GenotypePhenotypeComponent converts a raw genotype (allele 1 + allele 2) into the clearance-scaling factor stored in the associated phenotype template.
No equations are introduced—only an algebraic assignment—so you can drop it anywhere in a block diagram without changing the solver dynamics.
To use a different gene, just pick a different concrete record in the
  parameter dialog (e.g. SLC22A2Genotype, CYP2C19Genotype). And inside the Genotype change alleles or leave default ones