Gentics -Day 23

Multifactorial traits

traits determined by multiple genetic and environmental factors acting together

Continuous traits

continuous gradation from one phenotype to the next (height)

categorical traits

phenotype is determined by counting

thershold traits

only two, or a few phenotypic classes but their inheritance is determined bdiabetesy multiple genes and environment (adult onset diabets)

Mean

The average of all phenotypic values

Variance

the spread of distribution estimated by squared deviation from the mean

Standard deviation

square root of the variance

Standard deviation curves

small standard deviation suggests that most data are very close to the mean (narrow curve)

Large standard deviation suggests broad distrubution of data (broad curve)

Genotypic variance

s_g2 : Variation in phenotype causes by differences in genotype

environmental variance

s_e2 : Variation in phenotype caused by environment

Total vairance

S_p2 : combined effects of genotypic and enviormental variance

Sp2 = Sp2 + s_g2

How to produce ofspring with 0 genetic variance to measure og and oe

Cross between same inbread line or cross between two different imbreed lines

Genotype-enviorment interctions

genotype - environment interactions can change phenotypic distribution

• environmental effects on phenotype differ according to genotype

genotype by sex interaction

some genotype produces different phenotype in males and females (height)

Broad sense hertiablity

the proportion of phenotypic variation in a population due to all genetic effects combined

H²=S_g2/s_p2= s_g2 / (s_g2+s_e2)

narrow sense hertiablity

the proportion of phenotypic variation in a population due to additive affects of individual alleles

h²=s_a2/s_p2

Artifical selection

Allows for the estimation of hertiablity

M= means of starting population

m*= mean of selected breeders

m’= mean of progeny

selction differential (S): M*-m

response to selection (R)= m’-m

h²=R/S

Articifcal selction limits

Selectional limit: at which succesive generations shsow no futher imporvement can be reached becuse either all alleles have been fixed or lost or naturals elction counteracts artifical selection due to inderect harmful affects of selected traits

Correlated response: effect of selection for one trait on a non selected trait (effects another trait)

Quantitative Trait Loci (QTLs) and Genome-wide association studies (GWAS)

QTLs: Genes that affect a quantitative trait

Localized by determining associations between phenotype and alleles of polymorphic DNA markers

(typically SNPs)

ATLs identified by GWAS

Take DNa sequences ana dcomapre them with millions → can we find some correlation - QTLS

The Basics of Genome-Wide Association Studies

(GWAS)

Sample a large population consisting of unaffected and affected individuals.

• Determine genotypes at a large number (> 500,000) polymorphic markers (usually SNPs).

• Look for statistical correlations between presence of SNP alleles and the phenotype in question.

When you exame the traits, most traits are examined are affected by a large number of loci, loci typically explain only 10–20% of the total heritability of a trait.

Why the missing heritability?

• epistatic genes are not detected

• rare variants are not represented in sample populatiion

• effects of some varients are too small to be detected