( ( some alleles ) ( ( ( ( are dominant ) and ) ( are ( always ( ( expressed , ) ( while ( others ( ( ( are recessive ) and ) ( ( are only ) ( expressed ( when ( ( ( both alleles ) ( of ( a gene ) ) ) ( are recessive ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT some) (NNS alleles)) (VP (VP (VBP are) (ADJP (JJ dominant))) (CC and) (VP (VBP are) (VP (ADVP (RB always)) (VBN expressed) (, ,) (SBAR (IN while) (S (NP (NNS others)) (VP (VP (VBP are) (ADJP (JJ recessive))) (CC and) (VP (VBP are) (ADVP (RB only)) (VP (VBN expressed) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT both) (NNS alleles)) (PP (IN of) (NP (DT a) (NN gene)))) (VP (VBP are) (ADJP (JJ recessive))))))))))))) (. .)))
some alleles are dominant and are always expressed, while others are recessive and are only expressed when both alleles of a gene are recessive.
(ROOT (S (NP (NP (DT An) (NN allele)) (SBAR (WHNP (WDT that)) (S (VP (VBZ 's) (ADVP (RB always)) (VP (VBN expressed) (PP (IN over) (NP (DT another)))))))) (VP (VBZ is) (NP (DT a) (JJ dominant) (NN allele))) (. .)))