Chapter 13 Animal Science with Professor Larson
Chapter 13 Animal Science with Professor Larson ADS 1113
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This 4 page Class Notes was uploaded by Kaitlyn Notetaker on Friday September 30, 2016. The Class Notes belongs to ADS 1113 at Mississippi State University taught by Dr. Larson in Fall 2016. Since its upload, it has received 20 views. For similar materials see Animal Science in Animal and Dairy Science at Mississippi State University.
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Date Created: 09/30/16
Animal Science Chapter 13: Genetic Change through Selection Learning Objective o Explain the concept of genetic variation o Compare and contrast qualitative and quantitative traits o Describe the genetic model and its component (only genotype and environment) o Calculate adjusted 205-day weights (understand formula and reason for it) o Calculate the rate of genetic change (understand formula and how to calculate it) o Outline evidence of genetic change in the livestock industry o Describe the various approaches to selection and the role of selection tool Breeding Industry o There are differences among the species on how improvements in genetics are made, but also similarities: o Elite seedstock producers: create breeding animals as their primary business objective They use genetic information to select best genetic, and then use the reproduction technologies to propagate those genetics Make their business on selling this improved and predictable breeding stock o Multiplier seedstock producers: buy dam and sires from the elites and work to create a higher volume of improved sires and dams In both elite and multiplier, this “volume” might be semen and embryos, not just live animals Their customer is primarily the commercial breeders o Commercial breeders may retain breeding females, but often buy breeding sires from the multiplier seedstock producers Sell most of their offspring as feeder or finished animals that will be harvested, or eggs, wool, fiber, or milk Breeds o A breed is a population of animals that share a distinctive set of characteristics that have been established via a process of deliberate selection to “fix” these traits so that they are passed from generation to generation o Created to match specific functions to meet market requirements and/or environmental constraints o Can be categorized by region of origin or function o Keep in mind, a lot of genetic variation within breed o Beef cattle breeds: British (Angus, Hereford), or European (Charolais, Simmental), or Zebu (Brahman) o Goat breeds: meat (Boer), dairy (Nubian), or fiber (Angora) o Sheep breeds: fine wool (Merino), meat-type (Hampshire), or dual- purpose (Columbia) o Horse breeds: light horses, draft (Belgian, Clydesdale), or pony (Shetland) Light horse further classified: Hunter (thoroughbreds), Saddle (Arabian), or stock (Quarter Horse, Paint) Continuous Variation o The book gives a good description about how many traits are controlled by a large number of genes So although in the last chapter we used examples 1 or 2 traits to illustrate inheritance, in any cases this is much too simple For example: If 8 genes controlled a trait, there are 324 different genetic combination possible in offspring When measuring the trait (for example, weaning weight) the offspring born typically fall into a bell-shaped curve Traits o Quantitative traits Those that can be objectively measured and observations typically exist along a continuum Growth traits, skeletal size, speed, etc… Many gene pairs control these traits o Qualitative Traits Those that are descriptively or subjectively measured Hair color, horned vs. polled, etc.… Often a few, or even just one gene pair, controls these traits. o Phenotype: the observation or measurement of each trait o Phenotypic variation exists because of both genotype and environment Weaning weight as an example Environmental effects: summation of all non-genetic influences o Adjusted records help to remove environmental effects Adjusted weaning weight, or 205-day weight ((Actual weaning weight-birth weight)/age in days at weaning x 250)+ birth weight + age of dam adjustment Commonly used in beef cattle production Understand this calculation and why it, and other adjusted records, are used Pg. 210 Heritability o It is the portion of the total variation or phenotypic differences among animals that is due to heredity o Use heritability and select superior phenotypes to make genetic improvement (selection) o Factors that affect the rate of genetic improvement from section include selection differential, heritability, and generation interval Selection differential: superiority of the selected animals compared to the herd average Heritability: the portion of the selection differential that is passed from parent to offspring Generation interval: the average age of the parents when the offspring are born o Traits are generally categorized into: Highly heritable: 40% or higher Medium (or moderately) heritable: 20-39% Lowly heritable: <20% No need to memorized numbers in table 13.4, but they are interesting Predicting Genetic Change o Genetic change per year =(heritability x selectional differential)/ generation interval o Work through example in book , understand formula, and the factors that go into it o Multiple trait selection is often more important, because selecting for 1 trait may harm another trait unintentionally Amount of milk produced by dairy cattle You affect other things when you do this. Feet and Legs might be affected Selecting against everything else For example, selecting for yearling weight may unintentionally increase birth weight causing dystocia Evidence of Genetic Change o Interesting examples provided in book o Be able to “discuss examples of change in livestock species resulting from selection” Effective Selection o Requires that the traits in question: Be heritable Be relatively easy to measure Be associated with economic value That genetic variation be available o This is why breed associations came about and the utilizations of performance data Expected Progeny Differences (examples of this) o EPDs are calculated for a variety of traits Utilized information on the individual, siblings (half and full), on ancestors, and progeny As more information goes into the calculation, the accuracy of the estimate improves o Best linear unbiased prediction (BLUP) has allowed field data to be utilized to compute valid breeding values to compare animals across herds Much of this is done by breed associations or large seedstock companies Basis for Selection o Poultry and dairy industries have made the best use of sophisticated genetic information This has allowed for tremendous genetic progress o A lot of genetic information is available to beef producers, although fewer use it o Utilization of performance data is one of the most profit-oriented decisions a livestock producer can make Dairy Herd Improvement Association (DHIA) Many producers tend to rely the most on visual appraisal rather than more accurate EPDs that are available Summary Points o Phenotype (what is seen or measured) is determined by genotype (genetic makeup and the environment to which the animal is exposed) o Heritability measure the proportion (0-100%) of the total phenotypic variation that is due to genetics. Traits high in heritability are less than or equal to 40%, while low heritability traits are less than 20% o Selection differential is the superiority (or inferiority) of the selected animals compared to the average of the group from which they came. Generation interval is the average age of the parents when the offspring are born. o Genetic change per year= (heritability x selection differential)/generation interval o Independent culling level is the most common method of selection o Expected progeny differences ought to be the basis for an effective selection program. Questions: 1, 2, 3, 6, 7, 8, 10, 11, 12, 13, and 14 Possible Essays: 12
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