Genetic Screening and Ethics: GCSE Biology

What is genetic screening and why is it done?

Genetic screening is the analysis of an individual's (or embryo's) DNA to identify the presence of alleles or chromosomal variations associated with genetic diseases or increased disease risk.

Reasons for genetic screening:

• Prenatal screening: to detect chromosomal abnormalities or inherited conditions in a foetus during pregnancy.
• Pre-implantation genetic diagnosis (PGD): to select healthy embryos before implantation during IVF.
• Newborn screening: to detect metabolic or genetic conditions in newborns before symptoms develop, enabling early treatment.
• Carrier testing: to identify adults who carry one copy of a recessive allele (such as cystic fibrosis) and could pass it to their children.
• Predictive testing: for adults at risk of late-onset conditions (e.g. Huntington's disease, familial breast cancer).

What is amniocentesis?

Amniocentesis is a prenatal diagnostic test carried out between approximately 14 and 20 weeks of pregnancy.

Procedure:

1. Using ultrasound guidance, a fine needle is inserted through the mother's abdomen and uterine wall into the amniotic fluid surrounding the foetus.
2. A small sample (~15–20 ml) of amniotic fluid is withdrawn. This fluid contains foetal cells shed by the baby's skin.
3. The foetal cells are cultured (grown) in a laboratory and the chromosomes are examined (karyotyping) or the DNA is analysed.

Conditions detected: Down's syndrome (trisomy 21 — an extra chromosome 21), Edwards' syndrome (trisomy 18), Patau's syndrome (trisomy 13), cystic fibrosis, sickle cell anaemia, and other chromosomal and single-gene disorders.

Risk: carries a small risk of miscarriage (approximately 0.5–1%), so it is not offered routinely to all pregnant women — typically offered to those with a higher-risk result on non-invasive screening tests.

What is chorionic villus sampling (CVS)?

CVS is carried out earlier in pregnancy, between 10 and 13 weeks.

Procedure:

1. A small sample of the chorionic villi (finger-like projections of the placenta, which contain foetal cells) is removed by inserting a catheter through the cervix or a needle through the abdomen under ultrasound guidance.
2. The sample is analysed for chromosomal and gene abnormalities.

CVS can provide results earlier than amniocentesis, giving parents more time to consider their options. Like amniocentesis, it carries a small miscarriage risk (~1–2%).

What is pre-implantation genetic diagnosis (PGD)?

PGD is used alongside IVF (in vitro fertilisation):

1. Several eggs are fertilised in a laboratory to create multiple embryos.
2. When each embryo has developed to the 3–5-day stage (blastocyst), one or two cells are removed.
3. The DNA from those cells is tested for specific alleles or chromosomal abnormalities.
4. Only embryos that do not carry the condition are selected for implantation into the uterus.

PGD avoids the need for prenatal diagnosis and the difficult decision of whether to terminate a pregnancy. It is available through the NHS for couples with a high risk of passing on a serious genetic condition (e.g. both parents are carriers of cystic fibrosis). The procedure is regulated by the Human Fertilisation and Embryology Authority (HFEA) in the UK.

What is the newborn heel prick test?

The heel prick (Guthrie) test is offered to all newborns in the UK at 5 days old. A few drops of blood are collected from the baby's heel onto a card. The dried blood spots are screened for:

• Phenylketonuria (PKU) — inability to metabolise phenylalanine; treatable with a special diet
• Congenital hypothyroidism — low thyroid hormone; treatable with thyroxine
• Cystic fibrosis
• Sickle cell disease
• Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) — a metabolic disorder
• And several other rare but treatable conditions

Early detection allows treatment to begin before irreversible damage occurs — for PKU, for example, starting a low-phenylalanine diet in the first weeks of life prevents the severe intellectual disability that would otherwise develop.

What are the ethical arguments about genetic screening?

Genetic screening raises important ethical questions examined in GCSE biology:

There is no single "correct" answer in science to many of these questions — they involve values, cultural perspectives, and legal frameworks. GCSE exam questions ask students to evaluate arguments for and against, not to state a definitive personal view.

Frequently asked questions

What is the difference between genetic screening and genetic testing?

The terms are sometimes used interchangeably, but genetic screening typically refers to testing a whole population or defined group (e.g. all newborns, all pregnant women over 35) for a condition regardless of whether a family history exists. Genetic testing more often refers to testing an individual who already has a specific reason to suspect risk — for example, testing an adult whose parent had Huntington's disease. Population screening programmes aim to find conditions before symptoms develop, whereas targeted testing answers a specific clinical question.

What happens if a prenatal test reveals a chromosomal abnormality?

Parents are offered counselling by a genetic counsellor and a specialist doctor. They are given detailed information about the condition — its severity, available treatments, likely outcomes, and support available. The decision about how to proceed is entirely the parents' — options include continuing the pregnancy (with plans for appropriate medical care), or, in some cases, terminating the pregnancy. In the UK, the decision to terminate a pregnancy on grounds of a serious foetal abnormality is legal under the Abortion Act 1967. Genetic counsellors provide support but do not make decisions for families.

Can genetic screening identify all genetic conditions?

No. Screening tests are designed to detect specific, known genetic variants. They cannot predict all possible genetic problems, spontaneous new mutations (de novo mutations), or conditions with complex multi-gene and environmental causes (such as type 2 diabetes or most common cancers). A "normal" screening result does not guarantee a healthy baby — it means the specific conditions tested for are not detected. The panel of conditions screened for varies by country and test; new conditions are added as validated tests and treatments become available.

What is the difference between a dominant and recessive condition in the context of screening?

For dominant conditions (e.g. Huntington's disease), a single copy of the mutant allele causes the disease — heterozygotes are affected. Predictive testing for adults at risk (who have a 50% chance of inheriting it from an affected parent) raises difficult choices: knowing you will develop an incurable fatal disease decades before symptoms appear has serious psychological implications. For recessive conditions (e.g. cystic fibrosis, sickle cell), two copies of the mutant allele are needed for the disease. Carriers (one copy) are healthy but may pass the allele to their children. Carrier screening helps couples identify whether both partners are carriers and what chance their children have of being affected (25% if both parents are carriers).

---

For Socratic GCSE biology with Professor Darwin — linking inheritance, genetics and the ethical dimensions of modern biotechnology — visit aitutors.me.