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Principles of Selecting and Mating Farm Animals (Chapter 9)
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  1. 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)

  2. 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

  3. 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

  4. Frequency of Genes in a Population • Goal of genetic improvement • Increase frequency of desirable alleles (form of a gene) • Decrease frequency of undesirable alleles

  5. Total Number Genotype Red White 49 red RR 98 0 42 roan RW 42 42 9 white WW 018 Total 140 60 Freq R = 140/200 = .7 Freq W = 60/200 = .3 Frequency of Genes in a Population

  6. Forces that Change Gene Frequency • Mutation • Migration • Selection • Genetic drift

  7. 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)

  8. 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

  9. 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

  10. 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

  11. Phenotypic Variation • Phenotype = Genotype + Environment • Variance in phenotypes • Due to variance in genotypes and environments • Environmental effects • Effects other than genetic effects

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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)

  21. 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

  22. 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

  23. Genetic Correlation • Selection for one trait causes genetic change in another trait • Caused by pleiotropy (genes that affect more than one trait)

  24. National Performance Programs • Was need for uniform performance information • Dairy programs organized first • Beef programs followed • Swine and sheep programs came later

  25. 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

  26. Beef Cattle Performance Programs • Beef Improvement Federation • “Guidelines for Uniform Beef Improvement Programs” • Established standard recommendations for measuring growth, efficiency, reproduction, carcass traits

  27. 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

  28. 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

  29. 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

  30. 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

  31. National Genetic Evaluation • Expected Progeny Difference (EPD) • Measure of predicted genetic merit • Used for comparison between animals • BullWeaning 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

  32. 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

  33. Mating Systems • Inbreeding • Linebreeding • Linecrossing • Crossbreeding

  34. 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

  35. 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

  36. 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

  37. Mating Systems • Linecrossing • Mating unrelated individuals within a breed • Causes some increase in performance (less than what is seen with crossbreeding)

  38. 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

  39. Mating Systems • Heterosis • Individual heterosis – advantage of crossbred offspring • Maternal heterosis – advantage of crossbred mother • Paternal heterosis – advantage of crossbred sire

  40. 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

  41. 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

  42. 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 Crossbreeding Systems