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CNR SELECTION1 SELECTION Selection Selection is choosing or allowing some animals to be parents of next generation while depriving others of the privilege. There are two types of selection: 1) Artificial and 2) Natural selection In artificial selection the breeder chooses the parents of the next generation. Castration is one of the oldest forms of artificial selection. The strategies of genetic progress through selection are 1) Selection between breeds or strains which can achieve dramatic and rapid genetic change when there are large genetic differences between the breeds chosen for the characteristics of economic importance. 2) Selection within breed/strain involves comparing animals of the same breed and mating the preferred animals to produce the next generation. The natural selection operates through differences in fertility among the parents and differences in viability among the offspring. As discussed in the class, selection changes gene frequencies of the population. The gene controlling the trait favored by the nature/breeder will tend to get fixed (accumulated) in population. Selection response (R) Selection response is also called as genetic gain. The three factors that control genetic gain (selection response) in a trait are: 1. Heritability 2. Selection differential 3. Generation interval 1. Heritability (h2) This is a term used to describe the strength of inheritance of a character. A precise definition would be – For a given trait Heritability is the amount of the superiority of the parents above their contemporaries, which on average is passed onto the offspring. h2 is expressed on scale from 0 to 1 or 0 to100. The commonly used generalized grading is: 0-0.1 (0-10%) 0.1-0.3 (10-30%) 0.3 < (above 30%) -Low or weak -Intermediate or medium -High strong ______________________________________________________________________________________ 8/22/2010 1/5 CNR SELECTION2 Heritability (h2) is the ratio of additive genetic variance to the total phenotypic variance. h2 = VA/Vp It expresses the proportion of the total phenotypic variance that is due to the average effects of the genes. h2 of quantitative traits is one the important properties of that trait. h2 is not only the property of the trait but also of the population If h2 is high, selection will be effective, and if it is low selection will be ineffective (This will be clear to you when you see the equation of selection response later on ) 2. Selection differential (SD) Selection differential (SD) is a measure of how good the parents chosen to produce the next generation will be. It is superiority of the selected parents over the mean of the population from which they came. Average of whole group (0.5 Kg) Average of whole group (0.5 Kg) Average of best 50% (0.54kg) Average of best 10% (0.58kg) ______________________________________________________________________________________ 8/22/2010 2/5 CNR SELECTION3 If we have high variation, the SD will be high. In the examples that we have done in the class, we have assumed that the mean of selected males and females are same. There fore we have done it at one shot: For example: Population mean of gain per day= 0.25 kg/day Mean of the selected males = 1 kg/day Mean of the selected females = 1 kg/day Therefore the mean of the selected males and females = 1 So we have: SD = mean of the selected males and females – pop mean SD = 1-0.25 = 0.75 kg/day This can be done only if the mean of the selected females is equal to the mean of the selected males. So, generally we calculate the SD separately for males and females and average for the two. For example: To calculate selection differential for the males: Gain/day Mean of the selected males 2.00 kg/day Overall herd mean (pop mean) 0.24 kg/day Selection differential = 2 – 0.25 = 1.75 kg/day To calculate SD for the females: Mean of selected females Overall herd mean (pop mean) 0.75kg/day 0.25 kg/day - This should be same as for the males because it is the mean of the whole herd irrespective of whether it is male or female) Selection diffrential = 0.75-0.25 = 0.50 kg/day These SD are average to give : 1.75 (SD for males) + 0.5 (SD for females) = 2.25/2 = 1.13 2 kg/day Selection Intensity (i) Selection intensity also describes the superiority of the selected parents over the population mean and is selection differential in terms of standard deviation unit. I (i) = Selection Differential (SD) Phenotypic standard deviation (p) 3. Generation Interval This is the time interval between generations and is defined as the average age of the parents when their offsprings are born. It varies between species and breeds. The general average generation intervals for different livestock species are: ______________________________________________________________________________________ 8/22/2010 3/5 CNR SELECTION4 Horse Dairy cow Pig 9-13 4-5 2-2.5 Beef cow Sheep Chicken 4.5-5 3-4 1-1.5 The generation interval severely restricts the genetic progress. The generation interval adds onto the length of time the breeder has to wait until the sufficient data are available from an animal on which to make a decision. An example of this is – Waiting for completion of 1 st lactation milk yields of daughters before a bull is widely used in a herd or for AI. The way in which the three factors (Heritability, selection differential and generation interval) are put together to give an estimate of genetic gain is as follows: Genetic gain or selection response (R ) per generation = h2 X SD Genetic gain or selection response (R) per year = (h2 X SD) / GI i = (SD) (p) , Therefore R = (h2 * p * i) R=Response h2 = Heritability SD = Selection Differential GI = generation interval i = Selection intensity From the above equation you can see that: 1. Higher the heretability, higher will be the genetic gain. 2. Higher the Selection Differential, higher will be the genetic gain. 3. Higher the Generation Interval, lower will be the genetic gain. Selection Limits In a closed population, there will be good response to selection for few or several generations and will slow down and eventually stop. This point is referred to as selection limits. This phenomenon occurs because; the population is running out of usable genetic variation. However, if some new variation (animal from different herd or breed) is introduced, progress from selection continues until the population reaches another plateau and so on. ______________________________________________________________________________________ 8/22/2010 4/5 CNR SELECTION5 Aids to selection These are the source of information based on which we can decide whether to choose a particular or group of animal(s) or not: 1. Individual or mass selection This aid to selection is used for traits of high heritability where the animal’s own performance is an accurate guide as to how it’s progeny will breed. 2. Lifetime performance records Here the breeder has more than one record of animal’s performance, such as series of lactation yields in cow. A good animal will generally perform well each season and this will be seen in the above average yield despite the ups and downs in the herd due to environment. If a breeder looked at one of the cows past records he could make a fairly safe predictions of her future productions. 3. Pedigree information - Based on the performance of the parents and grand parents and great grand parents and so on… 4. Progeny performance - Based on the production performance of the progeny (sons and daughters) 5. Performance of other relatives (family selection) - Based on the production performance of the cousins, brothers, sisters, uncles, aunties, nieces, nephews… Selection methods 1. Tandem selection Selecting one trait and improving it for several generations till it satisfies the breeder’s goal. Then going for another trait for improvement. The traits should not be negative correlated. Or else the improvement in the 1st phase of selection will be cancelled by 2nd phase of selection. 2. Independent culling level Under this method, the animal has to reach a set standard for all the criteria set by the breeder. Or else the animal gets culled. This is similar to examination system where in you have to pass all the subjects to get to the next class level. 3. Index selection Under this selection method the animal is assessed on the overall score. This is similar to examination system where in you can fail in any number of subjects and you can still get to next class if you pass in the overall average. ______________________________________________________________________________________ 8/22/2010 5/5