Screening for genetic disorders

What you'll learn

This revision guide covers how genetic disorders are detected before and after birth, including screening methods, interpretation of family history, and the ethical considerations surrounding genetic testing. You'll understand when and why screening is offered, how results inform reproductive decisions, and the social and economic implications of screening programmes in the UK and internationally.

Key terms and definitions

Genetic disorder — A disease or condition caused by a mutation in one or more genes, which can be inherited from parents or arise spontaneously.

Genetic screening — Testing individuals or populations to identify those who carry alleles for genetic disorders, either as carriers or those who will develop the condition.

Carrier — An individual who has one copy of a recessive allele for a genetic disorder but does not show symptoms because they also have one dominant normal allele.

Embryo screening — The process of testing embryos created through IVF for genetic disorders before implantation in the uterus, also called pre-implantation genetic diagnosis (PGD).

Pre-natal screening — Testing a fetus during pregnancy to detect genetic disorders, using techniques such as amniocentesis or choriodonic villus sampling (CVS).

Pedigree analysis — Studying family trees to track the inheritance of genetic disorders across generations and predict the probability of offspring inheriting conditions.

Allele frequency — The proportion of a particular allele within a population, which affects how common a genetic disorder is.

False positive/negative — When a screening test incorrectly indicates a disorder is present (false positive) or fails to detect a disorder that is present (false negative).

Core concepts

What are genetic disorders and why screen for them?

Genetic disorders result from mutations in DNA that can be passed from parents to offspring. Some disorders are caused by:

• Dominant alleles — only one copy needed to cause the disorder (e.g., polydactyly, Huntington's disease)
• Recessive alleles — two copies needed to cause the disorder (e.g., cystic fibrosis, sickle cell disease)
• Sex-linked alleles — genes located on sex chromosomes, usually the X chromosome (e.g., haemophilia, red-green colour blindness)

Screening allows:

• Parents to make informed decisions about having children
• Early treatment or management of conditions
• Selection of embryos without genetic disorders during IVF
• Prevention of suffering in affected children
• Identification of carriers who could pass disorders to offspring

The NHS offers screening for certain conditions when there is a family history or when prospective parents come from populations with higher carrier frequencies. For example, sickle cell disease screening is routinely offered to pregnant women in the UK because certain ethnic groups (particularly those of African, Caribbean, Mediterranean, Middle Eastern and Asian descent) have higher carrier rates.

Types of genetic screening

Carrier screening

This identifies individuals who carry one copy of a recessive allele for a genetic disorder. Carriers are healthy but can pass the allele to their children. If both parents are carriers for the same recessive disorder:

• 25% chance each child will have the disorder (homozygous recessive)
• 50% chance each child will be a carrier (heterozygous)
• 25% chance each child will have two normal alleles (homozygous dominant)

Carrier screening is particularly important for:

• Cystic fibrosis — most common inherited disorder in white European populations
• Sickle cell disease — higher frequency in people of African, Caribbean, and Mediterranean ancestry
• Thalassaemia — more common in people of Mediterranean, South Asian, and Middle Eastern origin

Embryo screening (PGD)

Pre-implantation genetic diagnosis is used during IVF treatment:

1. Eggs are fertilised in a laboratory
2. Embryos develop to the 8-cell stage
3. One or two cells are removed from each embryo
4. DNA from these cells is tested for specific genetic disorders
5. Only embryos without the disorder are implanted into the uterus
6. Embryos with genetic disorders are destroyed or used for research

This technique is particularly useful when:

• Both parents are carriers of a recessive disorder
• One parent has a dominant genetic disorder
• There is a family history of a serious genetic condition
• Previous children have been affected by a genetic disorder

Pre-natal screening

Testing occurs during pregnancy to detect genetic disorders in the developing fetus:

Amniocentesis (usually performed at 15-20 weeks):

• A needle is inserted through the mother's abdomen into the amniotic fluid
• Fetal cells in the fluid are collected and cultured
• DNA is extracted and tested for genetic disorders
• Small risk (0.5-1%) of miscarriage

Chorionic villus sampling (CVS) (usually performed at 11-14 weeks):

• A sample of placental tissue (which contains fetal cells) is removed
• Can be done earlier than amniocentesis
• Slightly higher risk of miscarriage (1-2%)

Non-invasive pre-natal testing (NIPT):

• Analyses fetal DNA present in the mother's blood
• No risk of miscarriage
• Can detect chromosomal disorders like Down's syndrome
• Less reliable for single-gene disorders

Newborn screening

In the UK, all newborns are offered screening for nine genetic conditions through the heel-prick blood test at 5 days old. This includes:

• Sickle cell disease
• Cystic fibrosis
• Phenylketonuria (PKU)
• Congenital hypothyroidism

Early detection allows immediate treatment, preventing serious health problems or developmental delays.

