Topic 9 - Quantitative gentics

Polygenic traits

traits controlled by more than 1 genes, assort independently

Multifactorial (polygenic) traits

multiple factors, contribute genetic architecture of complex traits

Quantitative traits

continous phenotypic variation, described in units of measure, show continuum, homozygous are extremes, some have discrete values (meristic), genetic potential, complex inheritance

Qualitative traits (categorical)

discrete categories, discontinuous phenotypic range, simple inheritance, predictable phenotypic ratio

Qualitative traits that don’t have simple inheritance

gentic and enviromental factors, risk factors, threshold traits (have disease when threshold exceeded)

Meristic traits

quantitative traits with discrete values, falls into categories/can’t have half numbers

Genetic potential

genotype give maximum phenotypic expression, enviromental/developmental factors infulence phenotypes

Major genes

polygenic traits, contribute more to trait then others

Modifier genes

polygenic traits, contribute small effect to phenotype

Predict polygenic traits

predict if number alles/genes known, assuming 2 alles per gene and diploid use binomial expansion/Pascal triangle, p = contributing allele, q = alternative, n= number of alleles, n + 1 = # phenotypes

Condition for using binomonal expansion/Pascal triangle for predicting polygenic traits

Diploid, 2 alles per gene, even allel frequences (5050), random mating (no selection), contributing alleles additive, no linkage, no interaction, no enviromental effects, discrete categories

additive genes

incremental contributions from multiple genes, no enviromental influence, each allele own quantitative value

multi gene hypothesis

Hermann Nilsson-Ehle propsed, segregation of alleles from multiple genes, contributes to phenotypes

Gene environment interaction (GxE)

no interaction - discontinuous phenotypes, Some interaction - overlap in phenotypes, Lots of interaction - lots of phenotype overlap

frequency distribution

values on quantitative scale (graph), represent proportion of variation in sample to estimate of variation in population

Mean

average, sum of all/ population number

Mode

most common value in distribution

Median

middle value om distribution

Varaince

spread in distribution around mean

Standard Deviation

deviation from mean, in same unit as scale but squared

Vp

phenotypic variance of quantitative trait, = Vg+Ve

Vg (genes)

proportion of variation due to genotypes

Ve (environment)

proportion of variation due to enviromental factors, can control in lab/hard in nature, all variations in F1 from

Correlation

tendency of one variable to vary in proportion to another, positive (same direction) or negative (opposite direction)

Correlation coefficient - r

statistical measure of correlation, -1 to 1, 0 no connection to 1 strong connection

Familial traits

shared by family, any reason

Heritable traitss

similar in family, due to shared genes

Heritability

proportion of phenotypic variation due to genetic factors, specific to trait and environmental context, High - most variation due to genes (Vg>Ve) and strongly influenced by natural seletion, Low - most variation not inherited (Vg<Ve) and influenced by envionment, measure degree but not how, change if enviro changes (natural selection)

Broad sense heritability (H²)

estimate proportion variation due to total genetic variation (Vg/Vp), measure magnitude, don’t partition genetic variation (Vg = Va+ Vd+ Vi)

Narrow sense heritability (h²)

estimate proportion variation due to additive genetic variation (Va/Vp),estimate regression to show correlation, resemblance between parent and offspring, higher values = greater response to selection

Twin studies

broad sense heritability, compare MZ to same sex DZ to estimate heritability, prone to error, stronger maternal effect in identical (more similar enviroments), MZ have higher concordance than DZ if strong genetic influence

Identical (monozygotic) twins

single fertilisation, splitting of embryo, share all alleles, assume broad sene heritability (H²) solely enviromental (Vp = Ve)

Fraternal (dizygotic) twins

independent fertilisation, 2 zygotes, genetically same as siblings, smae ~50% of alleles(Vp = Ve + ½ Vg)

Concordance

% of twin pairs in which both have same phenotypes, MZ concordnat if 100% genetic, DZ twins ~50% concordant, equal between MZ and DZ if not solely gentic

Discordance

% of twin pairs in which both have different phenotypes

Behavioural conditions (mental health)

some genetic influence, traditional diagnosed based familiar patterns, starting to look at symptoms over familial patterns

Additive variance (Va)

additive effects, contributing alleles, produce heterozygotes with intermediate phenotypes

Dominant variance (Vd)

dominant relationship, non contributing alleles, heterozygote produce phenotype not inbetween

Interactive variance (Vi)

epistatic effects between alleles on different genes

Selection differential (S)

difference between population mean and mean of mating portion, = mean mating - mean population

Selection response (R)

extent difference between mating mean and progeny mean, + S(h²), under stable growth conditions mean progeny = population mean + R

Breeder equation

S = mean mating - mean original population, R = mean offspring - mean original population

Genetic correlation

influence response to selection, indirect response or restrict trait combination, important in evolution, important in breeding

Response to selection

M = mean phenotype of parental, Ms = Mean phenotype selected for mating, M’= mean phenotype of offspring after selection, higher heritability = higher selection response

Directional selection

mean shifted one direction, one extreme favoured

Stabilising selection

favours intermediate, reduce variation, mean maintained

Disruptive selection

both extreme favoured, increase phenotypic variation, mean maintained, potential phenotypic split