Q30 - Heteroplasmy Thresholds Behind Mitochondrial Replacement Therapy

Theoretical B Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Mitochondria & Chloroplasts: Structure & Endosymbiotic Origin Extranuclear & Non-Mendelian Inheritance

Sir Robert Edwards (1925-2013) invented in vitro fertilisation (IVF) and Sir Douglas Turnbull developed ‘3 parent IVF’: the nuclear DNA of a mother and father are transferred into an enucleated oocyte from a second female. The ethicist Baroness Mary Warnock (1924-present) has enabled the UK to pioneer the safest and most advanced reproductive medicines, to combat genetic diseases such as Leigh syndrome. Leigh syndrome is caused by mutations to the mitochondrial gene COX2. Mitochondrial DNA (mtDNA) in the muscles typically has the following distributions.

Figure 1 for Q30: Heteroplasmy Thresholds Behind Mitochondrial Replacement Therapy

Using the information and data, determine which of the statements are true or which are false.

A. 30% of fully functional mitochondria is sufficient to prevent Leigh's disease.

B. The father of a Leigh’s child should be screened for COX2 mutant mtDNA.

C. If only a small amount of cytoplasm is accidently transferred with the nuclear DNA, the three- parent IVF child is safe from Leigh syndrome.

D. Rapidly dividing tissues are typically more affected by mtDNA mutations than slowly dividing tissues.

E. Sampling a cell from an early IVF embryo could determine whether the foetus will develop Leigh syndrome when implanted.

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Question reproduced from IBO 2017, Theoretical Exam B, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key & worked solutions