Whistlejacket is the most recent of nine foals out of Adventure Seeker (Bering), who won the Listed Prix de Liancourt and was second in the Gr.3 Prix Cleopatre, and her second Group 1 winner. He is a full brother to Little Big Bear (No Nay Never), a Group 1 winner at two who took the Gr.1 Phoenix S. and now a sire at Coolmore, and a half-brother to Andrea Mantegna (Giant's Causeway), a winner who was Group 3-placed in Australia. Adventure Seeker is a half-sister to the winner Along Again (Elusive City), third in the Gr.3 Princess Margaret S.

Second dam American Adventure (Miswaki) was placed. She is a full sister to Arnaquer, a Listed winner who was twice Group 3-placed, and a half-sister to Along All (Mill Reef), winner of the Prix Greffulhe, then a Group 2, Group 1-placed and a sire. Two more half-sisters produced the sire Asolo, placed in the Gr.1 Prix Royal-Oak, and the Listed winner Juliette.

Third dam is the celebrated All Along (Targowice), a multiple Group 1 winner whose victories include the Gr.1 Prix de l'Arc de Triomphe.