Heredity is the likely cause. He should probably check the nipples of his family members.
very correct
just like asking why someone has a big nose? definitely somewhere from his/her lineage
between it might be from a great great great family memeber, and it could be from either of the parents, such things are difficult to explain esp. when the living family members don't have the feature
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WHAT DO YOU THINK IS THE CAUSE, because MY FRIEND GOT IT & HE'S WORRIED BOUT IT. 
