<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def run_genotyper(bam_file, ref_file, dbsnp_file, config_file):
 """Perform SNP genotyping and analysis using GATK. """

    cl = ["gatk_genotyper.py", config_file, ref_file, bam_file]
    if dbsnp_file:
        cl.append(dbsnp_file)
    subprocess.check_call(cl)