2.4 antenatal and postnatal screening

Question 1

What is the purpose of antenatal screening?

Answer 1

To identify the risk of a disorder, so further tests and a prenatal diagnosis can be offered.

Question 2

How many ultrasound scans are typically given to pregnant women?

Answer 2

Two scans: a dating scan and an anomaly scan.

Question 3

What is a dating scan, and when is it performed?

Answer 3

A scan to determine pregnancy stage and due date, performed between 8 and 14 weeks.

Question 4

What is an anomaly scan, and when is it performed?

Answer 4

A scan to detect serious physical abnormalities in the fetus, performed between 18 and 20 weeks.

Question 5

Why are blood and urine tests carried out throughout pregnancy?

Answer 5

To monitor the concentrations of marker chemicals.

Question 6

What can happen if a chemical is measured at the wrong time during pregnancy?

Answer 6

It could lead to a false positive result.

Question 7

What might an atypical concentration of a chemical indicate?

Answer 7

It may lead to diagnostic testing to determine if the fetus has a medical condition.

Question 8

What is the purpose of a karyotype?

Answer 8

A karyotype shows an individual’s chromosomes arranged as homologous pairs and can detect chromosome abnormalities.

Question 9

How are cells used in karyotyping?

Answer 9

Cells from samples are cultured to produce enough cells for a karyotype, which diagnoses a range of conditions.

Question 10

What is chorionic villus sampling

Answer 10

CVS is a prenatal test where fetal cells are obtained from the placenta to test for genetic abnormalities.

Question 11

When is chorionic villus sampling (CVS) performed compared to amniocentesis?

Answer 11

CVS is performed earlier in pregnancy than amniocentesis.

Question 12

What is a key risk of chorionic villus sampling (CVS)?

Answer 12

CVS has a higher risk of miscarriage compared to amniocentesis.

Question 13

What is amniocentesis?

Answer 13

Amniocentesis is a test where fetal cells are obtained from amniotic fluid for karyotyping to detect genetic conditions.

Question 14

What is an allele?

Answer 14

Different forms of a gene, e.g., for hearing and non-hearing.

Question 15

What does “dominant” mean in genetics?

Answer 15

A trait that always shows in a person’s phenotype if present

Question 16

What does “recessive” mean in genetics?

Answer 16

A trait that only shows in a person’s phenotype if two recessive alleles are inherited.

Question 17

What does “homozygous” mean?

Answer 17

Having two identical alleles present in a person’s genotype.

Question 18

What does “heterozygous” mean?

Answer 18

Having two different alleles present in a person’s genotype.

Question 19

Who is a “carrier” in genetic terms?

Answer 19

A person who has a recessive allele for a condition but does not show symptoms.

Question 20

What is “genotype”?.

Answer 20

The alleles a person has for a trait

Question 21

What is “phenotype”?

Answer 21

The physical expression of the alleles of a trait.

Question 22

What does “sex-linked” mean in genetics?

Answer 22

It refers to alleles carried on the X sex chromosome.

Question 23

How many alleles do females carry for a sex-linked trait?

Answer 23

Females carry two alleles (one on each X chromosome).

Question 24

From whom do sons inherit their X chromosome?

Answer 24

Sons always inherit their X chromosome from their mother.

Question 25

What does it mean if a son is affected by a sex-linked condition?

Answer 25

The affected son must have received the affected allele from his mother.

Question 26

How many alleles do males carry for a sex-linked trait?

Answer 26

Males carry only one allele (on their X chromosome).

Question 27

From whom do daughters inherit their X chromosome?

Answer 27

daughters always inherit a X chromosome from their father

Question 28

Why are males more likely to show a phenotype for a recessive sex-linked disease?.

Answer 28

Males only need to inherit one recessive allele to express the recessive phenotype, as they have only one X chromosome

Question 29

What might indicate a condition is sex-linked?

Answer 29

If more males are affected than females, it suggests the condition might be sex-linked.

Question 30

What is the purpose of postnatal screening?

Answer 30

To test newborns for metabolic disorders, such as phenylketonuria (PKU).

Question 31

What is phenylketonuria (PKU)?

Answer 31

A metabolic disorder caused by a substitution mutation that makes the enzyme converting phenylalanine to tyrosine nonfunctional.

Question 32

What happens in PKU when phenylalanine cannot be broken down?

Answer 32

Phenylalanine is broken down into toxic compounds, which can lead to mental disabilities and seizures.

Question 33

How is PKU managed in individuals with high phenylalanine levels?

Answer 33

They are placed on a restricted diet to limit phenylalanine intake.

Question 34

Can individuals with PKU still produce melanin? Why?

Answer 34

Yes, because they obtain tyrosine from their diet, which is needed to produce melanin.