Threshold II:
Heritability and the Threshold Model
This exercise applies the principles of heritability to the threshold model. You should do the exercises on heritability before you do this exercise. To understand the Applet, let's begin with a simple example. Set the heritability to .50, the threshold to 1.0, and N (the number of observations) to its maximum value of 5000. Press Submit.
A large number of dots will appear on the screen, and just as in the exercise Heritability I, each dot represents an observation or person. The dots plot the phenotypic value of a person as a function of the genotypic value of the person using the equation
.
Here, P is the phenotypic value, G is the genotypic value, and E is the environmental value for a person. The quantity h is the square root of heritability and e is the square root of environmentability. Because we set heritability to .50, the value of h is the square root of .50 or .7071. Because environmentability is 1.0  heritability, the value for e also works out to be .7071.
Notice the horizontal red line that crosses the axis where the phenotypic value is 1.0. This line is the threshold. All the dots above this line are affected and all the dots below the line are unaffected.
There are four colors to the dots:
Exercises:
Lifetime prevalence for several forms of psychopathology (from the Epidemiological Catchment Area study). 




Women 
Men 


Prevalence 
Threshold 
Prevalence 
Threshold 
Depression 
10.2 
1.27 
5.2 
1.63 
Panic Disorder 
2.1 
2.03 
1.0 
2.33 
Phobic Disorder 
17.4 
.94 
10.4 
1.26 
Antisocial Personality 
1.0 
2.33 
4.5 
1.70 
Alcohol Abuse/Dependence 
4.6 
1.69 
23.8 
.71 
Drug Abuse/Dependence 
4.8 
1.66 
7.7 
1.42 