Board Thread:Lore Discussion/@comment-26861588-20150814174339/@comment-13446185-20150815011642

Very few realize that being Dragonborn is not a simple matter of heredity - being the blessing of Akatosh Himself, it is beyond our understanding exactly how and why it is bestowed

-The Book of the Dragonborn

The Dragonblood is not hereditary it is a blessing from Akatosh.