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9700 · 19.2

Genetic technology applied to medicine — practice questions

Practice and worked examples for 9700 Genetic technology applied to medicine. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

A couple has a high risk of having a child with a severe genetic disorder. Explain how preimplantation genetic diagnosis (PGD) could be used in their situation and discuss two significant ethical concerns associated with this technology.

Show solution outline
  1. PGD Process: The couple would first undergo in vitro fertilisation (IVF) to produce several embryos.
  2. Biopsy: At the 8-cell stage (or blastocyst stage), a single cell (blastomere) or a few cells from the trophoectoderm are carefully removed from each embryo.
  3. Genetic Analysis: The DNA from these cells is then analysed for the specific genetic mutation causing the disorder using techniques like PCR and DNA probes.
  4. Selection and Implantation: Embryos identified as free from the disorder are then selected and implanted into the mother's uterus, while affected or aneuploid embryos are typically discarded.

Ethical Concerns:

  1. Destruction of Embryos: PGD involves the creation of multiple embryos, many of which will be discarded if found to carry the disorder or deemed unsuitable. This raises moral questions about the status of embryos and the destruction of potential human life, particularly for those who believe life begins at conception.
  2. Slippery Slope to 'Designer Babies': While PGD is primarily used to prevent serious diseases, there's a concern that it could lead to 'selection' for non-medical traits like intelligence, sex, or physical characteristics. This 'slippery slope' argument worries about a future where parents might seek to engineer their children, raising issues of parental autonomy versus societal values and potential discrimination against those with certain traits.

Worked example 2

A genetic screening test for sickle cell anaemia uses PCR to amplify a 350 bp region of the β-globin gene. The amplified DNA is then digested with the restriction enzyme MstII. The normal allele (HbA) contains an MstII recognition site, while the sickle cell allele (HbS) does not due to a point mutation. Digestion of the HbA PCR product yields two fragments of 200 bp and 150 bp. The HbS PCR product remains as a single 350 bp fragment. The results from a family (Mother, Father, Child 1) are analysed by gel electrophoresis.

  • The Mother's sample shows bands at 350 bp, 200 bp, and 150 bp.
  • The Father's sample shows bands at 350 bp, 200 bp, and 150 bp.
  • Child 1's sample shows a single band at 350 bp.

Determine the genotype and phenotype for each individual.

Show solution outline

The analysis relies on interpreting the DNA fragment patterns after restriction digest and electrophoresis. Smaller fragments travel further down the gel.

  1. Understanding the Alleles:
    • HbA (Normal): The 350 bp PCR product is cut by MstII into 200 bp and 150 bp fragments.
    • HbS (Sickle Cell): The 350 bp PCR product is not cut and remains as a 350 bp fragment.
  2. Analysing the Mother:
    • Bands: 350 bp, 200 bp, 150 bp.
    • Interpretation: The presence of the 200 bp and 150 bp fragments indicates she has at least one HbA allele. The presence of the 350 bp fragment indicates she has at least one HbS allele.
    • Genotype: HbA HbS (Heterozygous).
    • Phenotype: Carrier with sickle cell trait. Typically asymptomatic or with mild symptoms under extreme conditions.
  3. Analysing the Father:
    • Bands: 350 bp, 200 bp, 150 bp.
    • Interpretation: Same pattern as the mother. He has fragments corresponding to both the HbA and HbS alleles.
    • Genotype: HbA HbS (Heterozygous).
    • Phenotype: Carrier with sickle cell trait.
  4. Analysing Child 1:
    • Bands: A single band at 350 bp.
    • Interpretation: This pattern shows that the PCR product was not cut by the MstII enzyme. This means the child only possesses the HbS allele.
    • Genotype: HbS HbS (Homozygous recessive).
    • Phenotype: Has sickle cell anaemia.

Conclusion: Both parents are carriers, and they have had a child who inherited the sickle cell allele from both of them, resulting in the disease. This matches the expected Mendelian inheritance pattern for an autosomal recessive condition (1 in 4 chance).

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