( ( ( The head ) ( of ( the ( mature sperm ) ) ) ) ( ( ( consists mainly ) ( of ( ( ( the nucleus ) , ) ( which ( carries ( copies ( of ( ( the ( father 's ) ) chromosomes ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN head)) (PP (IN of) (NP (DT the) (JJ mature) (NN sperm)))) (VP (VBZ consists) (ADVP (RB mainly)) (PP (IN of) (NP (NP (DT the) (NN nucleus)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ carries) (NP (NP (NNS copies)) (PP (IN of) (NP (NP (DT the) (NN father) (POS 's)) (NNS chromosomes)))))))))) (. .)))
The head of the mature sperm consists mainly of the nucleus,  which carries copies of the father's chromosomes.
(ROOT (S (NP (NP (DT The) (NN head)) (PP (IN of) (NP (DT the) (NN sperm)))) (VP (VBZ contains) (NP (DT the) (NN nucleus))) (. .)))
The head of the sperm contains the nucleus.