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【单选题】
下列有关基因的叙述，正确的有 ①基因是具有遗传效应的DNA片段，全部位于染色体上 ②自然选择使基因发生定向变异 ③某基因的频率即该基因在种群个体总数中所占的比例 ④每一个基因都包含特定的碱基序列，具有特异性 ⑤人体内的肝脏细胞和成熟红细胞所含的基因相同 [ ]A.
①②④
B.
②③⑤
C.
只有④
D.
只有③
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