選擇強度和遺傳進展　　　　Selection Intensity and Genetic Improvement　　　　Dinesh Thekkoot博士　　　　Dinesh Thekkoot, PhD　　　　阿爾伯塔大學和加裕公司。　　　　University of Alberta and Genesus Inc.　　　　動物育種目的是利用個體之間的遺傳差異改良種群。遺傳進展是通過選留先進的動物作為親本，從而決定下一代的遺傳值。這個過程的基本機制是增加群體中有利基因的頻率，并防止劣勢基因動物通過下一代傳遞不利基因?！　　　∮N體系的遺傳進展取決于四個關鍵因素，計算公式如下：　　　　Animal breeding aims to improve livestock populations by utilizing the genetic differences among individuals. Genetic improvement is achieved by selecting superior animals as parents, and this determines the genetic value of the next generation. The basic mechanism underlying this process is to increase the frequency of the favorable form of genes in the population, and to prevent genetically inferior animals from passing their unfavorable forms to next generations.　　　　Genetic improvement per year from a breeding program depends on four key factors and is calculated as follows:　　　　∆G= (irs_g)/L (1)　　　　∆G是遺傳進展，i是選擇強度，r是選擇準確性，s_g是群體育種值的標準差，L是世代間隔?！　　　∏懊嫖恼乱呀浽敿毭枋隽诉x擇準確性的重要性，文章可以通過這個鏈接查閱。在本篇報告中，我們將討論另外一個因素-選擇強度?！　　　≡趧游镉N中，通常采用截斷的方式選擇動物，即只有當個體的指數高于一定的截止值時（截斷點），才能被選留作為親本；截斷選擇的結果是選留群體的平均指數和總群體的平均值之差，稱為選擇差（S）。選擇差的定義是超過總群體平均值的優(yōu)勢選留群體?！　　　here ∆G is the genetic improvement per year, i is the intensity of selection, r is the accuracy of selection, s_gis the genetic standard deviation of the trait under selection and L is the generation interval expressed in years.　　　　The importance of accuracy of selection was described in detail in a previous article, which can be read here. In this report, we will discuss another factor - The intensity of selection　　　　It is common in animal breeding to select animals by means of truncation, i.e. individuals are selected as parents only if their index values are higher than a certain cutoff value; the truncation point. Truncation selection results in a difference between the mean indexes of selected parents and the overall population mean, which is called Selection Differential (S). Selection differential is defined as the superiority of the selected parents over the population mean.　　　　選擇差= I ̅_選留群體-I ̅_總群體　　　　S= I ̅_selected-I ̅_population (2)　　　　I ̅_selected 是指選留群體平均指數，I ̅_population是指總群體平均指數。用標準差除以選擇差，可以將選擇差標準化。標準化的選擇差稱為選擇強度，用i表示?！　　　here I ̅_selected is the mean index of the selected parents and I ̅_population the mean index of the population. The selection differential can be standardized by dividing it with standard deviation of the index (s_I ). The standardized selection differential is called selection intensity, and is expressed as i.　　　　i= (I ̅_selected-I ̅_population)/s_I (3)　　　　根據以上的公式，我們可以看到，選擇差越高，遺傳進展（∆G）將會越高?！　　　rom the above equations, we can see that higher the selection differential, higher will be the genetic improvement (∆G).　　　　假設一個豬群的平均指數是100，標準差是25。當從這個群體中選擇時，我們通常從高于一個特定值的被選群體中去選擇，比如130。在這種情況下，130被稱為截斷點。圖1代表了整個群體的分布，選留群體為陰影區(qū)域所示。選留群體的平均指數是142.2，這種情況下選擇差等于42.2（142.2-100）?！　　　onsider a population of pigs with a mean index value of 100 and standard deviation of 25 index points. While selecting from this population, we typically select from within the chosen group of pigs that fall above a certain index value, say 130. In this case 130 is called the truncation point. Figure 1, represents the distribution of this population, where the selected parents are indicated by the shaded region. The average index of the selected parents will be 142.2 and the selection differential in this scenario will be 42.2 (142.2 – 100).　　　　圖1 Figure 1:　　　　理論上一個更大的選擇差將會得到更高的遺傳進展。因此為了提高選擇差，我們需要把截斷點向分布圖的更右側移動。這樣會得到了更高的選擇差，但是降低了選留的親本動物群體數量。圖2證明，把截斷點從圖1中的130向右移到140的情況下，選留群體的平均值增加到150.6，但是被選留的動物比例下降了（位于陰影區(qū)域部分）。　　　　In theory a larger selection differential will result in higher genetic improvement. Hence to increase selection differential, we need to move the truncation point further to the right of the distribution. This results in a higher selection differential, but a lower number of animals being selected as parents. Figure 2 demonstrates the scenario of moving the truncation point to 140 from 130 (Figure 1). Here the mean of the selected parents will increase to 150.6, but the proportion of animals selected goes down (indicated by the shaded region).　　　　圖2 Figure 2　　　　從公式1中我們可以看出，選擇強度i越高，遺傳進展越大。選擇強度i取決于選留群體的比例。例如，當選留的比例是100，也就是如果我們選擇了全部群體，選擇差和選擇強度都為0，則沒有任何遺傳進展。相反，如果我們選留了非常小的比例，假設是0.01%（從10,000個個體中選擇1個），選擇強度則是3.9，遺傳進展非常大。　　　　總的來說，為了遺傳進展最大化，育種體系需要保證盡可能低的選留比例。但是對于較小數量的核心群體來說，更高的選擇強度可能導致更高的近親交配，從而降低遺傳方差或者標準差。從公式1中我們可以看出，降低遺傳標準差將會降低遺傳進展。更大的群體，合理地管理核心群體數量將會增加遺傳變異。在對遺傳進展沒有任何負面效應的情況下提高選擇強度，較好的方式是增加核心群體數量。加裕公司管理著世界上最大的純種注冊核心群體，這樣龐大的群體數量有助于在沒有任何負面效應的情況下，保持我們育種體系中很高的選擇強度，確保較高的遺傳進展和加裕客戶的最大盈利?！　　　rom equation 1 we can see that, higher the value of i, the bigger will be the genetic improvement per year. The value of i depends on the proportion of animals selected as parents. For example, when the proportion selected is 100, i.e. if we select the whole population, both the selection differential and the selection intensity will be zero, and there won’t be any genetic improvement. Conversely, if we select a very small proportion, say 0.01%, (selecting 1 out of every 10,000 animals), the selection intensity will be 3.9, and the genetic improvement will be very large.　　　　In general, to maximize the genetic improvement, breeding organizations need to keep the selection proportion as low as possible. But for smaller nucleus populations, higher selection intensity can result in higher inbreeding and thereby reduced genetic variance or standard deviation. From equation 1 we can see that reduced genetic standard deviation will reduce genetic improvement. Larger, well managed nucleus populations will have increased genetic variation. The best way to increase the selection intensity without any negative effects on genetic improvement is by increasing the nucleus population size. Genesus manages the world’s largest registered pure bred nucleus herd, and this large population helps to maintain very high selection intensity in our breeding program without any adverse effects, assuring the highest genetic response and maximum profitability to Genesus customers.