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25 Cards in this Set

  • Front
  • Back
Phenotypes are the result of shared genetic AND environmental factors
Multifactorial Inheritance
How do we determine the relative importance of genes vs. the environment in disease etiology?
Monozygotic (identical): share 100%

Dizygotic (fraternal): share 50%
How often two individuals share the same trait/condition?
Concordance rates

"Not shared"

This measures the proportion of the total variance of a trait that is caused by genes.
What does heritability equal to?
Determined by the difference between concordance rates of MZ and DZ twins

H=2 (Cmz-Cdz)
The more significant the difference....what happens to H?

If concordance rates are similar
Higher the heritability

Heritability is low
Traits mostly determined by genes will have a heritability close to?
Does high concordance rate always equal "highly genetic?"
No, for example Measles have a high concordance rates for MZ and DZ twins, but heritability is low.
What does heritability not tell you anything about?
1. How many genes could be involved.

2. How much each gene contributes to phenotype

3. Mode of inheritance
What are some difficulties with twin studies?
1. The environment for MZ twins may be more similar than for DZ twins.

2. Post-fertilization somatic mutations may make MZ twins not entirely identical.

3. In-utero environment may differ among pairs of MZ twins.
What are the benefits of adoption studies?
1. Allow you to evaluate the effect on environment among individuals with shared genes.

2. Compare adopted offspring of affected parents to adopted offspring of unaffected parents.
What are the difficulties of adoption studies?
1. Effect of in utero environment (not completely separate environments)

2. May be placed in a home with similar environment.
Are quantitative traits multifactorial?

How are they represented in a graph?
Yes in terms of an individual's risk to develop the disease.

A bell shape curve
Does the disease follow a bell-shaped curve?
No, the disease is typically either "present" or "absent" and does not follow a bell-shaped curve.
The disease only manifests itself if a certain number of the predisposing factors are present
Exceeding the threshold of liability.
The risk that a relative will develop the same disease
Recurrence risk
The recurrence risk in a family will increase if
1. The affected family member is a close relative

2. more family members are affected

3. Disease has a more severe expression within that family

4. The affected individual is a member of the less commonly affected gender.
What is the effect of gender-based differences in thresholds?
If the affected individual belongs to the less commonly affected gender (higher threshold of liability), the recurrence risk is higher than if the affected individual belongs to theh more commonly affected gender (lower threshold)
If the more commonly affected gender is affected, what happens to the threshold and recurrence risk?
threshold: lower

Recurrence risk: lower overall

same gender has highest risk
Less commonly affected gender is affected, what happens to the threshold and recurrence risk?
threshold: higher

recurrence risk: higher overall

Opposite gender has highest risk
What percent of births have congenital malformations?
2-3% of all births
Environmental factors that can cause congenital malformations
Examples of multifactorial conditions
1. Neural tube defects

2. Coronary artery disease

3. thrombosis

4. Factor V Leiden

5. Cancer

6. Diabetes

7. Psychiatric disorders
-Bipolar affective disorder
What is the impact of multifactorial conditions?
1. Mendelian disorders often have the largest impact on specific families (less common)

2. Multifactorial disorders have largest impact on population as a whole (more common)