Multifactorial Traits Flashcards
multifactorial traits
traits that are arise from multiple genes as well as from environmental contributions
qualitative traits
the trait is either present or absent, ex. cleft lip, pyloric stenosis
quantitative traits
the trait varies in degree, ex. height, intelligence
relative risk
the reisk to an individual relative to the risk in the general population
risk = (incidence among relatives of affected)/(incidence in the population)
cleft lip +/- cleft palate
not a simple mendelian traits, recurrence risk to siblings of an affected individual is higher than predicted based on population incidence
most often unilateral (75-80%
more common ton the left side
bilateral cleft lip occurs and is associated with cleft palate in ~85% of cases
risk of CL +/- CP
incidence of siblings is higher than the population incidence
twice as common in boys than in girls
when index case is female, the recurrence risk is higher than when it is male
families with more affected individuals have a higher recurrence risk than families with one or a few affected individuals
multifactorial/threshold model
multiple genes and environmental factors contribute to an individual’s liability to the trait
to explain the dichotomy of the trait, a threshold is postulated, when one exceeds this threshold, the trait is expressed
recurrence risk in first degree relatives approximates the square root of the population incidence
with each additional affected family member, the risk of another family member expressing the trait increases
individuals with more severe phenotypes are more likely to transmit the trait as well as individuals of the less frequently affected sex
the risk of expressing the trait decreases logarithmically with the decreasing degree of relationship to the proband
recurrence for risk for a sibling expressing the trait is iften in the rang eof 3-10%
heritability
applies to quantitative traits
the fraction of total phenotypic variance that is attributable to genes
calculated from variance in MZ twins vs DZ twins
meausre the extent to which different alleles at various loci are responsible for a given trait in a population
limitations of heritability
applicable only to the population from which it is derived
close relatives also frequently share the same environment so some environmental effects are also included
should not be considered immutable since even genetically determined traits can be modified through environmental changes
mixed model
combines single gene effect with multifactorial threshold model
a major contribution from one or a few genes, a polygenic component, and an environmental component
