Lesson 27: Genetic Analysis of Quantitative Traits Terms

discontinuous variation

a phenotype distribution containing discrete or separable categories

continuous variation

in polygenic and multifactorial traits, the observation of phenotypic distribution over a contiuous range

major genes

a gene that has substantial effect on phenotypic variation

modifier genes

a gene that modifies the effect of a major gene

additive genes

genes contributing to a polygenic trait and producing their effect by their cumulative contributions that are approx. equal for each gene

multiple-gene hypothesis

the hypothesis that alleles of multiple genes contribute to the production of certain traits

threshold traits/threshold of genetic liability

in polygenic and multifactorial inheritance, a trait with different phenotypes (affected and unaffected) that are determined by whether individual organisms are above or below a particular critical value on the phenotypic scale

genetic liability

tendency of a genome containing numerous gene or chromosome mutations to acquire more mutations

first-degree relatives

a genetic relationship between individuals who share 50% of their genes in common (parent child)

quantitative trait loci (QTLs)/quantitative trait locus

a gene contributing to the phenotypic variation of a quantitative trait

QTL mapping

a method for locating quantitative trait loci in a genome

near isogenic lines (NILs)

one of two more experimental lineages of organisms that are nearly genetically uniform

introgression lines (ILs)

line of experimental organisms in which genome segments from two or more other lines are present due to repeated back crosses between hybrids and organisms of one parental line

genome-wide association studies

association analysis performed using genetic marker genes distributed throughout the genome, designed to locate genes that may influence the variation of quantitative traits

frequency distrbution

a visual display of quantitative data

variance (s²)

a statistical measurement of the variation of sample values around the mean value

phenotypic variance (Vp)

the total variance observed for a trait

genetic variance (Vg)

in polygenic and multifactorial inheritance, the proportion of total phenotypic variance contributed by genetic variation

environmental variance (Ve)

for quantitative traits, the proportion of the total phenotypic variance contributed by differences in the environment experienced by population members

additive variance (Va)

for quantitative traits, the component of genetic variance contributed by genes having an additive effect on phenotypic variance

dominance variance (Vd)

in polygenic and multifactorial inheritance, the portion of genetic variance attributed to the dominance effects of contributing genes

inheritance variance (Vi)

portion of the total phenotypic variation that is due to genetic differences among individuals

heritability

quality of a characteristic being transmissible from parent to offspring

broad sense heritability (H²)

the proportion of total phenotypic variance that is contributed by total genetic variance

narrow sense heritability (h²)

the proportion of total phenotypic variance that is contributed by additive genetic variance

concordance

in twin studies, the observation that both twins exhibit the trait

discordance

in twin studies, the observation that the traits exhibited by the twins are different

selection differential (S)

the difference between the population mean value for a phenotype and the phenotype value of population members selected as parents for the next gen

response to selection (R)

the amount of change in the phenotype of a trait between parental and offspring generations as a result of selection on the parents

directional selection

selection that goes far right or far left

stabilizing selection

selection that favors the middle

disruptive selection

selection that favors both extremes