# 双生子研究

## 方法

• 遗传效应 (遗传度、);
• 共同处在的环境 – 共同作用在双胞胎身上的环境因素，以同一方式影响两者;
• 各异的环境因素 – 不共同作用于双胞胎的环境因素。

ACE模型揭示了对于影响某一性状的因素来说，可遗传的多样性所占比例是多大，以及共同环境因素以及各异环境因素所占比例是多少。该研究是通过SEM系统例如OpenMx英语OpenMx来完成的，然而双生子设计的核心逻辑是不变的，如下所述：

rmz = A + C
rdz = ½A + C

A = 2 (rmzrdz)

E = 1 − rmz

C = rmzA

### 现代模型

#### 多群体和多变量模型

SEM 系统（比如OpenMx 以及其他适用于标准化数据及多群体的应用）使得这项新技术能够被有一定技能的人所使用。

#### 对环境建模: 单合子不一致设计

 “单合子不一致模型效应的例子” “MZ不一致测试的假设：运动能够保护我们免得抑郁症”

A cross-lagged longitudinal MZ discordant twin design. This model can take account of relationships among differences across traits at time one, and then examine the distinct hypotheses that increments in trait1 drive subsequent change in that trait in the future, or, importantly, in other traits.

### 假设

#### 相似性测定：双生子研究假设的直接测试

Visscher等 报道了一种针对双生子研究的强大的测试技术。他们利用了这一事实——兄弟姐妹平均有50%的基因相同,而对于特定的兄弟姐妹同胞对，相同基因数在这个数字附近,也就是说本质上是在家庭中创建一个连续的遗传相似性或“双胞胎”（填补普通同胞与双胞胎之间的空隙）。基于直接估计基因相似性来估计遗传力,为该方法的假设提供支持。

## 批评

### Physical few-character stereotypes causing behavioral correlations

Apparent correlation between heredity and behavior may be due to social stereotypes making people with certain physical characteristics (which may be hereditary physical characteristics) behave in certain ways, without any biological link between the genetic factor and the behavior. This factor has an especially high likelihood of being the case for behaviors that are culturally depicted in mass media as being more accepted by those who consider them to be biological than by those who consider them learned (e.g. sexuality and some psychiatric diagnoses) and for physical characteristics that are not culturally considered by mass media to be bigoted to associate with certain behaviors (e.g. not ethnic or racial). Such unequal environment biases may concentrate on a relatively small number of minor physical characteristics that are subject to stereotypes that are considered to be politically correct, with no statistically significant effect on overall ratings of physical similarity, let alone anywhere near being able to pass identical twins off as being fraternal.

## Terminology

### Pairwise concordance

For a group of twins, pairwise concordance is defined as C/(C+D), where C is the number of concordant pairs and D is the number of discordant pairs.

For example, a group of 10 twins have been pre-selected to have one affected member (of the pair). During the course of the study four other previously non-affected members become affected, giving a pairwise concordance of 4/(4+6) or 4/10 or 40%.

### Probandwise concordance

For a group of twins in which at least one member of each pair is affected, probandwise concordance is a measure of the proportion of twins who have the illness who have an affected twin and can be calculated with the formula of 2C/(2C+D), in which C is the number of concordant pairs and D is the number of discordant pairs.

For example, consider a group of 10 twins that have been pre-selected to have one affected member. During the course of the study, four other previously non-affected members become affected, giving a probandwise concordance of 8/(8+6) or 8/14 or 57%.

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