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标题: 如何控制隐形基因携带者 [打印本页]

作者: 晨光 时间: 2009-9-29 13:43
标题: 如何控制隐形基因携带者
前言
专家们很清楚地知道辛勤工作是没有捷径可以走的，但是我们可以想出好的替换方法来加以变通。如果饲养一只血统优良但疏于驯养，且社会化淡薄的幼犬，你将很难发掘出它血统的潜力，尽管这只幼犬品质纯正，但可能天性胆怯。这就意味着，这些坚持在家完成“家庭作业”——即训练幼犬，并努力解决犬只社会化问题的饲主，较之那些没有任何作为的犬只来说，显然会有许多优势，但并不一定完全就是如此。

作者: 晨光 时间: 2009-9-29 13:43
标题: 谱系——合适的工具
千百年来，谱系已经成为繁殖者用来绘制犬只基因特性代与代间继承遗传的工具。正是这种记录，成为了基因遗传的图形示例。只要你看一下图表，就会发现它了类似家族谱系的树系结构图，因为你可以在上面找到每一个家族的名称，以及在成功繁殖过程中的各个家庭成员的姓名。从这个树状图形中，我们可以追溯“堂兄弟”，“姨妈”，“叔辈”，“祖辈”及“外祖辈”。繁殖者们给它们冠以头衔，证书以及多项参加的获奖名次，以此来帮助他们更加清楚地认识到该犬只的价值，但是不幸的是这些信息对遗传性并没有反映。他们所学的是根据犬型来繁殖，而将冠军公犬与冠军母犬进行配对，并没有繁殖出更多的冠军后代。人们开始清楚的认识到，我们需要更多不同的繁殖信息。繁殖者们很快便开始了解他们其实并不需要变身为专业遗传学家去繁殖出高品种的犬只，他们真正需要的是要了解该犬祖先的相关信息。繁殖方式也会出现误导性，一些繁殖者为避免基因的遗传性障碍（遗传病），而使用异系繁殖。而另一些不通过这种繁殖方法的人发现，犬只本身是遗传基因的载体，因此仍然会有影响。但这些都没有办法证明他们各自繁殖方式的有效性。为减少隐性基因的不利影响，繁殖者不断重复使用异系繁殖（不同血系间的繁殖）的方法。通过此繁殖方式我们发现，异系繁殖往往是隐性携带者，但这只是暂时性的。由于基因不能被淡化，所以繁殖过程中所存在的问题就是无法减少或除去那么多余无用的基因；基因存在的形式就是分为“有”或“没有”两种。更重要的是，繁殖者们意识到，隐性基因可能会间隔一代或者几代后，又再次出现。

那些淘汰犬只的繁殖者们发现，要是不顾犬只本身的品质而进行繁殖，只会减少该犬种良好犬只的数量而已（这会很快影响到该犬种的基因库）。这并不会消除犬种自身的问题，因为一些犬种中基因携带者的频率高达百分之五十。例如，在二十世纪五十年代的时候，拍X光片成为当时最被广泛运用诊疗手段，用以确定髋关节发育不良的问题（即HD）。在繁殖计划中，通过照X射线观察髋部情况来进行品种的筛选，数以千计的没有通过检查的优秀犬只被处以安乐或是排除。经过多年的优胜劣汰，髋关节问题还是没有得到解决，且导致很多繁殖计划也停滞不前。一直到繁殖者们开始转移他们的焦点，错误才得以停止。取而代之的是，他们在繁殖个体犬只时，开始考虑该犬只的祖辈血系信息。通过对已知载体的谱系进行筛选，繁殖者不仅能够较长时间的保留理想的犬只特性，还能对问题加以有效控制。

