosine / metabolic pathway 1 cannot occur.
• Name the condition a person is said to have if they fail to convert

  phenylalanine to tyrosine.

  Phenylketonuria / PKU.
• Describe how PKU affects the metabolic pathway shown.

  Enzyme 1 is not produced / does not function OR Phenylalanine builds up OR Tyrosine is not produced.
• What term describes the testing of new born babies for inherited disorders such as PKU?

  Postnatal screening.
• Suggest why some inborn errors of metabolism, such as phenylketonuria (PKU), only have an effect after birth?

  Poisons / chemicals / phenylalanine is removed by the mother.
• Describe how the son’s phenotype differs from his parents.

  Son only has one antigen
• What term is used to refer to chromosomes which are not sex-linked?

  Autosomal / autosomes.
• Antenatal screening occurs before birth to identify risks so that further tests can occur.
• Dating scan scans at 8-14 weeks is used to determine stage of pregnancy
• Anomaly scan scans at 18-20 weeks for serious physical problems.
• Biochemical / chemical tests detect (normal physiological) changes of pregnancy
• Marker chemicals can indicate medical conditions / can give a false positive result.
• Diagnostic / further testing can follow from routine testing.
• Amniocentesis / fetal cells can be obtained from the amniotic fluid.
• Chorionic villus sampling / fetal cells can be obtained from the placenta
• Fetal cells can be used to produce a karyotype / chromosome

  abnormalities.
• CVS can be carried out earlier in pregnancy than amniocentesis.
• CVS has higher risk of miscarriage.