Genetic Selection, which way

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Genetic Selection,(which way?)
Teosinte corn (Zea mexicana) 6-9,000 yrs ago, So.
Mexico, Cobbs 3-4 cm long
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Phenotype The appearance, behavior, or
performance with respect to a given genotype.
Quantitative phenotype is
metric/continuous. Phenotype Genotype
Environment
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What Constitutes the Term, Environment ?
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Three Contributing Components to the
Genotype Additive Genetic Effect, GA - 2
alleles, A1 and A2 A1 1. A2 .5
(arbitrary values)
A2A2 A1A2 A1A1
Genotype. Value
1.0 1.5
2.0

• Net value for genotype is dependent upon the A
  alleles present,
• The alleles at B also contribute, but could be
  a nega. value,
• Each locus that contributes to the trait, makes
  an offering to GA

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Three Contributing Components to the
Genotype Dominance Deviation Effect, GD - 2
alleles, and pg, one locus Pygmy mice,
Falconer
pg pg pg
14g 12g 6g
Mid-point 10 g
d
d 2
Thus, combine all deviations of all loci
contributing to the quantitative trait and obtain
a or - effect.
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Three Contributing Components to the
Genotype Interaction Deviation, GI - GI refers
to epistatic-like interactions, etc. GI 0
if an interaction is present between loci GI
0 if genes at two loci do not interact So,
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P G E can also be expressed/written
as P GA GD GI E
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Genetic-Environment Interaction, GE
GenotypeEnvironment Interaction occurs when the
effects of a gene depends upon a specific
environment (e.g. AA in dry environment is better
than a wet)
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In the study of polygenic traits, we are
concerned with the variation about the mean of
the (phenotypic) trait. now, P becomes VP
V variance VP VG VE
VEG VP VA VD VI VE VEG
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Now, we can determine the relative importance of
variance components in heredity vs
environment....... VG / VP expresses the
extent to which individual phenotypes are
determined by their genotypes. This is
heritability, H2 broad sense. This value is not
reliable.
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Second form of heritability, heritability in the
narrow sense , h2 Percentage of the traits
phenotypic variation associated with
heredity. h2 VA / VP
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h2 VA / VP A ratio and not an absolute,
however VA is stable and passed from one
generation to the next. Dominance and epistasis
are not transmitted with this same guarantee. h2
is a ratio and is expressed as a decimal or a
percent, 0 to 1.0 or 0 to 100
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Characteristics about Heritability Percentage of
the traits phenotypic variation associated with
heredity. 0 - .1 low .2 - .4 medium .5
gt high h2 0 all variation is due to
environment 1. variation is all
genetic and no environmental
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Examples of Heritability Estimates
Beef Cattle - calving interval .1 birth weight
.40 feedlot gain .45 efficiency of gain
.40 cancer eye susceptibility .30
Sheep - twinning .13 birth weight .30 weanling
weight .27 no. of functional nipples .24 loin
eye area .50
Swine - live pigs farrowed .15 birth weight
.14 litter size _at_ weaning .17
Others - horse race finish time
.24 spondylolisthesis in boxer dogs .42 fertility
in chickens .05
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Heritability of Dairy Cattle Traits. Trait
Heritability Trait
Heritability Milk yield 0.2-0.3
Mature weight 0.4-0.6 Milk fat
yield 0.2-0.3 Wither height
0.4-0.6 Protein yield 0.2-0.3
Heat tolerance 0.0-0.2 Total solids
yield 0.2-0.3 Conception rate
0.0-0.1 Milk fat percentage 0.5-0.6
Reprod. efficiency 0.0-0.1 Protein percentage
0.5-0.6 Calving interval
0.0-0.2 Persistency 0.3-0.5
Life span 0.1-0.3 Peak milk yield
0.2-0.4 Feed efficiency
0.3-0.4 Milking rate 0.3-0.6
Mastitis resistance 0.2-0.3 Gestation length
0.3-0.5 Overall type score
0.1-0.3 Birth weight 0.3-0.5
Dairy character 0.1-0.3
( URL
http//hammock.ifas.ufl.edu/txt/fairs/3027 )
Why are there ranges of values?
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Heritability for Clinical Chemistry
Variables in Cats (adult,
short hair, domestic)
J. Hered., (2006) 79549.
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Heritability, Broad or Narrow ?
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Factors that Affect a Heritability Estimate

• Sex
• Category of the trait
• Breed of Animal
• Age when determined
• Trait
• Environmental Conditions
• The Investigator ?
• Size of Population Measured in

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• How is the Heritability Estimate Determined ?
• Comparison of Twin Data
• - Phenotypic differences are attributable to
  environment
• Regression Analyses
• - Relationship between parent(s) and offspring
  (full and half -sib) performances
• Correlation Coefficients
• Analysis of Variance
• Published Reference Values

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Example of Published h2 Estimates
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A Procedures Manual
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b .70 h2
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(No Transcript)
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Predicting Response to Selection
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Population Mean, Xp - phenotypic mean of the
animals of interest and expressed in measurable
units. Selected Mean, Xs - phenotypic mean of
those animals choosen to be parents for the next
generation and expressed in measurable units.
Selection Differential, SD - difference between
the phenotypic means of the entire population and
its selected mean.
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Selection Differential
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Genetic Gain or Reach SD x h2
the amount that the phenotypic mean in the next
generation should change by.......... - the
change can be or -
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Calculating Genetic Gain
Gain M h2 (M1 - M) Where, Gain resulting
mean of next generation M mean of
parental population M1 mean of selected
population h2 heritability of the trait
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Factors that can influence the Genetic Gain

• Size of the population that selection is being
  applied to
• Magnitude of the Selection Differential
• SD of the sexes, if different
• Generation Interval, GI
• The average time between the birth of one
  generation and the birth of the subsequent
  generation.
• Economics of the program, justify ??

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Examples of Generation Intervals
Human 20 y Bat ? Horse 2-3
y Trout ? Beef cow 2 - 3 y Wolf
? Dairy cow 2-3 y Ostrich 3-4 y Sheep
1-2 y Coturnix quail 9w Chicken 45
w Bear ?
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Gain SD x h2 , per generation Gain SD x h2
, per year GI
How long can genetic improvement in a trait be
realized ?
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Knowing the Selection Differential, and the
response to selection, an estimate of the traits
heritability can be calculated R /SD Realized
Heritability
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Realized Heritability can also be calculated
as Gain M h2 (M1 - M) rearranged,
Gain (M1 - M)
h2
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• Maximizing Genetic Gain
• Examples

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N48, Population Mean 109.7
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Goal Improve ( ) the Mean Select those in
red, N 6, Mean of Selected 119.5 Population
Mean 109.7
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Goal Reduce the Mean Select those in blue, N
8, Mean of Selected 100.4 Population Mean
109.7
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Example

• Flies trained to avoid certain olfactory cue
• Mean of popula. 8.5 trials
• A sub popula. 6.0 trials
• Progeny of this sub population were interbred and
  their offspring 7.5 trials.
• What is the h2 for olfactory learning ?
• What if selection was different for the sexes ?