作者: 晨光 时间: 2009-9-29 13:44
标题: 全新繁殖方式
研究谱系的基本原则之一就是收集相关信息。资料越详尽，那么繁殖者对该犬只问题及优点的认识便会更加清晰，这也有助于完成他们的繁殖计划。但是当繁殖者开始收集更多信息时，他们已有的许多问题依然存在，只是已较慢速度继续发展而已。不久以后，他们渐渐认识到解决问题的方法必须包含某些特定的犬只信息和一个更优秀的血系。历来最为常见的谱系称为“传统谱系”，它只对犬只的名字，它的头衔以及所获证书的情况进行记录。这些信息往往让人误以为是家族遗传。与此相关的另一个问题是，一小部分祖辈在三代谱系中都有出现，而事实上，它们的直系后裔及其兄弟姐妹一般不在谱系的研究范围之列。正是由于这些方面的限制，就需要有一种新型且更佳的描述谱系的方式，它应该能捕捉有血缘关系的犬只间特定的信息。为此所设立的谱系被成作是“符号谱系”。它不是用犬只名字或其获得的头衔来命名，而是用符号来识别每只祖辈。这个谱系是用来收集幼犬的公犬、母犬、其祖父母的曾祖父母的后代（即其兄弟姐妹）的信息。同时，符号谱系也取代了收录其祖先的名字、头衔以及获奖证书的做法，它对犬类资料收集方式有所不同。它是运用两种符号，而不是每个祖辈的名字。圆圈用来代表母犬，而方块则用以表示公犬。而每个符号都会有颜色来标识，这样一来就能对所关注的特定性状加以辨识，包括该性状的携带者，受影响个体或是正常个体。根据每个性状或是疾病都有分配不通的颜色。当确定一个受影响的个体时，繁殖者在挑选时就会在它的代表符号上就会覆盖上一个特殊的颜色，以表示它存在的某种性状或是疾病。而携带者则会用同种颜色的圆点的形式加以表示，以此来代表它存在的某种性状或疾病。在下面的图表示例中，假定种犬/或是繁殖标准而言是前臼齿缺失。图表1说明了如果前臼齿缺失是性状所导致的，那么通过符号谱系是如何解码的。在这个例子中，黑色是用来表示前臼齿缺失的犬只。在这个谱系中，公犬“ST”和母犬“LF”和他们所有的兄弟姐妹一样都拥有完整的齿列。尽管这些都是事实，但是在它所繁殖出来的六只幼犬中，其中五只（用全黑色表示）都出现前臼齿缺失的情况。有许多特性，包括前臼齿缺失在内，通过遗传性都是看不出来的。这样就使得问题变得更加棘手。在这种情况下，繁殖者必须借助那些已知的信息。通过对这些产自直接祖辈的后代信息的收集，我们能获知更多的信息以便确定可能出现的问题或是病症发生的根源所在。在图一的谱系中，双方都存在有牙齿缺失的祖辈。而最后的幼犬之中，只有一只公犬是拥有完整齿列的。由于幼犬是谱系中性状携带的最佳的体现个体，因此这只公犬则被视作为高风险的种公候选人。除了利用谱系关系来进一步了解，还应采用一些其他的方式加以辅助。例如，幼犬可以在8到12周成犬牙出现前的时候进行X光照检查。X光片会显示出“芽胚”，或是小的尚未出齐的前臼齿。如果有42个“芽胚”，那么就表明该只幼犬发育成熟后，牙齿会是完整的。

繁殖者有了这类详细的资料信息，就能更好的了解和评估繁殖的风险，便于从谱系中挑选出那些性状携带者。符号（LA）半阴标示这只公犬存在臼齿缺失的情况。而它相应符号的余下空白部分则能作为另一个特性的代码。

下面这个例子是用来进一步阐明，扩大祖辈的数量以及对他们进行编码，对繁殖有很大的利用价值。图二所表示的就是繁殖者如何利用红色标记，来对谱系中患有渐进性视网膜萎缩症（简称PRA）的犬只进行标识。PRA是一个带“保护伞”色彩的医学术语，它包括所有导致渐进性视网膜萎缩的遗产性疾病。然而不幸的是，这种疾病具有迟发性，且可能导致失明。并不是所有视网膜疾病都是PRA，也不是所有PRA都是由相同的基因缺陷所造成的。视网膜疾病的遗传形式可能是显性，或者隐形，亦或伴X染色体与另一个不同的基因相连接而导致。最常见的PRA形式是渐进性杆锥细胞退化症，简称PRCD，现作为常染色体隐性条件，在13个犬种/品种中被发现。一只犬只是正常犬，携带者还是受影响者，都能够通过基因检测技术来进行精准确定。到目前为止，受PRCD影响的犬种有：美国可卡犬，美国爱斯基摩犬，澳大利亚牧牛犬，澳大利亚粗短尾牧牛犬，切萨皮克海湾寻回犬，中国冠毛犬，英国可卡犬，恩特布犬，拉布拉多巡回猎犬，迷你及玩具贵宾犬，挪瓦斯克提亚猎鸭巡回猎犬和葡萄牙水犬。在爱尔兰赛特犬和斯卢夫猎犬中，能通过直接检查来诊断是否患有其他类型的PRA；而常染色体显性遗传常见于古牧犬，獒犬和斗牛马士提夫犬；萨摩耶犬和西伯利亚哈士奇犬中有伴X染色体的PRA；A型PRA则在迷你雪纳瑞中有被发现。

作者: 晨光 时间: 2009-9-29 13:45
在图二中，两只母犬（“g”和“i”）为受影响者，她们相应的符号被标识为红色。在这个例子中，繁殖者一开始并不知道它们中哪些是隐性基因携带者，直到交配后发现后代中有出现PRA病症时才注意到。一旦繁殖者发现父母中有携带者，就应该在它们相对应的符号上点上一个红点（如图中“a”“b”“c”“d”所示）。在随后“g”和“h”繁殖出的后代中，所有必然为携带者，因为它们的母亲（即“g”）携带有两个突变性基因。不管它将哪个基因遗传到它的后代，它们身上总会存在一个PRA的隐性基因，因此这些后代都是该疾病的携带者。所有幼犬“k”到“p”相应的符号上都应该标上红点。

