Very soon after fertilization, parental epigenetic methylation is reversed in the DNA.
Later, tissue-specific epigenetic modifications are made to the embryonic DNA. The graph follows the degree of methylation from different sources during embryonic development.

According to the graph, what are the changes in DNA methylation during embryonic development?

A. Only the paternal DNA becomes demethylated.
B.  The maternal DNA becomes demethylated first.
C. The methylation patterns of the parents’ DNA are erased before fertilization.
D. The methylation patterns of both parents are erased after fertilization.

D

There were many comments on the G2 forms about this question. The teachers were concerned that candidates would confuse embryonic development with embryo, but this did not seem to happen. The question was too wordy and rather confusing, but the answer was obtained out of common sense. Most candidates went for the correct answer D. This was the only possible answer, as when located in a gene promoter, DNA methylation acts to repress gene transcription. DNA methylation is typically removed during zygote formation and re-established through successive cell divisions during development.