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Q23 - Genomic Imprinting Pedigree

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Pedigree Analysis & Human Genetic Disorders Gene Regulation: Eukaryotic & Epigenetics

The pedigree shows the inheritance pattern of gene X, whose mutation leads to a disorder in adults.

  • The mutation leads to a loss-of-function allele.
  • In addition, gene X undergoes genomic imprinting.
  • In this case, the imprint prevents the allele from being expressed.

The expression of alleles that are subject to imprinting depends on whether they were inherited from the mother or the father. When the imprinting takes place during sperm formation, it is called paternal imprinting. If it takes place during formation of the egg, it is known as maternal imprinting. In each generation, the imprints are reset during gametogenesis.

Individuals who are carriers for the mutant allele are marked with a dot. Individuals showing the disorder are shaded. Individuals in generation IV have not reached adulthood and the presence of the mutant allele has not been analyzed.

Pedigree diagram showing four generations. Generation I: male I-1 (carrier, dot) married to female I-2 (unaffected). Generation II: male II-1, female II-2 (carrier), male II-3 married to II-2, female II-4, female II-5 (carrier) married to male II-6. Generation III: male III-1, female III-2 (affected, shaded), female III-3 (affected, shaded), male III-4 (carrier, dot), male III-5, female III-6, female III-7, male III-8 (carrier, dot). Generation IV: female IV-1 and male IV-2 (children of III-7 and III-8). Pedigree of gene X. Dots indicate carriers of the mutant allele. Shaded individuals show the disorder.

Q23.1.1. Individual II-2 has inherited an imprinted allele.
Q23.1.2. Gene X is paternally imprinted.
Q23.2. Calculate the probability that individual IV-1 will be homozygous for the mutant allele. Give your answer as a decimal to two decimal places.
Q23.3. Calculate the probability that individual IV-1 will show the disorder. Give your answer as a decimal to two decimal places.
Q23.4. If the individuals III-7 and III-8 had a third child, calculate the probability that it would be a son showing the disorder. Give your answer as a decimal to two decimal places.
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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF