Why are methylation values bimodal rather than trimodal?

Question

[ning]

Correct me if I'm wrong, but it seems like most methylation arrays have bimodal distributions of beta, M, or raw methylation signals. Why aren't these signals trimodal instead, corresponding to methylation on both, just one, or none of each chromosome? (At least for the autosomal ones).

Answer 1

[jared.andrews07]

Hemi-methylation is rare, and methylated sites on one allele are copied to the other by DNMT1 when a new DNA strand is synthesized during replication. See this review, especially figure 6, for more info.