primogeniture
type of inheritance or succession in which the first-born qualifying child inherits the entire or main estate
en-US/ˌpɹaɪməˈd͡ʒɛnɪt͡ʃɚ/
en-GB/ˈpɹɪməʊd͡ʒˌnɪt͡ʃə/
type of inheritance or succession in which the first-born qualifying child inherits the entire or main estate