正如先前所提到的，通过颜色来标识不仅仅只是用来确定性状所处的个体，同时还能检测其发生的频率。有了这份详细的信息，繁殖者们就能更好的了解他们所要面对的潜在风险和谱系中的弱点。例如，若将两只有相同不良特性的犬只进行交配，那么其后代所产生的不良性状的机率就会增加。如果通过基因测试能确定携带者，那么应在其后代交配前对它们进行检测。然而如果没有必要的检测手段可以协助挑选最佳幼犬来进行繁殖，那么对于它们的后代和同窝幼犬的追踪则可能会出现混乱。在下篇文章（第二部分），我们将继续讨论和研究如何使用新方法，运用测试手段来控制这些携带者。

作者简介：

Carmen L Battaglia拥有美国佛罗里达州立大学的博士和硕士学位。他不仅是一名AKC的裁判，还是犬类研究者和作家。他在犬类繁殖业上也是个开拓者和领导者，著有多篇相关文章和书籍，同时也是电台和电视台频繁邀请的做客嘉宾，常会出现在类似《动物星球》的节目中。他在全国各地的俱乐部都有举办过关于繁殖犬只的研讨会，也协助俱乐部挑选种公，幼犬等等。

原文选自犬展年鉴

作者: 晨光 时间: 2009-9-29 13:45
标题: 原文：

By Dr. Carmen Battaglia

INTRODUCTION

Experts know that there is no substitute for hard work, but consider the alternative. Take a well-bred superior pup that is badly-reared and not properly socialized. It will not achieve the potential of its pedigree and it is likely to be cowardly despite its good breeding. This means that those who do their homework and strive to manage their problems will continue to have the advantage over those who don’t, but there are no guarantees.

PEDIGREES – THE RIGHT TOOL

Over the centuries, pedigrees have been used as the breeder’s tool to map the inheritance of genetic traits from one generation to the next. It is this record that serves as a graphic representation for genetic inheritance. At a glance, a pedigree looks very much like a family tree, because you are able to trace a family’s name and the various family members through successive generations. Through the tree, cousins, aunts, uncles, grandparents and great grandparents can be traced. Animal breeders added titles, certifications and the results of performance to help them understand a dog’s value, but unfortunately this information was not heritable. What they learned was that type breedings and the mating of champions to champions did not produce more champions. It became clear that more and different kinds of information were needed. Breeders soon learned that they did not need to become a geneticist in order to breed the better dogs but they did need to know more about the ancestors. Misguided about breeding methods, some used outcross breedings in order to avoid genetic disorders. Others excluded those found to be affected or known to be a carrier. Neither approach proved to be effective. Those who repeated the use of outcross breedings in an attempt to dilute the detrimental effects of the recessive genes learned that outcross breedings tend to hide the carriers but only temporarily. The risk of producing the unwanted traits does not diminish because the genes cannot be diluted; they are either present or not. More importantly, breeders learned that the recessives can skip one or more generations before they reappear.

Those who eliminated dogs found to have problems regardless of the dog’s qualities only reduced the number of good dogs in their breed, (which quickly affected their breed’s gene pool). It did not rid the breed of its problems because the frequency of carriers in some breeds is as high as 50% (Bell). For example, in the 1950s when radiographs became the popular diagnostic tool to identify hip dysplasia (HD), thousands of good quality dogs were euthanized or excluded from breeding programs when they did not pass the phenotypical standard set for evaluating hips (X-rays). After years of exclusion, HD did not go away and many breeding programs were devastated in the process. These mistakes continued until breeders began to change their focus. Instead of thinking about the breeding of an individual dog, they began to think about the dog as a pedigree of ancestors. By selecting against pedigrees that had known carriers, breeders were able to retain the desirable traits that took years to acquire and also gain control over their problems.