Interpreting family history and pedigree diagrams

Pedigree analysis helps predict the probability of offspring inheriting genetic disorders. Key features to identify:

Recessive inheritance patterns:

• Disorder can skip generations
• Affected individuals have unaffected parents (who are carriers)
• More likely when parents are related (consanguinity)
• Approximately equal numbers of males and females affected

Dominant inheritance patterns:

• Disorder appears in every generation
• Affected individuals have at least one affected parent
• Cannot skip generations
• Approximately equal numbers of males and females affected

Sex-linked recessive inheritance (X-linked):

• Much more common in males than females
• Affected males often have unaffected parents
• Can pass through carrier females who show no symptoms
• Affected males cannot pass the condition to sons (only daughters become carriers)

When analysing family trees:

1. Identify the pattern of inheritance
2. Determine the genotypes of individuals where possible
3. Use genetic diagrams (Punnett squares) to calculate probabilities for future offspring
4. Remember that each pregnancy is an independent event with the same probability

Economic, social and ethical considerations

Genetic screening raises important questions that extend beyond biology:

Economic arguments for screening:

• Early detection reduces long-term NHS treatment costs
• Prevents lifelong care costs for severely affected individuals
• Reduces absence from work for parents caring for affected children
• Population screening programmes can be cost-effective if targeting high-risk groups
• Genetic counselling services require funding

Economic arguments against screening:

• Screening programmes are expensive to run (testing, counselling, infrastructure)
• False results lead to unnecessary anxiety and further testing costs
• Not all detected conditions have available treatments
• May reduce genetic diversity in the population

Social and ethical considerations:

Arguments supporting screening:

• Reduces suffering for affected individuals and families
• Allows informed reproductive choices
• Enables early intervention and treatment
• Parents can prepare for a child with special needs
• Respects autonomy — individuals can make their own decisions

Arguments opposing or raising concerns about screening:

• May lead to termination of pregnancies, which some find morally unacceptable
• Risk of discrimination against carriers (employment, insurance)
• Potential for creating a "designer baby" culture
• Could reduce acceptance and support for people with disabilities
• Questions about what conditions are "serious enough" to screen for
• Pressure on parents to make difficult decisions
• Psychological impact of knowing you carry a harmful allele
• May reduce genetic diversity
• Religious or cultural objections

Specific considerations:

• Who decides which embryos are implanted or which pregnancies continue?
• Should embryo screening be allowed for non-medical characteristics?
• What happens to embryos that test positive for genetic disorders?
• How accurate must tests be before making life-changing decisions?
• Should screening be mandatory or optional?
• How do we ensure informed consent, especially in diverse communities?

In the UK, the Human Fertilisation and Embryology Authority (HFEA) regulates embryo screening, permitting PGD only for serious genetic conditions. The decision to screen and what to do with results remains with individuals, supported by genetic counselling services.

Worked examples

Example 1: Carrier screening probability calculation

Question: Cystic fibrosis is caused by a recessive allele (f). Both Sarah and James are carriers (Ff) for cystic fibrosis. They want to know the probability that their child will have cystic fibrosis. Show your working. (4 marks)

Mark scheme answer:

Parental genotypes: Ff × Ff (1 mark)

Punnett square:

	F	f
F	FF	Ff
f	Ff	ff

(1 mark for correct Punnett square)

Offspring genotypes:

• FF = 1/4 (25%)
• Ff = 2/4 (50%)
• ff = 1/4 (25%)

(1 mark for genotype ratios)

Probability child will have cystic fibrosis = 1/4 or 25% or 1 in 4 (1 mark)

(Must show the ff genotype causes the disorder)

Example 2: Interpreting embryo screening data

Question: A couple used IVF and embryo screening because both are carriers for sickle cell disease. Five embryos were tested. Two had sickle cell disease, two were carriers, and one had no sickle cell alleles.

