Linebreeding: A Mating Decision. Richard Browning Jr., PhD Tennessee State University Nashville, TN

Transcription

1 Linebreeding: A Mating Decision Richard Browning Jr., PhD Tennessee State University Nashville, TN

2 Breeders have two types of decisions to make: 1. Which bucks and does to keep and use as breeding stock? SELECTION decisions 2. Which buck(s) and does to match? MATING decisions

3 Selection Selection is aimed at altering allele frequencies in a population by picking animals with the best set of genes. As allele frequencies changes, so will genotypic frequencies.

4 Selection Through selection, we split a set of goats into two general groups: 1. Those we like and will keep as breeding stock. 2. Those we dislike and will sell as market animals.

5 Selection Selection decisions are based on some means of comparison. Recorded performance data is recommended to form the basis of selection decision. Unfortunately, looks and pedigree have taken precedence in many goat selection programs.

6 Selection We may be choosing among individual goats within a population. OR We may be choosing among different breeds.

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8 After selection, mating is designed to match up particular sires and dams to achieve the desired change in the next generation. Mating systems vary.

9 Mating Decisions for Simply-inherited Traits Simply know what genotypes (homozygous and heterozygous) produce which phenotypes. Examples: TT = wattles Tt = wattles tt = no wattles PP = no horns Pp = no horns pp = horned

10 If two wattled goats are mated: T = wattles t = smooth neck Wattled Buck T t Wattled Doe T TT Tt t Tt tt

11 Unfortunately, economically important production traits are POLYGENIC. In other words, they are affected by many genes. Phenotypic outcomes are not easy to predict for growth and reproductive traits.

12 Mating Schemes for Polygenic Traits may be based on: 1. Goat performance a. Random b. Assortative 2. Goat pedigree a. Inbreeding b. Outbreeding

13 Performance-based Strategies A. Random mating is the lack of any preconditions on sire-dam combinations. Convenient. Used in genetic evaluations (such as the TSU meat goat project). Not an inherently inferior approach because of the randomness of inheritance.

14 Performance-based Strategies B. Assortative mating is pairing of sires and dams based on being similar or dissimilar for performance/type. Requires performance assessment. Can introduce bias in genetic evaluations. Positive or negative assortative mating sire and dam similar or different.

15 Pedigree-Based Mating Systems Inbreeding Outbreeding Linebreeding Outcrossing Grading Crossbreeding Intense inbreeding Linecrossing up Species crossing

16 Outbreeding Examples include linecrossing, outcrossing, crossbreeding Crossbreeding Takes advantage of complementarity & heterosis. Crossbreeding increases heterozygosity in the population.

17 Inbreeding Examples include linebreeding and intense inbreeding Creates uniformity but causes inbreeding depression. Increases homozygosity in the population.

18 Seedstock producers often inbreed, usually linebreeding. Not concerned about complementarity or hybrid vigor. Commercial producers typically outbreed, usually crossbreeding. Seek advantages of hybrid vigor and complementarity.

19 Mating of animals more closely related than usual for the population. Intense Inbreeding Parent-offspring Sibling matings Linebreeding Mating of cousins or other moderate relatives. Mating animals related by a featured, desirous ancestor.

20 1. Increased homozygosity One or more common ancestors on BOTH sides of pedigree. Increased uniformity of performance (prepotency) sought because of reduced genotype possibilities. Environment may offset prepotency for low- to moderately-inherited traits.

21 Effects of inbreeding on heterozygosity/homozygosity (Cundiff and Gregory, 1977) H E T E R O Z Y G O S I T Y (%) Aa Homozygous line Inbred Line Sire Daughter or Full sib Pure Breed Random mated Cross breeding H O M O Z Y G O S I T Y (%) AA or aa

22 Average expected performance of crossbred, purebred, and inbred lines with additive and non additive gene effects. Frequency (%) per /genotype Partal Dom. Additive Complete Dom. AA Aa aa Crossbred Purebred Sire-dau. F= Inbred line F= Homozygous line (Cundiff, 2008)

23 2. Uncovering of Lethal Recessives Drives many producers away from inbreeding. The bad alleles are not created in the population. They are just exposed with an increased occurrence of homozygosity. Remember the simply-inherited ( onegene ) traits scenario?

