Figure 1.
Diploid two- and three-locus regulatory pathways with competitive transcription factor (TF) binding. (A) TF molecules and promoter sites are represented by bitstrings of arbitrary length n, and the strength of binding is proportional to the extent of their match; m represents the proportion of mismatched bits (m = h/n, where h is the Hamming distance), darkened for emphasis. In diploids, each gene copy is assigned a bitstring to each of its variable sites. (B) Two-locus pathway. TF locus A codes for the TF protein that regulates the expressed locus B. Locus A can vary at its own promoter (alleles 1A and 2A), its coding region (alleles A1 and A2), or both; locus B varies only in its promoter (1B and 2B). The dosages of each of the TF alleles ([1A] and [2A]) are determined by their promoter sequences. Subscripts are dropped for homozygotes. (C) Three-locus pathway. As in the two-locus pathway, except that locus B codes for a second TF that goes on to regulate expression of locus C.