Coefficient of Relatedness: the proportion of "genes" that two people related in a certain way share. For example, a parent and a child have a coefficient of relatedness of 50% (or 1/2), because they share 50% of their genes. (Lewis, 138) Editor’s note - siblings share 50% of their genes. A grandparent and grandchild share 25% (1/4), an uncle and a niece share 25%, and first cousins share 12.5% (1/8). Also referred to as 'percent shared genes.'

Concordance: the proportion of genes that two related people share. Used to calculate “heritability.” Used to evaluate “multifactorial traits.” Twins who differ in a trait are said to be ‘discordant’ for it. (Lewis, 138) For a “single gene”  “dominant” or “recessive” trait among “MZ twins,” concordance approaches 100%. For a single gene trait among “DZ twins,” concordance generally is 50% for the dominant trait and 25% for the recessive trait. For a (multiple gene) trait with little “environmental” input, concordance values for MZ twins are significantly greater than for DZ twins. (Lewis, 139-140) Editor’s note - for "Alzheimer's disease,” the concordance is 78% for MZ twins and 39% for DZ twins. For “schizophrenia," the concordance is 40-50% for MZ twins and 10% for DZ twins. For acne, the concordance is 14% for MZ twins and 14% for DZ twins.

Empiric Risk: the likelihood that a hereditary trait or disorder present in one family member will occur again in other family members. This is distinguished from “recurrence risk” for cancer, which is the chance that a cancer that has been treated will recur. (NCI3) The probability that a trait will recur based on its incidence in a specific population. Expressed by and varies by relationship to the affected person. Used to evaluate multifactorial traits. Not a calculation, but a 'population statistic' based on observation. Population may be broad, such as an ethnic group, or narrow, such as families that have "cystic fibrosis.” (Lewis, 137) Also referred to as ‘recurrence risk.’ Editor’s note - an example is the empiric risk of recurrence of 'cleft lip.’ For an identical twin it is is 40%, for a sibling it is 4%, for a child 3.5%, a first cousin 0.3%, and for the general population 0.1%.

Genetic Risk: risk of developing a disease based on a patients heredity. (NCIt) The probability that an individual will be affected by a particular “genetic disorder.” Both genes and environment influence risk. An individual's risk may be higher because they inherit genes that cause or increase susceptibility to a disorder. Other individuals may be at higher risk because they live or work in an environment that promotes the development of the disorder. (NHGRI) 

Cancer Risk: the probability that exposure to some agent or substance will adversely transform cells to replicate and form a "malignant" "tumor." (NCIt)

Heritability: degree of variation in a trait that is due to genetic differences. Used to evaluate multifactorial traits. Heritability equals 1.0 for a trait whose variability is completely the result of gene action, such as in a population of laboratory mice whose environment is controlled. Variability of most traits, however, reflects a combination of differences among genes and environmental components. The distinction between ‘empiric risk’ and ‘heritability’ is that ‘empiric risk’ could result from non-genetic influences, whereas ‘heritability’ focuses on the genetic component of the variation in a trait. (Lewis, 137-138) Editor’s note - ‘height’ has a ‘heritability of 0.8.’ Therefore, 80 percent of the difference in height between individuals is determined by genetic factors, whereas 20 percent can be attributed to environmental effects, mainly nutrition. United States heritability for schizophrenia is 0.8, obesity 0.75, total serum cholesterol 0.6, body mass index 0.5, and for drug addiction 0.4 to 0.6.

Incidence: the number of new cases of a disease diagnosed each year. (NCIt) The rate at which a certain event occurs, such as the number of new cases of a disorder diagnosed per year in a population of known size. (Lewis, 137) It also is used for the rate at which new events occur in a defined population. It is differentiated from "prevalence," which refers to all cases, new or old, in the population at a given time. (MeSH)

Cancer Incidence: the number of new cancers of a specific site/type occurring in a specified population during a year. Usually expressed as the number of cancers per 100,000 population at risk. (NCI3)

Lifetime Risk: an estimate of cumulative risk calculated over a lifetime. (NCIt)

Population Risk: how often a particular variant occurs in a particular population. The percentage of people in the population who have a disorder (or who do not). (Lewis, 261) The proportion of individuals in the general population who are affected with a particular disorder or who carry a certain gene; often discussed in the genetic counseling process as a comparison to the patient's personal risk given his or her family history or other circumstances. The ratio of the number of observations in a statistical category to the total number of observations. (NCIt) Also referred to as ‘population frequency.’

Prevalence: the number of cases alive on a certain date. (NCI4) The proportion or number of individuals in a population who have a particular disorder at a specific time, such as during one year. (Lewis 137) The total number of cases of a given disease in a specified population at a designated time. It is differentiated from "incidence," which refers to the number of new cases in the population at a given time. (MeSH) The prevalence of behavioral disorders in the U.S. population includes: anxiety 8%, major depression 6%, Alzheimer’s disease 4%, drug addiction 4%, eating disorders 3%, ADHD 2%, schizophrenia 1.3%, bipolar disorder 1%, and autism spectrum disorder 0.1%. (Lewis, 151)

Cancer Prevalence: the number or percent of people alive on a certain date in a population who previously had a diagnosis of the disease. It includes new and pre-existing cases, and is a function of both past incidence and survival. Information on prevalence can be used for health planning, resource allocation, and an estimate of cancer survivorship. (NCI4)

Sex Ratio: the number of males per 100 females. (MeSH) The proportion of males to females in a human population. Calculated as the number of males divided by the number of females multiplied by 1,000. A sex ratio of equal numbers of males and females would be designated as 1,000. In the United States for the past six decades, the sex ratio averages 1,050. (Lewis, 115-116)