CHAPTER 7 VOCABULARY DEFINITIONS Polygenic- Traits determined by more than one gene. Multifactorial- A trait or illness determined by several genes and the environment. Quantitative trait loci- Genes that determine polygenic traits. Empiric risk- Probability that a trait will recur based upon its incidence in a population. Incidence- The number of new cases of a disease during a certain time in a particular population. Prevalence- The number of cases of a disease in a population at a particular time. Heritability- An estimate of the proportion of phenotypic variation in a group due to genes. Coefficient of relatedness- The proportion of genes that two people related in a certain way share. Concordance- A measure indicating the degree to which a trait is inherited; percentage of twin pairs in which both member express a trait.