Exercises: Screening in Pregnancy and Infancy

Name three types of screening used before, during or after birth in Ireland, and give one example of a condition each can detect.

Phenylketonuria (PKU) is a genetic disorder in which the enzyme phenylalanine hydroxylase is missing. State how PKU is inherited and name the substance that builds up in the blood.

In Ireland, a blood sample from the heel of newborn babies is tested within the first days of life. State the name commonly given to this test and explain why early detection matters.

A couple in Cork are both carriers of the recessive allele for PKU.

1. a) Using the symbols $P$ and $p$, give the genotypes of the parents.

1. b) Draw a Punnett square and state the probability that their child has PKU.

1. c) State the probability that a child will be a carrier.

Compare ultrasound scanning, amniocentesis and chorionic villus sampling (CVS) in terms of what they detect, when they are carried out and any risk to the fetus.

A child diagnosed with PKU is placed on a diet low in phenylalanine.

1. a) Explain how the diet prevents damage to the child.

1. b) Name two foods that must be restricted and explain why.

1. c) Explain why a woman with PKU must keep to the diet before and during pregnancy.

A screening test for a genetic condition has a false-positive rate of 5% and a detection rate of 95%. The condition affects 1 in 1,000 pregnancies. In a group of 100,000 pregnancies:

1. a) Calculate the number of affected pregnancies and the number correctly detected.

1. b) Calculate the number of false positives.

1. c) Calculate the proportion of positive results that are true positives and explain why a positive screening result is followed by a diagnostic test.

Down syndrome is caused by trisomy 21.

1. a) Explain how non-disjunction in meiosis produces a gamete with an extra chromosome 21.

1. b) Describe how a karyotype of cells from amniocentesis would show the condition.

1. c) Discuss two ethical issues linked to prenatal screening for conditions like Down syndrome.

Explain, at the level of metabolism and brain development, why untreated PKU causes intellectual disability. Evaluate the benefits and limitations of population-wide newborn screening for PKU in terms of cost, health outcome and ethics.