读下面的古诗，完成题目。

                                                                                                                                               望岳
                                                                                                                               岱宗夫如何？齐鲁青未了。
                                                                                                                               造化钟神秀，阴阳割昏晓。
                                                                                                                               荡胸生曾云，决眦入归鸟。
                                                                                                                               会当凌绝顶，一览众山小。

低磷酸酯酶症是一种遗传病。一对夫妇均正常，他们的父母也都正常，但丈夫的妹妹和妻子的哥哥患有低磷酸酯酶症。这对夫妇生患病儿子的概率是（　　）