24 Abnormal teeth beta-mannosidosis Cleft lip or palate Cryptorchidism G-6-Sulfase deficiency Hernia Missing mandible Prolapse Inverted eyelids Poor jaw alignment Supernumerary teats

25 3. Inbreeding Depression The somewhat hidden but significant consequence in performance traits. Results from a series of unfavorable recessive homozygous combinations. Inbreeding depression is the B-side of the same phenomenon that yields hybrid vigor in crossbreds (GCV).

26 FITNESS TRAITS are most affected by inbreeding depression and hybrid vigor. Reproduction and survival traits. These traits tend to be lowly heritable. Heritability and Inbreeding Depression/Hybrid Vigor tend to be inversely related for traits.

27 (Cassell, 1999)

28 Inbreeding depression for changes in lactation yield by dairy goat breed. Breed Milk(lb) / %inbreeding Fat(lb) / %inbreeding Protein(lb)/ %inbreeding Alpine a a a LaMancha b b bc Nubian b b b Saanen c bc ac Toggenburg a ac a (Gipson, 2002)

29 Inbreeding coefficient (F x ) is used to indicate the level of inbreeding in an animal. Probability that two alleles in a gene pair are the same by inheritance. Range is 0 to 1. F x values less than 6.25% desirable. F x values greater than 10% problematic.

30 INBREEDING COEFFICIENTS (F x ) FOR OFFSPRING OF POSSIBLE MATINGS Mating F x Parent x Kid 0.25 Full Siblings 0.25 Half Siblings Grandparent x Grandkid Uncle x Niece First Cousins

31 Determining Inbreeding Coefficients using Arrow Pedigrees and the Path Method (Northcutt et al., 2004)

32 F N = (0.5) 5 + (0.5) 4 + (0.5) 4 F N = F N = (TAVE, 1999)

33 0.0% 0.0% 0.0% 8.6% 25.0% 21.9% 12.5% Some Registries Provide F-values

34 Litter size weaned Swine (Buchanan, 1991)

35 Beef Cattle (Northcutt et al., 2004)

36 Various practices can lead to increase F-values without immediate notice. Selection strategy Positive assortative mating Assisted reproductive technologies

37 F-values in Dairy Goats (Gipson, 2002)

38 Linebreeding is a mild, controlled form of inbreeding. Allows for a herd to maintain a significant influence from a highly regarding ancestor. Slow form of inbreeding in which selection may offset inbreeding depression.

39 Why do breeders linebreed? 1. Increase herd uniformity. 2. Maintain high level of influence from an outstanding individual in the herd. 3. Create opportunities for purebred heterosis (linecrossing).

40 Many breeds of livestock are the result of linebreeding. The Kiko is not one of them. However, some Kiko producers may be linebreeding at the current time.

41 Linebreeding Some of the most notable family lines in livestock breeds are the result of sustained linebreeding. Since inbreeding depression vanishes with crossbreeding, seedstock producers can practice linebreeding.

42 Half siblings from Doe 409 mated.

43 TSU KK 2121 TSU KK 8114 TSU KK KK Fndn 409 KK Fndn 470 KK Fndn 409 KK Fndn

44 TSU SS 6015 bred back to 656 sire.

45 656 SS Fndn TSU SS 2194 TSU SS SS Fndn TSU SS 3003

46 Bucks KK 2121 and SS 2194 are examples that linebred animals are not destined to be poor performers

47 Linebreeding is most effective when used in outstanding herds and when featuring a buck or doe PROVEN to be exceptional. How is exceptional or outstanding defined?

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