Lecture 7: Polygenic and Multifactorial Inheritance Flashcards
What are complex traits?
Traits that do not show clear patterns of Mendelian inheritance, but do cluster in families, indicating a genetic component. The phenotype of the disease is contributed to by multiple genes and is often influenced by environmental factors; multifactorial. Show continuous variation.
What did R.A Fisher show?
Showed that continuous variation can arise even if a trait is controlled by a number of genes (polygenes) with discrete effects.
What are additive alleles?
Additive alleles show combined effects; both dominant and recessive genotypes and phenotypes are expressed.
Describe the position of the complex trait alleles on chromosomes
Complex trait alleles can be either on the same or different chromosome. If the alleles are on the same chromosome, they are distant enough apart from each other to be independent (not linked). Each allele codes for a protein which contributes to the phenotype in an additive fashion, meaning the more of the affected alleles an individual has, the more severe the trait is (the darker the phenotype is represented).
How can gene and environmental interactions complicate the predictive Mendelian inheritance of a complex trait?
Deterministic, highly penetrant monogenic traits can be inherited in a non-Mendelian manner due to other interactions because some individual alleles may contribute very little to the phenotype, compared to others.
What is the effect of environmental factors on multifactorial inheritance?
Environmental factors interact with many genes to generate a normally distributed susceptibility curve. Individuals are affected by a disorder if they lie at the wrong end of the distribution curve, due to the presence of many of the affected alleles or particular combinations of the affected alleles.
What is the liability/threshold model?
All factors which influence the development of a multifactorial disorder, whether genetic or environmental, can be considered as a single entity known as liability. The liabilities of all individuals in a population form a continuous variable, which has a normal distribution in both the general population and in relatives of affected individuals.
What is heritability?
Heritability is a statistic used to estimate the degree of variation in phenotypic trait (VP) in a population that is due to genetic variation (VG) between individuals in that population. Measure of genetic variation in the population rather than an individual.
Heritability is estimated from the degree of resemblance between relatives expressed in the form of a correlation coefficient.
What is the equation that explains total phenotypic variation?
VP= VG + VE
What is broad sense heritability (H2)?
The proportion of variance of a phenotype that is due to genetic variation in a sample, including dominance and gene-gene interactions.
H2= VG/VP
What is narrow sense heritability (h2)?
The proportion trait variation that is due to additive genetic factors.
h2= VA/VP
What are susceptibility alleles?
Genes and alleles that predispose an individual to a particular disease. The more susceptibility alleles in the genome, the more likely an individual is to tip over the threshold and develop the disease. Individual genes can either have the susceptibility alleles or not: binomially distributed factor.
What happens when the frequency of inheriting a susceptibility allele increases?
The greater the frequency of inheriting a susceptibility allele, the greater the likelihood of inheriting more than one susceptibility allele, which therefore increases the percentage of individuals that tip over the threshold and express the disease. More likely to inherit multiple susceptibility alleles so have greater risk of developing associated disease.
