選擇強(qiáng)度和遺傳進(jìn)展

　　

　　University of Alberta and Genesus Inc.

　　

　　動(dòng)物育種目的是利用個(gè)體之間的遺傳差異改良種群。遺傳進(jìn)展是通過(guò)選留先進(jìn)的動(dòng)物作為親本，從而決定下一代的遺傳值。這個(gè)過(guò)程的基本機(jī)制是增加群體中有利基因的頻率，并防止劣勢(shì)基因動(dòng)物通過(guò)下一代傳遞不利基因。

　　

　　育種體系的遺傳進(jìn)展取決于四個(gè)關(guān)鍵因素，計(jì)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(jìn)展，i是選擇強(qiáng)度，r是選擇準(zhǔn)確性，s_g是群體育種值的標(biāo)準(zhǔn)差，L是世代間隔。

　　

　　前面文章已經(jīng)詳細(xì)描述了選擇準(zhǔn)確性的重要性，文章可以通過(guò)這個(gè)鏈接查閱。在本篇報(bào)告中，我們將討論另外一個(gè)因素-選擇強(qiáng)度。

　　

　　在動(dòng)物育種中，通常采用截?cái)嗟姆绞竭x擇動(dòng)物，即只有當(dāng)個(gè)體的指數(shù)高于一定的截止值時(shí)（截?cái)帱c(diǎn)），才能被選留作為親本；截?cái)噙x擇的結(jié)果是選留群體的平均指數(shù)和總?cè)后w的平均值之差，稱(chēng)為選擇差（S）。選擇差的定義是超過(guò)總?cè)后w平均值的優(yōu)勢(shì)選留群體。

　　

　　Where ∆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?cè)后w

　　

　　S= I ̅_selected-I ̅_population  (2)

　　

　　I ̅_selected  是指選留群體平均指數(shù)，I ̅_population是指總?cè)后w平均指數(shù)。用標(biāo)準(zhǔn)差除以選擇差，可以將選擇差標(biāo)準(zhǔn)化。標(biāo)準(zhǔn)化的選擇差稱(chēng)為選擇強(qiáng)度，用i表示。

　　

　　Where 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(jù)以上的公式，我們可以看到，選擇差越高，遺傳進(jìn)展（∆G）將會(huì)越高。

　　

　　From the above equations, we can see that higher the selection differential, higher will be the genetic improvement (∆G).

　　

　　假設(shè)一個(gè)豬群的平均指數(shù)是100，標(biāo)準(zhǔn)差是25。當(dāng)從這個(gè)群體中選擇時(shí)，我們通常從高于一個(gè)特定值的被選群體中去選擇，比如130。在這種情況下，130被稱(chēng)為截?cái)帱c(diǎn)。圖1代表了整個(gè)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(shù)是142.2，這種情況下選擇差等于42.2（142.2-100）。

　　

　　Consider 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(gè)更大的選擇差將會(huì)得到更高的遺傳進(jìn)展。因此為了提高選擇差，我們需要把截?cái)帱c(diǎn)向分布圖的更右側(cè)移動(dòng)。這樣會(huì)得到了更高的選擇差，但是降低了選留的親本動(dòng)物群體數(shù)量。圖2證明，把截?cái)帱c(diǎn)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(dòng)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(qiáng)度i越高，遺傳進(jìn)展越大。選擇強(qiáng)度i取決于選留群體的比例。例如，當(dāng)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(qiáng)度都為0，則沒(méi)有任何遺傳進(jìn)展。相反，如果我們選留了非常小的比例，假設(shè)是0.01%（從10,000個(gè)個(gè)體中選擇1個(gè)），選擇強(qiáng)度則是3.9，遺傳進(jìn)展非常大。

　　

　　總的來(lái)說(shuō)，為了遺傳進(jìn)展最大化，育種體系需要保證盡可能低的選留比例。但是對(duì)于較小數(shù)量的核心群體來(lái)說(shuō)，更高的選擇強(qiáng)度可能導(dǎo)致更高的近親交配，從而降低遺傳方差或者標(biāo)準(zhǔn)差。從公式1中我們可以看出，降低遺傳標(biāo)準(zhǔn)差將會(huì)降低遺傳進(jìn)展。更大的群體，合理地管理核心群體數(shù)量將會(huì)增加遺傳變異。在對(duì)遺傳進(jìn)展沒(méi)有任何負(fù)面效應(yīng)的情況下提高選擇強(qiáng)度，較好的方式是增加核心群體數(shù)量。加裕公司管理著世界上最大的純種注冊(cè)核心群體，這樣龐大的群體數(shù)量有助于在沒(méi)有任何負(fù)面效應(yīng)的情況下，保持我們育種體系中很高的選擇強(qiáng)度，確保較高的遺傳進(jìn)展和加?？蛻?hù)的最大盈利。

　　

　　From 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.

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