Now consider a genome consisting of genes A and B, meaning three different transcription profiles-A is transcribed, B is transcribed, A and B are transcribed-requiring three different TFs {assuming you activate only one at a time}. Three genes, seven transcription profiles: A, B, C, A + B, A + C, B + C, A + B + C. Seven different TFs. Four genes, fifteen profiles. Five genes, thirty-one profiles.fn7 As the number of genes in a genome increases, the number of possible expression profiles increases exponentially. As does the number of TFs needed to produce those profiles.
( Robert M. Sapolsky )
[ Behave: The Biology of Humans ]
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