Your text reveals that if one identical twin is gay, there is almost a 50 percent chance that the other identical twin will also be gay. On the other hand, if a fraternal twin is schizophrenic, there is only about a 20 percent chance the other fraternal twin will be schizophrenic. The measure of likelihood that twins will share this mental illness in common is called a(n)

Concordance rate.

Explanation:

This can be defined to be a quantitative index of statistics that is been used by researchers in determining the relative influence of nature and nurture. Naturally, concordance is said to means 'to agree,' therefore concordance rate is explained to be rate of agreement. Formally, the concordance rate is the percent of cases in which both members of a pair have a particular attribute. Also in some cases, it is been seen to have relative influence of genes and environment on a mental illness, for example, the attribute is the particular mental illness, and the pairs studied are usually twins. Also in some studies that trades in line with identical twins result when a single fertilized egg splits into two eggs, and each egg develops into an embryo.