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incidence rate

number of new cases of a disease reported during a specific period (typically 1 year) divided by the number of individuals in the population

prevalence rate

proportion of the population affected by a disease at a specific point in time. This is determined by both the incidence rate and the length of the survival period in affected individuals

relative risk

incidence rate of a disease among individuals exposed to a risk factor divided by the incidence rate of a disease among individuals NOT exposed to a risk factor

polygenic trait

variation in traits caused by the combined effects of multiple genes

multifactorial trait

variation in traits caused by genetic and environmental or lifestyle factors

quantitative traits

traits that are measured on a continuous numeric scale are multifactorial (blood pressure)

liability distribution

those individuals on the low end of the curve have little chance of developing the disease. Individuals who are closed to the high end have more of the disease-causing genes and environmental factors and are at greater risk.

threshold of liability

the disease is either present or absent. Below-disease is absent. Above-disease is present.

multifactorial inheritance

threshold model, liability distribution, threshold of liability

recurrence risks

these can change substantially because gene frequencies as well as environment and lifestyle factors can differ among populations


if more than one family member is affected


if the expression of the disease in the proband is more severe


if the proband is of the less commonly affected sex


recurrence risk in more remotely related relatives





nature and nurture

family members share genes and a common environment. Few traits are influenced only by genes or only by environment or lifestyle factors. Most are influenced by both.


embryo divides to form two identical embryos. Natural clones


double ovulation followed by fertilization of both eggs by a different sperm. It is possible to have two different fathers.

concordant trait

both members of a twin pair share a trait (always in monozygotic twins)

discordant trait

a twin pair does not share a trait


children born to parents who have a disease but are then subsequently adopted by parents lacking the disease are studied for the recurrence of the disease.

congenital malformations

these diseases are presents at birth or shortly after birth (2% of newborns have these). Mother having healthy pregnancy can help prevent some defects.

congenital diseases

most are multifactorial: cleft lip, clubfoot, heart defects, hydrocephaly, neural tube defects, pyloric stenosis

coronary heart disease

can be caused by atherosclerosis. Risk increases if there are more AFFECTED relatives, affected relatives are FEMALE rather than male, Age of onset is younger than 55, Autosomal dominant familial hypercholesterolemia, high fat diet, lack of exercise, smoking and obesity

familial hypercholesterolemia

autosomal dominant: 1 in 500 is heterozygous for the FH gene, 1 in 1 million is homozygous for the trait. Serum 300-400mg/dL in heterozygote, 600-1200mg/dL in homozygote. Deposits in arteries and skin (xanthomas) Caused by a reduction in the number of LDL receptors on cell surfaces


risk factor for heart disease, stroke, and kidney disease. Studies show that 20% to 40% of blood pressure variations are genetic. This means the 60-80% are environmental. Causes:SODIUM INTAKE, lack of exercise, stress, obesity, smoking and high fat intake

breast cancer

affects 12% of American women who live to be 85. If a woman has a first degree relative with this, her risk doubles. Recurrence risk increases if the age of onset in the affected relative is early and the cancer is bilateral.

breast cancer

an autosomal dominant form of this (5%) has been linked to chromosomes 17 (BRCA1) and 13 (BRCA2). Women with this gene have a 50-80% risk of developing cancer. Other genes are implicated. Can test for BRCA genes to know risk.

colorectal cancer

1 in 20 Americans will develop this cancer. Second only to lung cancer. Risk factors: genetics (the risk of this in people with one affected first-degree relative is two to three times higher than the general population). High fat and low fiber diet are contributors.

diabetes mellitus

leading cause of blindness, heart disease, and kidney failure. Two major types: Type 1:(insulin-dependent) Type 2:(non-insulin-dependent)

Type 1 DM

autoimmune destruction of insulin-producing beta cells in the pancreas. T cell activation and autoantibody production. Onset before 40 years of age. High incidence with offspring of fathers affected with this disease. Recurrence risk: identical twins of individuals with this type range from 30-50%. Genetics are not the only thing responsible. The association of specific HLA class II alleles and type I demonstrates this to be autoimmune. Sibling recurrence risk 1-6%

Type 2 DM

80-90% of all cases of this. Mostly >40 yo. Neither HLA nor autoantibodies are commonly seen in this type. Pt has insulin resistance or diminished insulin production. Risk factors: high carb diet and obesity. Recurrence: MZ twin concordance .90 and sibling risk is 10-15%


a substantial risk factor for heart disease. stroke and type 2 diabetes. Adoptive studies: body weights of adopted individuals correlated significantly with their natural parents body weights. Twin studies: higher concordance in MZ than DZ twins. BMI>30.

alzheimer disease

progressive dementia and loss of memory. Formation of amyloid plaques and neurofibrillary tangles in the brain. Risk of developing this doubles in individuals who have an affected first-degree relative. Mutations in any of these thee genes that affect amyloid-beta deposition: Presentin 1 (PS1) Presentin 2 (PS2) Amyloid-beta precursor protein gene (APP)


risk is 3-5 times higher in individuals with a parent with this problem. Adoption studies offspring of parents without this, when reared by parents with this problem, did NOT have an increased risk. Twin Concordance rates MZ>60%, DZ<30%


severe emotional disorder characterized by delusions, hallucinations, and bizarre, withdrawn, and inappropriate behavior. Recurrence risk among offspring of one affected parents is 10x higher than the general population. Twin and adoptive studies indicate that genetic factors are likely to be involved.

Bipolar affective disorder

form of psychosis with extreme mood swings and emotional instability. Incidence of this disorder is 0.5% but rises to 5-10% among those with an affected first-degree relative. genetics, minimal environmental influence

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