Why is Ben not half selkie?

Well, I assumed that it was genetic - he has a 50/50 chance of becoming a selkie since he has non-selkie genes from his dad and selkie genes from his mom. He then took on his dad's traits, such as the blond hair. His sister, meanwhile, more closely follows their mother.

Of course, there can be dominant/recessive genes and all that, but I don't think the filmmakers intended this to become that scientific.

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Magic? The movie really doesn't give a reason, so perhaps selkieness is genetic, or maybe the fact that Ben was born on land (presumably) is what made him human while his sister was born in the water/ while his mother was suffering from selkie sickness or perhaps even transformed into a seal.

It's not too far fetched to have the male and female offsprings show different features. See third paragraph of http://tvtropes.org/pmwiki/pmwiki.php/Main/GenderEqualsBreed.

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Last movie watched: Song of the Sea (9/10)