Apparently, the head gene is related to the penis and asshole gene. That is why you only get one of each. The arm, leg, and ears gene are also related. The amazing thing (genetically speaking) is that you have 10 finger genes, but 20 digits (including your toes). So they somehow double-up their workload. That is, apparently, sometimes mixed up with the asshole gene, which is why some people both HAVE an asshole and ARE an asshole also. That is a common genetic mis-fire.