What evidence suggests that epigenetic effects may cause these phenotypic differences in monozygotic twins

Epigenetic marks (e.g., DNA methylation patterns) in older twins are more pronounced than in younger twins  

Explanation:  

Epigenetics refers to any heritable phenotypic change that does not involve alterations in the DNA sequence but impacts gene expression. Epigenetic modifications can be classified into three types: DNA methylation, histone modifications (i.e., acetylation, methylation, phosphorylation, etc) and regulatory non-coding RNA (ncRNAs) pathways. Epigenetic modifications in (epi)genome enable to explain how two individuals with identical genetic material (such as monozygotic twins) can have different phenotypes. In older twins, epigenetic differentiation will be higher than in younger ones due to the divergence between epigenomes as a result of age.

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what do the different brain waves do?

explanation:

brainwaves are produced by synchronised electrical pulses from masses of neurons communicating with each other. brainwaves are detected using sensors placed on the scalp. brainwave speed is measured in hertz (cycles per second) and they are divided into bands delineating slow, moderate, and fast waves.