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

Total 140 60

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 animalsBull Weaning Weight EPD

A +40B +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