Lecture 13: Complex Traits
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Genetic differences in individuals in a population contribute to
heterogeneity of a phenotype in the population
Heritability =
proportion of variation of a trait in the population that is due to genetic differences between people
If h^2 is close to 0, then the trait is primarily due to
environmental differences
If h^2 close to 1, then trait is primarily due to
genetic differences
Typical values for genetic complex disease = 40% to 70% suggests
the variability is due to combination of genetics and environment
Relatives of an affected person are at
higher risk for disorder
Person affected with complex genetic disorder inherited
multiple risk alleles
Closer relatives are at highest risk d/t
share more risk alleles
Polygenic risk scores in relatives of proband
Higher
Quantify familial risk for a disease
• frequency of disease in relatives of affected proband vs frequency of disease in general population • relative risk ratio (compared to general population) • Lifetime risk (probability that a condition occurs in a person’s lifetime)
Relative risk ratio formula
likelihood of the disease in the relatives of an affected person divided by likelihood of the disease in the general population
A value of relative RR = 1 indicates
that a relative is equally likely to develop the disease (compared to any individual in the population)
A value of relative RR > 1 indicates that a
relative is more likely to develop the disease than another individual in the population.
If disease is common in population, then individual could have the disease
by chance
Siblings of proband also share
environmental factors
Increased risk to relatives may be due to
increased sharing of disease-associated genes, increased sharing of environmental factors, or both.
Closer degree of genetic relationship =
higher % alleles identical by descent more risk (more potential risk alleles shared)
Higher risk in
closer (degree) relatives
Increased relative risk for MZ twins vs
normal siblings
MZ twins vs DZ twins/siblings/parent-child expected correlations
1 vs 0.5
Liability threshold higher with
2 affected parents
Each parents likely has different
risk alleles for disease
For rare complex diseases, we need ___ risk alleles to become affected
more - more risk alleles to be inherited by family members
more risk alleles means higher in
family