Polygenic and multifactorial traits.

Determining Heritability.

Assessing VE in plants or model animals-use genetically identical organisms, vary environment Assessing VG in plants or model animals - use genetically distinct organisms, keep environment consistent.

liability threshold model

For discontinuous polygenic traits, there is an underlying continuously variable susceptibility (displays normal distribution) There is a susceptibility/ liability threshold. Below liability threshold: trait not observed. Beyond liability threshold: trait observed; may display varying degrees of severity.

Influence on recurrence risk: For a trait that affects more boys than girls... recurrence risk s higher for family with affected girls- why?

This is because the parents have more susceptibility alleles.

Sex-specific thresholds.

Threshold traits (e.g. common diseases) often occur more frequently in one gender than the other. These traits display sex-specific thresholds. Females have to have a greater number of susceptibility alleles to display that trait.

Some polygenic traits display discontinuous variation (Threshold traits)

Threshold traits- polygenic trait with a small number (typically 2) of distinct phenotypic classes; observed frequently in common diseases. The individual will either have the disease or not have the disease. Examples: Type 11 diabetes cleft palate spina bifida coronary artery disease cancer asthma

what is a Monogenic trait?

Trait controlled by a single gene.

Multifactorial (complex) traits.

Traits in which the phenotype results both from gene action (often polygenic) and environmental influence. Examples include: Physical traits (skin, color, height) Behavioral traits common (complex) diseases.

Polygenic inheritance (Quantitative inheritance)

Transmission of a trait whose expression depends on the additive effect of several genes.

Many polygenic traits display:

continuous (quantitative) variation, rather than discontinuous (dichotomous) variation

How many genes are involved in determining kernel color? Determine genotypes.

there are 2 genes involved in determining this trait. when we look at an intermediate, we cannot determine its genotype, just when looking at the extremes can you do that.

Some polygenic traits display discontinuous variation (Meristic traits)

these are phenotypes described by whole numbers. Examples include: Number of peas in a pod Number of vertebra Number of animals in a litter.

what is a polygenic trait?

trait controlled by multiple genes.

why might we observe a threshold zone? instead of a distinct threshold?

we are not gonna see a number of susceptibility alleles required to display the trait, instead we are going to see a threshold zone. sometimes people with the susceptible alleles that fall within the category will show the trait and sometimes people with the same susceptible allele will not show the trait. this is because polygenic traits are not only determined by the genes but also by environmental factors.

Polygenic Risk Score

"score" that reflects a person's inherited susceptibility to a common chronic disease (e.g. coronary artery disease. type 11 diabetes, inflammatory bowel disease, breast cancer) Determined based upon assessment of common genetic variations associated with the trait ( identified via genome wide association studies, GWAS)

what is the relationship between number of genes involved and number of ratio parts?

4^n

Things observed in the polygenic theory

- As number of loci increases, distribution looks increasingly like a Gaussian curve. - Except for the extreme phenotypes, it is not possible to infer genotype from phenotype.

Characteristics of a common (complex) disease/ disorder.

-Non-mendelian pattern of inheritance; may observe familial aggregation (due to common genetic variants or common environmental factors) -Individual mutations associated with the trait are not completely penetrant -Environmental influences can increase or decrease risk. -trait often occurs more frequently in one gender than the other -concordance rates in monozygotic and dizygotic twins contradict mendelian proportions -often occurs more frequently in a specific ethnic group

Characteristics of monogenic trait

-Observe discontinuous (dichotomous) phenotype variation-trait present or not present. -Follow rules of mendelian inheritance. -"Rare" diseases.

Characteristics of polygenic trait

-often (but not always) observe continuous phenotype variation, -Do not follow rules of mendelain inheritance. -"common"diseases, complex diseases.

heritability in broad sense and narrow sense.

1. means that we have big contribution from genetics 2. small contribution from genetics

what is the relationship between the number of genes involved and the number of distinct phenotypes?

2n + 1

Polygenic theory

A trait that depends upon the additive action of a large number of individually small independent causes will show normal (Gaussian) distribution in the population.

how much of observed variation in a trait is due to genetic factors? environmental factors?

Heritability estimate- estimate of the portion of phenotypic variation observed in a certain population in a particular environment that is due to genetic factors. note: does not indicate how much of a trait is determined by genetic factors (how much of an individual's phenotype is determined by genotype)

some examples of polygenic traits include:

Human height Skin color (you would see a variation between the extremes of the two traits)

Primary genes display quantitative variation

OCA2-involved in pigment production in iris HERC2-regulates OCA2 expression more than 5 genes are seen, because although the genes above are primary, more genes contribute several other genes involved ( up to 16 total)

Threshold theory and recurrence risk

Polygenic threshold traits tend to run in families; affected individuals ( and relatives) have more liability (susceptibility) alleles than the general population. -parents with several affected children likely have more susceptibility alleles than parents with only affected child -Recurrence risk increases with an increasing number of previous affected children

Multiple-gene hypothesis.

many genes, each individually behaving in a Mendelian fashion, contribute to the phenotype in a cumulative or quantitative way. Each gene has one potential additive (dominant) allele and one potential non additive (recessive) allele.