BIOL 2500 - Quantitative Genetics (Topic 9)

Polygenic traits

traits controlled by one or more gene

• assort independently, producing a large number of possible genotypes

T/F: phenotypes are quantitative

TRUE

quantitative traits

characteristics that exhibit continuous variation and are influenced by multiple genes and environmental factors, making them essential for understanding genetic diversity and evolution.

• genetic potential due to complex inheritance

eye colour - _____ genes involved, but 2 ______ genes: OCA2 and HERC 2

15, major

• OCA2 alleles influence iris colour and skin colour (how much melanin present)

• HERC2 alleles regulate teh expression of OCA2

• other 13 genes are modifier genes

assumptions/conditions of binomial expansion

• diploid, 2 alleles/gen with even allele freq.

• random mating or cross multi-locus heterozygotes

• contributing alleles only have additive effect on the phenotype

• no linkage between genes

• no interactions between genes

• no envr. effects

• discrete phenotypic categories

additive genes

traits that have a continuous phenotypic range that results from incremental contributions from multiple genes (contributing genes)

multi-gene hypothesis - Hermann Nilsson-Ehle

segregation of alleles from multiple genes (each gene segregates and independently assorts on its own) contributes to phenotypic variation

more phenotypes that occur along the phenotypic range = ______ the freq. distribution of each category

narrower

correlation

tendency of one variable to vary in proportion to another

• can be positive: as one increases, other does as well

• can be negative: as one increases, other decreases

• e.g. taller people have larger feet

correlation coefficient - r

statistical measure of correlation

• ranges from -1 to 1

familial traits

shared by family members for any reasons

heritable traits

similar in family members due to shared gene

heritability

proportion of phenotypic variation that is due to genetic factors

T/F: heritability differs from trait to trait, and in the same trait between different environments

TRUE!

high heritability means most of phenotypic variation is due to ______ _____

genetic variation

• trait can be strongly influenced by natural selection

low heritability means most of phenotypic variation is not due to ______ ______ ______

inherited genetic variation

• mostly influenced by envr. and not so much by natural selection

two kinds of heritability

1. broad sense heritability (H²)

2. narrow sense heritability (h²)

broad sense heritability (H²)

estimates the proportion of phenotypic variation that is due to total genetic variation

• H² = Vg / Vp

• ranges from 0.0 to 1.0

narrow sense heritability (h²)

estimates proportion of phenotypic variation that is due to additive genetic variation (Va)

• h² = Va / Vp

• ranges from 0.0 to 1.0

what does heritability measure

measure of degree to which genetic differences contribute to phenotypic variation of a trait

what is heritability NOT

heritability is NOT an indication of the mechanism by which genes control a trait or how much of the trait is controlled by gene action

T/F: heritability can change if envr. factors change

TRUE!

identical twins

monozygotic twins (MZ)

• single fertilization event

• followed by splitting of fertilized embryo into 2 zygotes

• share all of their alleles

• can assume that broad sense heritability is solely due to environment (Vp = Ve)

fraternal twins

dizygotic twins (DZ)

• two independent fertilization events

• produced two independent zygotes

• born at the same time, but genetically no different than normal siblings

• share an average ~50% of their alleles

• Vp = Ve + 1/2Vg

what do twin studies usually compare

MZ twins with same-sex DZ to estimate heritability

errors of heritability from twin studies

• stronger shared maternal effects in identical twins than fraternal twins (more similar developmental envr.s)

• great similarity of treatment of identical twins than of fraternal twins

• great similarity of interactions between genes and environmental factors in identical twins than in fraternal twins

concordance

% of twin pairs in which both members of the pair have the same phenotype for a trait

discordance

% of twin pairs in which both members have different phenotypes

V_G

genotypic variance of a quantitative trait

additive variance (V_A)

additive effects of all alleles contributing to a trait

• can result from incomplete dominance at a locus or with alleles of equal effect, producing heterozygotes with phenotypes intermediate to the homozygotes

dominant variance (V_D)

variance resulting from dominant relationships in which alleles of a heterozygote produce phenotype not in between the homozygotes (i.e. non-additive/non-contributing alleles)

interactive variance (V_I)

epistatic effects between alleles on different genes

what contributes directly to resemblance between parents and offspring

narrow sense heritability

• higher values of h² correlate to greater degree of response to selection - can use this for inform selective breeding programs

selection differential (S)

measures the difference between the population mean value for a trait and the mean value for a trait for the mating portion of the population

• S = mean mating - mean population

selection response (R)

depends on the extent to which the difference between the mating trait mean value and the population trait mean value can be passed to progeny

directional selection

mean phenotypic value is shifted in one direction because one extreme of the phenotype is favored

• narrows phenotypic range

stablishing selection

favors an intermediate phenotype over the extreme phenotypes

• reduces phenotypic variance without changing the mean

disruptive selection

both extreme phenotypes are favored over the intermediate

• increases phenotypic variation without changing the mean

• potential phenotypic split in the population