Heritability

What are traits influenced by?

By genetic and environmental factors

Therefore what is the equation for phenotype?

Phenotype = Genotype + Environment

(P) = (G) + (E)

What makes up genotype and give examples?

Genotype = Additive effects + Non-additive effects

• Additive effects = Individual effects/genes that add up (predictable and heritable so passed down to generations)

• Non-additive effects = Genetic interactions that don’t simply add up

  • Dominance effects (interaction between SAME locus)

  • Epistasis effects (interaction between DIFFERENT locus)

So what is the full equation for phenotype?

P = A + D + I + E

• Genotype =

  • A = Additive gene effect

  • Non-additive gene effect =

    • D = Dominance

    • I = Interaction/Epistasis

• E = Environment

Heritability

• What

• Based on

• Symbol

• Types

• What: Measures how much of the variation in a trait (weight, height) is due to genetics rather than environment

  • Degree of resemblance between relatives for a trait in a population

• Based on: Breeding values or additive gene effects

• Symbol: h2

• Types:

  • Narrow sense heritability #00be3b

  • Board sense heritability #0084ff

Heritability: Narrow Sense Heritability #00be3b

• What

• Formula

• Tells us

• Used in

• What does it mean if

  • h² is high

  • h² is low

What: Proportion of phenotypic variance (V_P) that is due to additive genetic variance (V_A)

Formula: h² = V_A / V_P

Tells us:

• How much of a trait’s variation is passed from parents —> offspring

• How much of variation in animal’s performance is caused by breeding values (additive genetic differences)

Used in: Breeding and selection

• Because only additive effects (V_A) are predictable and inherited

What does it mean if:

• h² is high: Most trait variation is due to additive genetics (what you see = what’s inherited)

• h² is low: Most trait variation is due to environment or non-additive genes

Heritability: Broad Sense Heritability #0084ff

• What

• Includes

• Formula

• Shows how

• Used in

What: The proportion of phenotypic variance (V_P) that is due to total genetic variance (V_G)

Includes:

• Additive (V_A)

• Dominance (V_D)

• Epitasis/Interaction (V_I)

Formula: H² = (V_G) / (V_P)

• H² = (V_A) + (V_D) + (V_I) / (V_P)

Shows how: Genetically influenced a trait is (but not all can be inherited predictably)

Used in: Research

Heritability Estimates

• What is the value of low heritability

  • Means

  • How can improve herd

• What is the value of moderate heritability

  • Means

• What is the value of high heritability

  • Means

  • How can improve herd

What is the value of low heritability: < 0.2

• Means: Trait is mostly influenced by environment so selection is slow and difficult

• How can improve herd: Through improving management/environment

What is the value of moderate heritability: 0.2 - 0.4

• Means: Genetics have moderate influence

What is the value of high heritability: > 0.4

• Means: Trait is strongly controlled by genetics so selection is very effective

• How can improve herd: By selecting the best animals

What is the slope on the graph is all phenotype is heritable

Regression slope = 1

h² = 1

What is the slope on the graph when one half of the phenotype is heritable

Regression slope = 0.5

h² = 0.5

What are 2 ways to estimate heritability?

• Resemblance between relatives

  • By using information on relatives

• Realised heritability

  • From selection carried on individuals and observe the response to selection

What are the 3 classes of relatives?

• Ancestral relatives (parent and offspring)

• Collateral (siblings only; full sibs and half sibs)

• Both (pedigree; both ancestral and collateral)

Covariance

• What

• Why do relatives resemble each other

What: Between relatives is the extent the phenotype of pair of relatives or siblings vary together

Why do relatives resemble each other: Because they have more genes in common and share same environment

What does interclass (between-class) covariance mean?

Comparing values between generations

What does intraclass (within-class) covariance mean?

Looking within a group or class

• Such as comparing siblings within the same family

What is the regression value for

• Mid-parent & offspring

• Parent & offspring

• Half-siblings

• Mid-parent & offspring

  • Regression value: h²

  • Why: When the average of both parents is compared to the offspring’s trait then get direct estimate of h²

• Parent & offspring

  • Regression value: ½ h²

  • Why: If only one parent (sire or dam) is used, estimates only gives you half the heritability

• Half-siblings

  • Regression value: ¼ h²

  • Why: Half siblings only share 25% of their genes on average

What are the 4 ways to improve heritability estimates

• Make environment as uniform as possible

• Have accurate measurement to minimize error

• Adjust for known environmental effects

• Use contemporary group (compare animals born within the same season and coming from the same herd)