Both to the genealogy
yachas  (yaw-khas')
to sprout; to enroll by pedigree -- (number after, number throughout the) genealogy (to be reckoned), be reckoned by genealogies.
of the priests
kohen  (ko-hane')
literally, one officiating, a priest; also (by courtesy) an acting priest (although a layman) -- chief ruler, own, priest, prince, principal officer.
by the house
bayith  (bah'-yith)
a house (in the greatest variation of applications, especially family, etc.)
of their fathers
'ab  (awb)
father, in a literal and immediate, or figurative and remote application) -- chief, (fore-)father(-less), patrimony, principal. Compare names in Abi-.
and the Levites
Leviyiy  (lay-vee-ee')
a Levite or descendant of Levi -- Leviite.
from twenty
`esriym  (es-reem')
twenty; also (ordinal) twentieth -- (six-)score, twenty(-ieth).
shaneh  (shaw-neh')
a year (as a revolution of time) -- + whole age, long, + old, year(-ly).
ben  (bane)
a son (as a builder of the family name), in the widest sense (of literal and figurative relationship, including grandson, subject, nation, quality or condition, etc.
and upward
ma`al  (mah'al)
the upper part, used only adverbially with prefix upward, above, overhead, from the top, etc. -- above, exceeding(-ly), forward, on (very) high, over, up(-on, -ward), very.
in their charges
mishmereth  (mish-mer'-reth)
watch, i.e. the act (custody), or (concr.) the sentry, the post; obj. preservation, or (concr.) safe; fig. observance, i.e. (abstr.) duty or (obj.) a usage or party -- charge, keep, or to be kept, office, ordinace, safeguard, ward, watch.
by their courses
machaloqeth  (makh-al-o'-keth)
a section (of Levites, people or soldiers) -- company, course, division, portion.