Quantitative Selection - ABG Exam 2

Simply Inherited Traits

These traits are qualitative and can be grouped into categories.

Controlled by only a few genes

• phenotypic selection

• test mating

• probability of detection

Differ from polygenic traits because those can be measured and are controlled by many genes

problem we run into with polygenic traits

Cannot select for them because to many genes control the trait. we want to select for the phenotype (performance) rather than the specific genotype to do this we use “breeding values”

Breeding values

an abstract, mathematical value that is highly subjective, that is identified for specific traits of importance such as weight, liter size, and milk production amount.

Individuals with the BEST breading values are selected.

Called “Quantitative genetics”

Applies to polygenic traits (including threshold genes)

Not used in simply inherited traits

Most basic genetic model

P = Mew + G + E

p - phenotypic value

mew - population mean

G - Genotypic value of individual for the trait

E - Environmental effect on the individual’s performance

things to consider when determining BV

These represent different factors affecting a single animal’s performance.

the avg value of G and E are always zero

G and E are always independent from one another.

How is BV different from genotypic value

The BV is the value of an individual as a genetic parent and is not the same as genotypic value (G)

BV only represents the part of the genotypic value that is transmittable from parent to offspring.

G = BV + something else

Genotypic value

a combination of breeding value and the effects of the genes interacting with one another in the living parent.

Breeding value

The sum of all the independent effects of all an animal’s genes on a trait

• value of an individual’s genes to its offspring

Helps us to make selection decisions when dealing with polygenic traits.

Breeding value for mature weight

is the sum of all independent effects of an animal’s genes at the B locus and at all other loci affecting mature weight.

Determining breeding value

We’re not considering the interaction between the alleles at the locus of interest Aa isn’t in between

BV in determining progeny differences (PD)

Progeny Difference is (PD)^4

PD = 1/2BV(parent)

Due to a parent passing on average half of its breeding value down to the offspring

It is impossible to predict if some halves passed down are better that the other.

Mew

average value of that population

BV (offspring)

BV = .5(BV-sire) + .5(BV-dam)

average performance of offspring

= Mew + BV(offspring)

= Mew + .5(BV-sire) + .5(BV-dam)

independent gene effects

additive, and breeding values are composed of independent gene effects making breeding values additive

Gene combination value

the value of an individual’s genotype that is due to effects of gene combinations (such as dominance) this cannot be passed to offspring

G = BV + GCV

G = genotypic value

BV = breeding value

GCV = gene combination value

GCVs are also called

Nonadditive gene effects

All genes

All genes have independent effects, but only some have non-independent effects

Updated model

P = Mew + BV + GCV + E

inbreeding depression

animals with low GCVs may have great BVs

Heterosis

Hybrids may have high GCVs but low BVs

Environmental effects

Permanent and Temporary ways of effecting an animals performance

Temporary vs Permanent

temp = weather, sleep, location

perm = training, nutrition

Producing ability (PA)

the performance potential of an individual for repeated traits

repeated traits = a trait for which there is more than one performance record for the animal

examples:

milk production

wool production

twinning

weaning wight

PA = BV + GVC + Ep

MPPA

Most Probable Producing Ability

• prediction of producing ability

Threshold traits

polygenic traits but are expressed qualitatively measured with bell curve.