Basic Genetics and Selection Part 2

Mating Plans and Breeding Systems Mating Plans • Random – no effort to control relationship of parents. • Inbreeding – mating animals more closely related than the average of the population. Main use is linebreeding, to increase genetic influence of superior sires or dams. • Outbreeding – mating animals less closely related than the average of the population. Outcrossing is outbreeding within a breed. Crossbreeding is outbreeding of animals of different breeds. Effects of Mating Plans • Inbreeding is often detrimental to overall vigor and fitness. It tends to result in more homozygous gene pairs. Does not increase the number of possibly undesirable recessive genes, just increases the chance they will be expressed in progeny. • Outbreeding, especially crossbreeding, generally improves vigor and fitness. Tends to result in more heterozygous gene pairs. Continuous Systems • Straightbreeding - same breed of sire and dam, can be carried out in one breeding group. • True rotation - two or more breeds, same number of breeding groups, females bred to breed of sire to which they are least related. • Sire rotation - two or more breeds, one breeding group, one sire breed used at a time, changed periodically, before breeding back to daughters. Example 2-Breed True Rotation • Start with Angus females, breed to Hereford. • Rotate saved Hereford-sired heifers to another breeding group using Angus sires. • Rotate saved Angus-sired heifers to original breeding group using Hereford sires. • Females rotated once in their life, to group using sire breed they are least related to. Once the rotation is completed, both breeding groups are maintained. After several generations, each group contains about 2/3 of the influence of breed of sire and 1/3 of the other breed. Benefits of Crossbreeding 1) Heterosis or “hybrid vigor” • Difference in performance of crosses relative to the average of purebred parents. • Highest in progeny of parents least related genetically (for example, higher in Hereford X Brahman than Hereford X Angus) • Highest in reproduction/maternal traits, medium in production traits, lowest in product traits. • Can not be transmitted from parent to offspring. Choosing Applicable Genetic Types and Breeds Genetic Classes • Cattle: Genetic Classes – Bos taurus - non-humped types, originating in Continental Europe and the British Isles, most cold tolerant – Bos indicus - humped types, also called Zebu; originating in south central Asia, most heat tolerant • Sheep: Genetic Classes – Hair Sheep – Wool Sheep – Over 200 breeds world-wide Types and Breeds • Can be placed into functional-type breed groups based on genetic classification, body size, milking potential, and body composition (lean-to-fat ratio). Functional Breed Groups of Cattle  BRITISH BEEF - Bos taurus, moderate size, low to moderate milk, low to medium lean-to-fat. (Examples: Angus, Hereford, shorthorn)  CONTINENTAL BEEF - Bos taurus, moderate to very large size, low to moderate milk, high to very high lean-to-fat. (Examples: Chianina, Charolais, Limousin) Functional Breed Groups DAIRY - Bos taurus, small to large size, high to very high milking, very low to low lean-to-fat. (Examples: Holstein, Jersey) Functional Breed Groups BOS INDICUS - Bos indicus, moderate to large size, low to medium milking, low to medium lean-to-fat. (Example: American Brahman) Functional Breed Groups AMERICAN - Bos taurus X Bos indicus, moderate to large size, medium to high milking, low to medium leanto-fat. (Examples: Beefmaster, Brangus, Santa Gertrudis, Simbrah) Functional Breed Groups: Example Goats • Dairy – Examples: Alpine, Nubian, Saanen, Damasus, Toggenburg