18: Repeatability and Correlations Flashcards
Slide 11, 12
Repeatability as a function of variability
Order from largest to smallest: r, H^2, h^2
h^2 </= H^2</= r
Why is narrow sense heritability the smallest value
Least amount of factors in the numerator
slide 18
With high h^2…
With high r…
prediction of BV will be more accurate
prediction of PA will be more accurate
Higher h^2 and higher r lead to fewer mistakes in… respectively
replacement selection, culling
What is correlation
Function of covariance between two values and their standard deviations
Indicates consistency and reliability of the relationship
Range of correlation
+1 (very strong and positive) to -1 (very strong and negative)
What is phenotypic correlation
Association between two traits that can be directly observed
Strength of relationship btw phenotypic value in one trait and phenotypic value in another
What is environmental correlation
Measure of the strength of the relationship between environmental effects on one trait and environmental effects on another trait
Environmental correlation considers…
environmental deviations together with non-additive genetic deviations between two characters
What is environmental correlation used for? Example
Management purposes
e.g. environmental correlation between average daily gain and back-fat thickness in swine has been estimated at +0.40
Suggest environments conducive to rapid weight gains tend to produce fatter pigs
What is genetic correlation
Quantify the overall genetic similarity between complex traits
Measures the strength of the relationship between breeding values for one trait and breeding values for another trait
Slides 25-27
Types of correlation
Two ways genes would be gentically correlated? Describe
Pleiotropy: a single gene affects two or more different traits
Linkage: different loci influencing distinct traits are situated close together on the same chromosome, preventing independent segregation during meiosis phase of sexual reproduction
Example of pleiotropy
Two loci variants (IGF-1 and HMGA2) for small body size associated with separation anxiety, touch sensitivity, owned directed aggression and dog rivalry
Selected for size; unintentionally selected for behaviour
Why is understanding genetic correlation important? E.g.
If two traits are genetically correlated, selection for one will cause genetic change in the other
e.g. selection to improve scrotal circumference will result in reduced age at puberty (female trait)
Describe the selective breeding for tameness project in foxes
Started in 1959 by Dr. Dmitri Belyaev
Wanted to understand process of domestication by direct selection of behavioural traits on silver foxes
Would cross tame to tame and aggressive to aggressive
Saw behavioural changes in tame (approachability, tail wagging) and also morphological changes (floppy ears, curly tail, coat colour variations)
Different ways correlation between traits can be classified
Strength: strong or weak
Sign/direction: positive or negative
Favourable or unfavourable
Example of positive correlation and negative correlation
Pos: positive correlation between birth weight and calving dystocia is unfavourable since heavier calves are associated with higher calving problems
Neg: negative correlations between yearling weight and age at puberty is favourable since heavier animals at yearling will likely have lower age at puberty
Correlation varies from… E.g.
population to population
Correlation between milk production and fertility may be quite different in two different breeds of cattle
Different levels of correlation strength
None: 0-0.1
Weak: 0.1-0.3
Moderate: 0.3-0.5
Strong: 0.5+
(same in negative direction)
Additional applications of correlation
- predict genetic change in one or more traits from selection for another trait (correlated response to selection)
- provide additional source of info for the prediction of BVs
- used for construction of multi-trait selection indices
