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    【题目】果蝇红眼对白眼为显性，控制该性状的基因位于X染色体上。果蝇缺失1条Ⅳ号染色体仍能正常生存和繁殖，缺失2条则胚胎致死。一对都缺失1条Ⅳ号染色体的红眼果蝇杂交（亲本雌果蝇为杂合子），则F1中（ ）

    A．缺失1条Ⅳ号染色体的红眼果蝇占3/8

    B．染色体数正常的白眼果蝇占1/4

    C．缺失1条Ⅳ号染色体的白眼果蝇占1/4

    D．染色体数正常的红眼果蝇占1/4

    【答案】D

    【解析】亲本缺失染色体组成及基因型表示为ⅣOXBXb、ⅣOXBY，由于缺失2条染色体致死，则F1中缺失1条Ⅳ号染色体的概率为2/3，红眼果蝇概率为3/4，则A选项为1/2；B选项为（1/3）×（1/4）=1/12；C选项为（2/3）×（1/4）=1/6；D选项为（1/3）×（3/4）=1/4。

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