A NEW APPROACH

One of the basic principles associated with the study of pedigrees involves the collection of information. The more detailed the information the more likely it is that the breeder will be able to understand a problem and reach a goal. But even when breeders began to collect more information, many of their problems persisted and progress continued to be slow. It wasn’t long before they learned that the solution to many of their problems had to include specific kinds of information and a better kind of pedigree. The most popular pedigree has historically been the Traditional Pedigree which only collects the names of dogs, their titles and certifications. Too often these characteristics give the false impression that this kind of information is heritable. Related to this problem is the small sample of ancestors involved in a three-generation pedigree (N=14) and the fact that the littermates of these direct descendents and their siblings are generally not included in the study of a pedigree. Because of these logistical limitations, a new approach and a better pedigree was needed, one that would capture specific kinds of information about more relatives. The pedigree best suited for this purpose is called the Symbols Pedigree. It gets its name because symbols, rather than names and titles, are used to identify each ancestor. This pedigree is designed to collect detailed information about the littermates of the sire and dam and the siblings of the grandparents and great grandparents. It also replaces the requirement to collect names, titles and certifications earned by the ancestors. The Symbols Pedigree approaches the collection of information differently. It relies on the use of two symbols rather than the names of each ancestor. Circles are used to identify the females and squares are used to identify the males. Each symbol is then color-coded in order to identify the specific traits of interest including the carriers, those affected and the normals. Specific colors are assigned to each trait and disease of interest. When an affected individual is identified, its symbol is shaded with a specific color that is selected by the breeder to indicate the presence of a trait or disease. Carriers are coded using a dot of the same color that was used to code the presence of a trait or disease (Battaglia, 1986). In the example that follows, the assumption is that the breeder and/or breed standard will fault missing premolars. Figure 1 illustrates how the Symbols Pedigree would be coded if missing premolars were the trait of interest. In this example, the color black is used to designate missing premolars. In this pedigree, sire “ST” and dam “LF” and all of their siblings are coded to show they have full dentition. In spite of these facts, this breeding produced a litter with 5 of 6 offspring (coded black) with missing premolars. There are many traits including missing premolars whose heritability is not known. This makes the problem more difficult to solve. In these instances the breeder must rely on what is known. By collecting information about the littermates of the direct ancestors, more information can be used to determine the source of the problem and the likelihood of its appearance. In Figure 1, there are ancestors with missing teeth on both sides of the pedigree. In this litter only one puppy (male) had full dentition. This male should be considered a high-risk candidate as a breeding animal because littermates are good indicators of the traits carried in a pedigree. In addition to using pedigree analysis to understand a problem other tools should be employed. For example puppies can be radiographed at 8-12 weeks of age before their adult teeth appear. The radiograph will show the “buds”, or small round premature adult teeth, that have not erupted. If 42 “buds” are present, the pup should have full dentition when it is an adult.

FIGURE 1

With this kind of detailed information breeders can better understand and assess risk should they choose to breed dogs that have carriers in their pedigrees. The partial shading of symbol (LA) indicates that this individual (male) has a missing premolar. The remaining space in his symbol can be used to code another trait.

A second example is used to further illustrate the value gained by expanding the number of ancestors and coding what is learned about them. Figure 2 illustrates how a breeder, using the color red, would code a pedigree for the disease called progressive retinal atrophy (PRA). The term PRA is an “umbrella” word that includes all inherited diseases that cause progressive degeneration of the retina. Unfortunately this disease has a late onset and causes blindness. Not all retinal disease is PRA and not all PRA is caused by the same genetic defect. The inheritance pattern of retinal disease in a specific breed might be dominant, recessive, or X-linked – a different gene. The most prevalent type of PRA, progressive rod cone degeneration or prcd, has been identified in 13 breeds/varieties as an autosomal recessive condition. A dog’s status as normal/clear, carrier, or affected, can be determined with the highest degree of accuracy available when genetic testing is used (Felix, 2005). A list of the canine genetic tests is provided in Part II. To date, breeds affected with prcd are: American Cocker Spaniel, American Eskimo, Australian Cattle Dog, Australian Stumpy Tail Dog, Chesapeake Bay Retriever, Chinese Crested, English Cocker Spaniel, Entelbucher, Labrador Retriever, Miniature and Toy Poodle, Nova Scotia Duck Tolling Retriever and Portuguese Water Dog. Other types of PRA can be diagnosed by direct tests in the Irish Setter and Sloughi; autosomal dominant PRA can be detected in the Old English, Mastiff and Bullmastiff, X-linked PRA in the Samoyed and Siberian Huskies; and Type A PRA in the Miniature Schnauzer (Felix, 2005).

In Figure 2, two females (“g”, “i”) are affected and their symbols are shaded red. In this example the breeder did not know that carriers were bred to each other until they produced affected offspring with PRA. Once the breeder knows that the parents are carriers they should be coded with a red dot (“a”, “b”, and “c”, “d”). In the subsequent breeding (“g” to “h”), all of the resultant offspring (k-p) will be carriers because their dam (“g”) carries two mutant genes. Regardless of which gene she passes on to her offspring they all will receive one recessive gene for PRA, which results in making them all a carrier. A red dot should be placed in the symbol of each of these pups (k-p).

翻译供稿卡卡

作者: JACK 时间: 2009-12-31 10:48
哇，太专业！最喜欢这个板块，增长了很多知识！

作者: 天使爱虫虫 时间: 2010-2-6 16:19
这也太专业了啊！

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