noun
only child; precious only child
Idiomatic expression for a family's sole child, often with a nuance of being cherished or precious. Literally 'one-grain seed'.
彼女は一粒種だから、両親に大事に育てられた。
She is an only child, so she was raised with great care by her parents.
一粒種の息子が留学して、家の中が静かになった。
Their precious only son went to study abroad, and the house became quiet.