&quot;It may seem strange that we inherited disease-causing genes at all. After all, only beneficial genes should survive, so surely over hundreds of generations we should have shaken off these unwanted contributions?&quot;<p>Certainly not my field, but my understanding is that unless a mutation has a direct impact on survival and fertility... then there is no evolutionary drive for it to go away.<p>All animals have negative genes that just stick around as random &quot;junk&quot; which has accumulated over the years. It&#x27;s not surprising that we have picked them up too.