( ( In general ) ( , ( ( when ( ( ( only one ) ( of ( two alleles ) ) ) ( is ( expressed ( in ( the phenotype ) ) ) ) ) ) ( , ( ( the ( expressed allele ) ) ( ( is ( called ( the ( dominant allele ) ) ) ) . ) ) ) ) ) )
(ROOT (S (PP (IN In) (NP (NN general))) (, ,) (SBAR (WHADVP (WRB when)) (S (NP (NP (RB only) (CD one)) (PP (IN of) (NP (CD two) (NNS alleles)))) (VP (VBZ is) (VP (VBN expressed) (PP (IN in) (NP (DT the) (NN phenotype))))))) (, ,) (NP (DT the) (JJ expressed) (NN allele)) (VP (VBZ is) (VP (VBN called) (NP (DT the) (JJ dominant) (NN allele)))) (. .)))
In general, when only one of two alleles is expressed in the phenotype, the expressed allele  is called the dominant allele.
(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))) (. .)))