Adding to what Kalimac said, when the common family carries a recessive trait for a gene for a disease like hemophilia, cystic fibrosis, etc., then it is a problem for a descendant of many cousin-cousin unions, because the likelihood that they will inherit both recessive genes increases, with more cousin-cousin unions.
(To make this post on-topic: [img]smilies/biggrin.gif[/img])
Drogo and Primula wouldn't have had a problem because, to my knowledge, there is no record of any sort of genetic disease among Hobbits. Ergo, they could marry their cousins all they want, with some inherent risks of birth defects and suchlike. Those would be the only thing that they would have to worry about, if the physiology of the Hobbits was generally the same as that of Humans.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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