Genes and IQ

The Scientific American reports a new idea on what triggers Alzheimer's.

This paragraph triggered me, however:

But at start of the 21st century, researchers uncovered a harbinger for GR {gene recombination}. We discovered that DNA sequences vary from cell to cell, meaning that our brains are a vast mosaic of distinct genomes, a phenomenon aptly referred to as “genomic mosaicism.” These changes are distinct from epigenetic changes that do not directly affect DNA sequences. Scientists have now identified multiple sequence changes that are quite varied and seemingly random, consisting—in order of decreasing size—of entire chromosomes (aneuploidies), smaller copy number variations, even smaller LINE1 retrotransposon repeat elements and “single nucleotide variations that alter individual nucleotides.

How genes determine IQ becomes a bit mysterious if "our brains are a vast mosaic of distinct genomes".   One question presumably would be that are these "vast mosaic of distinct genomes in the brain" the same in some statistical sense in identical twins raised together and those raised separately?