病率就可以得出阈值距离均数有几个标准差,这只要查阅正态分布表 即可(表6-2)。易患性正态分布曲线右侧尾部的面积代表发病率。例 如,冠心病的群体患病率为2.3%~25%,其阈值与易患性平均值距 离约2δ;而先天性畸形足的群体患病率仅为0.13%,其阈值与易患性 平均值距离约3δ。 可见,一种多基因病的易患性的平均值与阈值越近,表明易患性 高阈值低,群体患病率高;相反,易患性的平均值与阈值越远,表明 易患性低阈值高,群体患病率低(图6-6)。 图66易患性的平均值和阈值距离与患病率关系 Characteristics of Multifactorial Inheritance 1. Although the disorder is obviously familial, there is no distinctive pattern of inheritance within a single family 2.The risk to first-degree relatives, determined from family studies, is approximately the square root of the population risk As a consequence, the lower the population incidence, the greater the relative increase in risk for first-degree relatives. However only rather large differences in population frequency make an appreciable difference to recurrencerisk 3.The risk is sharp ly lower for second-degree than for first-degree relatives, but it declines less rapidly for more remote relatives This characteristic distinguishes multifactorial inheritance from autosomal dominant inheritance, in which the risk drops by half with each step of more distant relationship. It is also different from the autosomal recessive pattern, in which virtually no relatives other than sibs are at risk 4.The recurrence risk is higher when more than one family member is6 病率就可以得出阈值距离均数有几个标准差，这只要查阅正态分布表 即可（表 6-2）。易患性正态分布曲线右侧尾部的面积代表发病率。例 如，冠心病的群体患病率为 2.3％～2.5%，其阈值与易患性平均值距 离约 2；而先天性畸形足的群体患病率仅为 0.13％，其阈值与易患性 平均值距离约 3。 可见，一种多基因病的易患性的平均值与阈值越近，表明易患性 高阈值低，群体患病率高；相反，易患性的平均值与阈值越远，表明 易患性低阈值高，群体患病率低（图 6-6）。 图 6-6 易患性的平均值和阈值距离与患病率关系 Characteristics of Multifactorial Inheritance 1.Although the disorder is obviously familial, there is no distinctive pattern of inheritance within a single family. 2.The risk to first-degree relatives, determined from family studies, is approximately the square root of the population risk. As a consequence, the lower the population incidence, the greater the relative increase in risk for first-degree relatives. However, only rather large differences in population frequency make an appreciable difference to recurrence risk. 3.The risk is sharply lower for second-degree than for first-degree relatives, but it declines less rapidly for more remote relatives. This characteristic distinguishes multifactorial inheritance from autosomal dominant inheritance, in which the risk drops by half with each step of more distant relationship. It is also different from the autosomal recessive pattern, in which virtually no relatives other than sibs are at risk. 4.The recurrence risk is higher when more than one family member is