17: Heritability and Repeatability Flashcards
What is heritability
Measures the degree to which offspring resemble their parents in performance for a trait
Highly heritable vs lowly heritable traits
High = animals with high performance tend to produce high performing offspring and vice versa
Low = performance records of parents reveal little about progeny performance
Genetic model for quantitative traits
P = u + BV + GCV + E
When is performance a good indicator of BV
With highly heritable traits
Definition of heritability with respect to the relationship between P and BV
Measure of the strength of the relationship between performance and breeding values for a trait in a population
If phenotypic value and breeding value are similar (strong, consistent relationship) then…
there is high heritability
Heritability (h^2) equation
h^2 = COV(P,BV)/Vp
Additive genetic variance/phenotypic variance
Heritability is the proportion of…
differences in performance for a trait that are attributable to differences in BV for the trait
High h^2, diff in performance…
are largely attributable to differences in BV
What is narrow-sense heritability (h^2)
Strength of the relationship between phenotypic values and breeding values for a trait in a population
Broad-sense heritability (H^2)
Strength of the relationship between phenotypic values and genotypic values for a trait in a population
H^2 equation
Broad-sense H = V(A)+V(GCV)/V(P)
Slide 16**
When is broad-sense heritability useful
For traits with low h^2, use of GCV effects through outbreeding is a viable option and H^2 assesses magnitude of total genetic contribution
Result of outbreeding is…
Gain in GCV
Why is H^2 not a particularly useful concept
GCV cannot be inherited, and H^2 does not reflect the relationship between the performance of animals and their potential as parents
Alternative definition for heritability
Amount of change in BV expected per unit change in phenotypic value
(regression of BV on P)
When h^2 is high (regression of BV on P)
There is greater change in BV per unit change in P
Range of heritability value
0 to 1, always positive
Moderately heritable values
between 0.21 and 0.40
Below = lowly, above = highly
How is animal performance improved for lowly heritable traits?
Through management, NOT selection
e.g. improve fertility by providing good nutrition
What is repeatability
Measure the strength of relationship between repeated records (repeated phenotypic values) for a trait in a population
Examples of repeated traits
Milk yield in dairy animals, racing performance in horses
Repeatability is the…
correlation between repeated records for a trait in a population
r = rP1,P2
What is producing ability
Performance potential of an individual for a repeated trait
PA is a function of…
All those factors that permanently affect an individual’s performance potential (genotypic value and permanent environmental effects)
Range of repeatability
-1 to +1
With high repeatability…
First record on an animal is, on average, a good indicator of that animals second record
Single record of performance on an animal is, on average, a good indicator of that animal’s producing ability
With low repeatability…
First record typically not good indicator of second
Single record of performance on an animal tells us very little about the animals producing ability
High r, differences in performance…
are largely attributable to differences in PA
Repeatability as a function of variability
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r indicates the strength of the relationship between…
P and PA
Alternative definition of repeatability
Amount of change in producing ability expected per unit change in phenotypic value
Regression of PA on P
When r is high (regression)
there is greater change in producing ability per unit change in phenotypic value
Repeatability depends on population-specific factors such as…
allele frequencies, gene variants, environmental factors
How does repeatability help make culling decisions?
For highly repeatable traits, cull poor producing animals based on performance
Wait for more records for lowly repeatable trait
