( ( The ( X ( and ( Y chromosomes ) ) ) ) ( ( determines sex ) . ) )
(ROOT (S (NP (DT The) (NNP X) (CC and) (NNP Y) (NNS chromosomes)) (VP (VBZ determines) (NP (NN sex))) (. .)))
The X and Y chromosomes determines sex.
(ROOT (S (NP (NNP Mammalian) (NN sex) (NN determination)) (VP (VBZ is) (ADJP (VBN determined) (RB genetically) (PP (IN by) (NP (NP (DT the) (NN presence)) (PP (IN of) (NP (NP (NNS chromosomes)) (VP (VBN identified) (PP (IN by) (NP (NP (DT the) (NNS letters)) (UCP (NP (SYM x)) (CC and) (ADJP (JJ y.))))))))))))))
Mammalian sex determination is determined genetically by the presence of chromosomes identified by the letters x and y.