- - AGRICULTURE CORE CURRICULUM - - (CLF3000) Advanced Core Cluster: ANIMAL SCIENCE (CLF3250) Unit Title: LIVESTOCK BREEDING AND GENETICS ____________________________________________________________________________ (CLF3254) Topic: BREEDING SYSTEMS Time Year(s) AND SITUATIONS 2 hours 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (D-6) - Identify an advantage and a disadvantage of the following breeding systems and give a situation when each could be used: inbreeding (close breeding), linebreeding, outcrossing, and crossbreeding. (D-7) - Define hybrid using the cross between a horse and a donkey as an example and explain the genetic effects that make the offspring sterile. (D-9) - Define prepotency as it relates to genetics and name a famous sire that possessed these characteristics. Special Materials and Equipment: Pedigrees of animals showing cases of linebreeding, inbreeding, and outcrossing. References: California Vocational Agriculture Curriculum Guidelines: ANIMAL PRODUCTION, "Livestock Breeding and Genetics". Ensminger, M.E. (1983). THE STOCKMAN'S HANDBOOK (6th ed.). Roediger, Roger D. (1977). LIVESTOCK BREEDING. Evaluation: Quiz by instructor. TOPIC PRESENTATION: BREEDING SYSTEMS AND SITUATIONS A. Breeding Systems 1. Inbreeding (Close breeding) - Definition: The mating of animals more closely related than the average of the population from which they came. a. Advantages: 1) It generates more pairs of pure genes (homozygous). 2) It allows hidden, undesirable recessive traits to show up and be eliminated. A good example is dwarfism in cattle. b. Disadvantages: 1) It tends to decrease the traits of reproductive efficiency and vigor of young stock. 2) Since traits will tend to be made pure homozygous, rigid culling is necessary to remove poor traits. This requires large numbers of animals, care recordkeeping, and extra expense. 3) It requires great skill and considerable training to continually make correct matings. c. Situations: 1) When we want to increase the proportion of the type of the parent animals. 2) It is used in the laboratory to make animals genetically identical. 3) When combined with selection, we obtain animals with more homozygous traits making the desired phenotypic results more predicable (e.g., coat color, size, etc.). 2. Linebreeding - Definition: The mating of distantly related animals; usually directed toward keeping the offspring closely related to an admired/desirable ancestor. a. Advantages: 1) Can be accomplished by a small scale breeder. 2) It generates more pairs of pure genes (homozygous). b. Disadvantages: The same as for inbreeding discussed above. c. Situation: 1) When we want to keep the offspring closely related to an admired ancestor. 2) In horses, linebreeding is often practiced when we have a multi-million dollar stallion (as in race horses). 3. Outcrossing - Definition: Mating animals that are members of the same breed, but which show no close up relationship in the pedigree. a. Advantages: 1) Relatively safe as there are few problems with decreased growth and vigor unlike inbreeding and linebreeding. 2) It may help an inbreeding program by decreasing the chance of undesirable traits. b. Disadvantages: It may require the breeder to go outside his herd to locate animals which increase costs. c. Situation: It is used in commercial herds to improve the production level of the animals (growth, rate, vigor, etc.) . 4. Crossbreeding - Definition: The mating of animals of different breeds. a. Advantages: 1) It tends to increase vigor (hybrid vigor). 2) The desirable (and dominant) genes of both tend to combine while undesirable genes tend to be recessive. b. Disadvantages: A producer cannot maintain a purebred flock or herd and predicting the genetic make-up of the off spring become much more complicated. c. Situations: It is used in commercial production to combine beneficial traits of different breeds. For example: Angus cattle are naturally polled and therefore eliminate the management process of dehorning, so by crossing horned cows (Hereford, Shorthorn) with an Angus bull, we "genetically" dehorn our cattle. __________________________________________________________ ACTIVITY: 1. Observe actual pedigrees and try to detect incidences of linebreeding, inbreeding, and out- crossing. __________________________________________________________ B. Hybrids 1. In general, the wider the cross within a species, the greater the increase in vigor. example - crossing Brahman (Zebu) and European breeds of cattle. 2. Crossing species may be advantageous. example - mules result from mating a horse and a donkey. Mules are a superior work animal, being stronger than either parent. However, the offspring resulting from these cross-species matings are usually sterile because each species of animal has a specific number of chromosomes and when we mate across species the chromosome numbers are incompatible. C. Prepotency - Definition: The ability of an animal to pass on its characteristics to its offspring. The characteristic must be genetically dominant, and must be homozygous. example: "Figure", the legendary horse that brought the Morgan horse breed into existence. This stallion was pre- potent in that his characteristics were dominant; therefore, his foals resembled him and thus established a new breed. 4/16/90 #%&C