(-JUL 1733)
+---Richard Trethewy (ABT 1720-)
| +---Isabel Beaubery 
(-FEB 1732)
+---Robert Trethewy (ABT 1759-)
| +---Ann Jewell
+---Robert Trethewy (ABT 1802-BEF 1861)
| +---Elizabeth Sarah
+---William Trethewy (ABT 1833-07 DEC 1870)
| | +---William Trevithick
| +---Mary Ann Trevithick (ABT 1808-1877)
| +---Elizabeth Harry
+---Robert Hugh Trethewy (31 JAN 1868-04 APR 1946)
| | +---Hugh Long
| +---Martha Long (04 MAY 1828-13 APR 1911)
| +---Elizabeth (Betty) Hodge
+---Robert Hugh Trethewy (17 OCT 1902-)
| | +---Theodore Turnbull
| +---Louisa Turnbull (ABT 1874-1902)
+---Alan Trethewy (30 JUN 1932-)
| +---Isabelle Nancarrow
Jennifer Trethewy (03 AUG 1960-)
+---Gwenda Caldwell