Board Thread:Wiki Discussions and Support/@comment-995176-20141123043523/@comment-25708482-20141126234720

Red hair is a recessive gene meaning both parents only had to carry the gene, not have red hair, just the gene for their offspring to have the potential to have red hair. Minato must have had someone in his family (mom, dad, grandmother, grandfather) had red hair. Thus Boruto got red hair through simple luck but totally possible.