Transcript Chapter 1: Animal Agriculture

Principles of Selecting and Mating
Farm Animals (Chapter 9)
• Genetic improvement of farm animals
–Involves selection (choosing the best to be
parents)
–Involves mating systems (combining sires and
dams to maximize efficiency)
Quantitative Inheritance
• Quantitative traits – traits that can be
measured
–Have continuous variation – any two values could
have an intermediate value
–Generally controlled by many gene pairs
• Qualitative traits – traits that can be classified
–Frequently controlled by few gene pairs
Phenotypic Variation in Quantitative
Traits
• Distribution of performance traits generally
normal (bell curve)
• Majority of values near the mean
• Fewer values far away from the mean
Frequency of Genes in a Population
• Goal of genetic improvement
–Increase frequency of desirable alleles
(form of a gene)
–Decrease frequency of undesirable alleles
Frequency of Genes in a Population
Total
Number
Genotype
Red
White
49 red
RR
98
0
42 roan
RW
42
42
9 white
WW
0
18
140
60
Total
Freq R = 140/200 = .7
Freq W = 60/200 = .3
Forces that Change Gene Frequency
• Mutation
• Migration
• Selection
• Genetic drift
Mutation
• Change in the base sequence
• Some mutations occur at regular frequency
• Mutation rate is low and regular change due to
mutations is very small
• By chance, some mutations end up making a
difference in livestock (dwarfism in beef cattle
in the 1950s)
Migration
• Importing new genes into a population
–Purchasing new sire
–Opening up breed to new animals
–Importing European breeds of cattle
• Very powerful force for changing gene
frequency
Selection
• Choosing best young animals to be parents
• Eliminating inferior parents from population
• Progress is gradual but steady
• Should select on a balance of characteristics
Genetic Drift
• Change in gene frequency due to chance
• Each sperm and egg contains random sample
of genes from parent
• Sample may be above or below average
• Some offspring better than average of parents
• Some offspring worse than average of parents
Phenotypic Variation
• Phenotype = Genotype + Environment
• Variance in phenotypes
–Due to variance in genotypes and environments
• Environmental effects
–Effects other than genetic effects
Genotype x Environment Interaction
• Differences between genotypes may not be
constant in all environments
• Example
–Brahman crosses superior to British crosses in
southern states
–British crosses superior to Brahman crosses in
northern states
Heritability
• Proportion of phenotypic variation that is due
to genetic variation
• Describes how easy to make progress through
selection
• May be any value from 0 to 1
• Usually between 0 and .60
Heritability
• Generally:
• Reproductive traits – low heritability (0-.2)
• Growth traits – moderate heritability (.2-.4)
• Carcass traits – high heritability (.4-.6)
• There are some exceptions to these
generalizations
Selection with Different
Types of Gene Action
• Effectiveness depends on whether gene action
is additive or non-additive
• Additive
–Easy to make selection improvement
–Each gene has differential effect
Selection with Different
Types of Gene Action
• Non- additive (dominance or epistasis)
–Some alleles may mask other alleles
–Some gene pairs may affect other gene pairs
–Reduces effectiveness of selection
–Selection may move toward some intermediate
gene frequencies instead of 0 or 1
Progeny Testing for Recessive Alleles
• Important to identify carriers
• Mate suspected carrier to known carriers or to
daughters
• If enough matings without affected offspring:
–Can establish low probability that individual is a
carrier
Gene Action with Heritability,
Inbreeding and Heterosis
• Additive effects large
–Heritability high, effect of inbreeding and
heterosis low
• Non-additive effects large
–Heritability low, effect in inbreeding and heterosis
high
Selection of Superior Breeding Stock
• Selection on individual performance
–If available – individual performance is single
most important piece of information
–Selection on individual performance most
effective for traits with moderate to high
heritability
Selection of Superior Breeding Stock
• Selection on performance of relatives
–Sibs, progeny, pedigree, other collateral relatives
–Useful especially for traits with low heritability
–Some traits not measured on potential parent
•carcass traits
•traits measured in only one sex (eg milk)
Predicting Selection Response
• One generation of selection
–Response = heritability x selection differential
–Selection differential = difference between those
selected to be parents and average of group
–Selection differential larger for males
• smaller proportion of young males need to be kept
Predicting Selection Response
