(a) Antenatal screening

• Antenatal screening identifies the risk of a disorder so that further tests and a prenatal diagnosis can be offered.
• A variety of techniques can be used to monitor the health of the mother, developing fetus and baby.

Ultrasound imaging

• Pregnant women are given two ultrasound scans.
• Anomaly scans may detect serious physical abnormalities in the fetus (done between 18 and 20 weeks).
• Dating scans, for pregnancy stage and due date, are used with tests for marker chemicals which vary normally during pregnancy (done between 8 and 14 weeks).

Blood and urine tests

• Routine blood and urine tests are carried out throughout pregnancy to monitor the concentrations of marker chemicals.
• Measuring a chemical at the wrong time could lead to a false positive result.
• An atypical chemical concentration can lead to diagnostic testing to determine if the fetus has a medical condition.

Diagnostic testing

• Amniocentesis is used to diagnose potential congenital abnormalities by examining cells found in the amniotic fluid.
• Chorionic villus sampling (CVS) is used to diagnose potential congenital abnormalities by examining cells from the chorion in the placenta.
• CVS can be carried out earlier in pregnancy than amniocentesis, although it has a higher risk of miscarriage.
• Cells from samples can be cultured to obtain sufficient cells to produce a karyotype to diagnose a range of conditions.
• A karyotype shows an individual’s chromosomes arranged as homologous pairs.
• In deciding to proceed with these tests, the element of risk will be assessed, as will the decisions the individuals concerned are likely to make if a test is positive.

(b) Pedigree charts

• Pedigree charts are used to analyse patterns of inheritance in genetic screening and counselling.
• Patterns of inheritance include –
  • autosomal recessive,
  • autosomal dominant,
  • incomplete dominance
  • sex-linked recessive single gene disorders.

Autosomal recessive

A disorder such as albinism which is caused by a defective recessive autosomal allele:

• is expressed relatively rarely
• usually skips generations
• affects both males and females in equal numbers
• requires the affected individual to be homozygous recessive
• can be carried by a heterozygous individual
• can result from two unaffected parents who are heterozygous.

Autosomal dominant

A disorder such as Huntington’s disease which is caused by a defective autosomal dominant allele:

• affects both males and females in equal numbers
• means anybody affected will have an affected parent
• no longer appears in future generations if a branch of the pedigree does show the disorder
• results in all non-affected individuals being homozygous recessive
• means anybody affected is either double dominant or heterozygous.

Incomplete dominance

A disorder such as sickle-cell anaemia which is caused by an autosomal defective allele:

• affects both males and females in equal numbers
• in the homozygous state produces the maximum expression of the phenotype
• is rarely expressed maximally
• in the heterozygous state produces a reduced expression of the phenotype.

Sex-linked recessive single gene disorders

A disorder such as haemophilia which is caused by a defective sex-linked recessive allele:

• affects many more males than females
• is not transmitted to a male from his affected father
• needs an individual to be homozygous if they are female
• will be expressed in a male who has one copy of the defective allele
• means all daughters of affected fathers will either be carriers or be affected themselves.

(c) Postnatal screening

• Diagnostic testing can take place for metabolic disorders, include phenylketonuria (PKU).
• In PKU a substitution mutation means that the enzyme which converts phenylalanine to tyrosine is non-functional.