(a) Explain which embryo(s) could be safely implanted. (2 marks) (b) Suggest why the couple chose embryo screening rather than pre-natal testing. (2 marks)

Mark scheme answer:

(a)

• The embryo with no sickle cell alleles could be safely implanted (1 mark)
• The two carrier embryos could also be implanted as they would not develop sickle cell disease / they are heterozygous (1 mark)

(b)

• Any two from:
• Avoids termination of pregnancy / avoids abortion, which they may find morally unacceptable (1 mark)
• Ensures the baby will not be affected before pregnancy begins (1 mark)
• Less stressful than ending a wanted pregnancy (1 mark)
• Reduces risk of having an affected child (1 mark)

Example 3: Sex-linked inheritance

Question: Haemophilia is a sex-linked recessive disorder. The allele for haemophilia (h) is located on the X chromosome. A woman who is a carrier (X^H X^h) has children with an unaffected man (X^H Y).

What is the probability that their sons will have haemophilia? Show your working. (3 marks)

Mark scheme answer:

Parental genotypes: X^H X^h × X^H Y (1 mark)

Genetic diagram showing:

• X^H X^H (unaffected female)
• X^H X^h (carrier female)
• X^H Y (unaffected male)
• X^h Y (affected male with haemophilia)

(1 mark for correct offspring genotypes)

Probability = 1/2 or 50% or 1 in 2 sons will have haemophilia (1 mark)

(Accept: out of male children only, 50% affected)

Common mistakes and how to avoid them

• Confusing carriers with affected individuals — Remember carriers (heterozygous) for recessive disorders do not show symptoms but can pass the allele to offspring. Only homozygous recessive individuals show symptoms of recessive disorders.

• Incorrect probability calculations — Each pregnancy is an independent event with the same probability. If a couple has a 1 in 4 chance of an affected child, this applies to every pregnancy, not just the first one. Having three unaffected children doesn't change the probability for the fourth child.

• Mixing up sex-linked and autosomal inheritance — Sex-linked disorders (usually X-linked) predominantly affect males. Autosomal disorders affect males and females equally. Always check the question carefully.

• Forgetting that embryo screening requires IVF — Embryo screening can only be used when embryos are created outside the body through IVF. Not all couples use or can access IVF.

• One-sided ethical arguments — Exam questions often ask you to discuss or evaluate screening. Always present balanced arguments showing different perspectives (economic, social, ethical, religious) unless specifically asked for one viewpoint.

• Vague ethical statements — Avoid phrases like "it's wrong" or "it's good." Instead, explain why someone might think screening is beneficial (reduces suffering, informed choice) or problematic (leads to termination, discrimination, designer babies) with specific reasoning.

Exam technique for "Screening for genetic disorders"

• Command words matter: "Describe" requires factual statements about what happens; "Explain" needs reasoning with because/therefore; "Discuss" or "Evaluate" requires arguments for and against with a conclusion. For 6-mark questions, use the "point-evidence-explain" structure.

• Show genetic diagrams clearly: When calculating probabilities, always show parental genotypes, use a Punnett square or genetic diagram, identify offspring genotypes, and state the probability clearly. This ensures method marks even if the final answer is incorrect.

• Ethics questions need balance: For questions worth 4-6 marks about ethics or screening decisions, structure your answer with at least two arguments supporting screening and two arguments against, using scientific and social reasoning. Refer to specific stakeholders (parents, affected individuals, NHS, society).

• Use correct terminology: Use precise scientific vocabulary — write "embryo screening/PGD" not "testing babies," "heterozygous carrier" not "someone with the gene," and "allele" not "gene" when referring to different versions.

Quick revision summary

Genetic screening identifies disorders caused by inherited alleles through carrier testing, embryo screening (PGD during IVF), or pre-natal testing (amniocentesis/CVS). Screening enables informed reproductive decisions but raises ethical questions about embryo selection and pregnancy termination. Recessive disorders require two copies of an allele; dominant disorders need only one. Sex-linked disorders mainly affect males. Pedigree analysis predicts inheritance probabilities. Economic benefits (reduced treatment costs) must be weighed against screening programme costs and social concerns about discrimination and genetic diversity.