An Sci 361 1st Edition Lecture 18 Outline of Last Lecture I. Phenotypic Valuea. HeritabilityII. Heritabilitya. Traitsb. Usesc. What it isd. What it is note. How to increase itf. Narrow Senseg. Broad SenseIII. Genetic DifferencesOutline of Current Lecture I. Genetic GainII. Selection Intensitya. Criterionb. Processc. Measuringd. Theoretical ApproachIII. Genetic VariationIV. Generation IntervalV. Selection Pathsa. Maleb. FemaleCurrent LectureGenetic Gain- Four factors determine the rate of genetic gain from selectiono Accuracy of Selectiono Intensity of Selectiono Genetic Variationo Generation interval- The Key Equationo ΔBV / Yr = Accuracy x Intensity x Variation / Generation IntervalThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Accuracy: The correlation between True Breeding Value and Predicted Breeding Valueo When accuracy is low, many of the selected animals will have poorer BV than many of those not selectedo With high accuracy fewer such errors will occuro When selection is based on a single phenotypic record, accuracy is:  rTBV,EBV = rBV,P = h = √h2o Accuracy has no unitso In meat animal evaluations, accuracy is the Accuracy of EPDo In dairy genetic evaluations, the measure of “accuracy” is the reliability of PTA. Reliability is the square root of Accuracy.Selection Intensity- Intensity measures how “choosy” we are in selecting breeding animalso Choosiness or intensity depends on: How many animals are performance tested How many (or what percentage) of the performance tested animals are selected for breeding How strictly the performance test data are used when selecting breeding animals- Selection criteriono The measures on which selection decisions are basedo Alternative selection criteria: Phenotypic values of individual animals for a single trait Estimated BVs of individuals based on phenotypic values of self and relatives for a single trait Selection Index based on phenotypes or EBVs of several traits from several individuals- Selection processo Rank candidates (potential future parents) on selection criteriao Select the highest ranking animals for breeding- Selection differential (S):o Difference in value between the mean of selected (breeding) animals and mean of performance tested candidates- Measuring Selection Intensityo Selection Intensity (i) = Selection differential divided by standard deviation of the selection criteria (Intensity is in standard deviation units) Measure all animals; select best for breeding  Calculate selection differential (S); divide it by standard deviation (sSC)- Selection Intensity – Theoretical Approacho Selection intensity (i) is average performance of selected parents, relative to the mean performance of all potential parents, in standard deviation units.o For traits with Normal frequency distribution and when truncation selection is used, selection intensity can be derived from Normal probability tables. o Select strictly the highest ranking animalso Selection intensity depends on portion of population selected as parentsGenetic Variation- The measure of genetic variation used in the Key Equation is the standard deviation of breeding value (Units are trait units - e.g., lbs, inches, counts)- Genetic variation, heritability, and phenotypic variation are natural characteristics of a trait in a populationo Genetic variation is not easily changedo Breeders have less control over σBV than other components of the key equation Generation Interval- Generation Interval (L) is average age of parents when offspring are born - Typically measured in years of ageo To calculate L, simply add across all progeny the ages of their sires and dams; calculate average age of parents L = (average age of sires + average age of dams)/2o Using younger parents reduces generation interval and increases genetic gain peryear- Is limited by reproductive characteristics of species, e.g. age at puberty, gestation length,litter size, etc.- Can be managed by breeders- To determine generation interval, consider:o Life Cycleso Number of Paritieso Ages of parents when progeny are borno Reproductive cycleso Herd or flock age structureo Gestation LengthsCommon Generation Intervals for Farm AnimalsSpecies Generation Interval (yr.)Horses 8 to 12Dairy Cattle 4 to 6Beef Cattle 4 to 6Sheep 3 to 5Swine 1.5 to 2Chickens 1 to 1.5Refined definition of Generation Interval (L) = average age of parents when offspring that are potential replacements are born- Which herd has the shorter generation interval?o A dairy herd that breeds heifers to Angus bullso A dairy herd that uses only dairy bulls2 Selection Paths: Males and Females- Accuracy, intensity, and generation interval may differ between sires and damso Fewer males are needed than females, i.e. greater selection intensity in maleso If selection is based on progeny test results, males will have more progeny than females, i.e. greater accuracy in male selectiono Progeny tested males may be older than females, i.e. greater generation interval for males - ΔBV / YR = Sum of male & female selection responses / Sum of male & female generation