• For several years
–Yearly selection response
= heritability x selection differential
generation interval
–Generation interval
•average length of time to replace parents
•swine 2-3 years, cattle 4-6 years
Genetic Correlation
• Selection for one trait causes genetic change in
another trait
• Caused by pleiotropy (genes that affect more
than one trait)
National Performance Programs
• Was need for uniform performance
information
• Dairy programs organized first
• Beef programs followed
• Swine and sheep programs came later
Dairy Cattle Performance Programs
• Dairy Herd Improvement Association
• Cooperative with United States Department of
Agriculture
• Standardized lactation length for measuring
milk production at 305 days
• Huge genetic increase in milk production in
last 50 years
Beef Cattle Performance Programs
• Beef Improvement Federation
• “Guidelines for Uniform Beef Improvement
Programs”
• Established standard recommendations for
measuring growth, efficiency, reproduction,
carcass traits
Swine Performance Programs
• National Swine Improvement Federation
• “Guidelines for Uniform Swine Improvement
Programs”
• Established standard recommendations for
measuring growth, efficiency, reproduction,
carcass traits
• Recommends indexes to use for selection
Sheep Performance Programs
• National Sheep Improvement Program
• Established standard recommendations for
measuring growth, efficiency, reproduction,
carcass traits
• Although slower to develop than other classes
of livestock, programs are well organized
National Genetic Evaluation
• Problem – how to make fair comparisons
between potential breeding stock raised in
different environments?
• Solution – use ties between herds that are
established because many sires are used across
several herds due to artificial insemination
National Genetic Evaluation
• Breed associations maintain large databases of
performance records for their herd
improvement programs
• Data used to compare genetic merit of animals
across entire breeds
National Genetic Evaluation
• Expected Progeny Difference (EPD)
–Measure of predicted genetic merit
–Used for comparison between animals
Bull Weaning Weight EPD
A
+40
B
+10
–Means that Bull A is expected to sire calves that
weigh 30 pounds more than the calves from Sire B
National Genetic Evaluation
• Expected Progeny Difference (EPD)
–EPD is called the PTA for dairy cattle (Predicted
Transmitting Ability)
• Dairy – conducted by USDA
• Beef – conducted by breed associations
• Swine – organized within STAGES program
(Swine Testing and Genetic Evaluation
System) directed by Purdue University
Mating Systems
• Inbreeding
• Linebreeding
• Linecrossing
• Crossbreeding
Mating Systems
• Inbreeding
–Mating of related individuals
–Increases homozygocity
–Does not cause mutations
–Does increase homozygous recessive frequency so
increases frequency that mutant genes are
expressed
Mating Systems
• Inbreeding
–Inbreeding depression
• recessive alleles tend to be inferior
• causes decline in performance due to increase in
frequency of recessive homozygotes
• most decline in reproduction and livability
Mating Systems
• Linebreeding
–Mating system that causes large relationship to one
outstanding ancestor while keeping inbreeding low
–Useful to retain genes of outstanding individual
who is not longer available for breeding purposes
–Outstanding individual must appear in pedigree
several times at least 3-4 generations back
Mating Systems
• Linecrossing
–Mating unrelated individuals within a breed
–Causes some increase in performance (less than
what is seen with crossbreeding)
Mating Systems
• Crossbreeding
–Mating of individuals from different breeds
–Benefits
• heterosis – advantage of crossbred individual compared
to the average of the component purebreds
• breed complementarity – using benefits from breeds
while hiding the flaws
Mating Systems
• Heterosis
–Individual heterosis – advantage of crossbred
offspring
–Maternal heterosis – advantage of crossbred mother
–Paternal heterosis – advantage of crossbred sire
Mating Systems
• Heterosis
–Opposite of inbreeding depression
–Results from increase in heterozygocity
–Reproduction – large advantage from heterosis
–Growth – moderate advantage from heterosis
–Carcass – little advantage from heterosis
Crossbreeding Systems
• Terminal
–Specific breed(s) of sire mated to specific breed(s)
of dam
• Rotational
–Breeds used in a regular cycle, daughters of one
breed of sire mated to next breed of sire
Crossbreeding Systems
• Terminal
–Uses maximum breed complementarity
–Uses maximum heterosis
–Must bring in replacement breeding stock
• Rotational
–Replacement females retained by system
–No breed complementarity
–Some loss of heterosis