(-JUL 1733)
+---Richard Trethewy (ABT 1720-)
| +---Isabel Beaubery 
(-FEB 1732)
+---Robert Trethewy (ABT 1759-)
| +---Ann Jewell
+---Robert Trethewy (ABT 1802-BEF 1861)
| +---Elizabeth Sarah
+---Richard Thomas Trethewy (ABT 1836-1908)
| | +---William Trevithick
| +---Mary Ann Trevithick (ABT 1808-1877)
| +---Elizabeth Harry
+---William Trethewy (1864-29 JUL 1927)
| +---Elizabeth Coombe (ABT 1837-14 JUN 1919)
Ida Trethewy (02 JAN 1895-30 JUL 1962)
+---Margaret Copeland (ABT 1868-07 NOV 1952)