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|Inbreeding, Linebreeding, and Outcrossing in Texas Longhorns|
© David M. Hillis, Double Helix Ranch
Professor, University of Texas at Austin
The reduced-switch syndrome as it affects the relative hair length of white and wild-type colored hair of the winter coat of a Texas Longhorn. Note that the white hair is long (normal for winter) but the wild-type colored hair is short. In some animals and in the warmer months, this genetic defect is easiest to see in the tail of the affected animals (see below), because it produces a short or absent tail switch (unless the tail is white). This photograph was taken in winter in Nebraska; in warmer climates and on some animals, the trait may not be as obvious.
This article discusses the genetic effects of inbreeding, linebreeding, and outcrossing. It also discusses how inbreeding can result in the fixation of both deleterious as well as advantageous genetic traits. Examples are given of the general reduction of fitness that results from inbreeding (known as inbreeding depression), as well as traits that have been improved through inbreeding (such as long horns).
All breeders who maintain a single breed of cattle are practicing some degree of inbreeding. However, the level of inbreeding varies considerably with different breeding strategies and goals. This article suggests that breeders who wish to maintain a single "family" of Texas Longhorns or linebreed using a particular bull need to be careful to avoid breeding schemes that increase inbreeding to undesirable levels.
If you have comments or questions about this page, please e-mail me.
[Author's note: This article is intended for a general audience of Texas Longhorn breeders, rather than a technical audience. However, some scientific jargon is unavoidable, so if any of the terms are unfamiliar, please see the Glossary. If you have suggestions for other terms that should be in the glossary, please e-mail me. If the article is not clear to you, then I want to know where and why, so that I can fix it.—DMH]
Texas Longhorns are noted for being remarkably free of genetic defects that plague some other breeds of cattle. This is largely a result of the natural selection against deleterious traits that occurred when Texas Longhorns lived wild on the open range. Most other breeds of cattle have undergone intensive artificial selection and inbreeding, as breeders have developed breeds to express certain traits in a uniform manner. One of the easiest ways to accomplish this goal is to use the bulls that express the desired trait to breed many cows, and then cross the progeny until the trait becomes fixed or uniform. Breeding closely related animals thus reduces genetic variation. This process is known as inbreeding.
Unfortunately, there is a serious down-side to inbreeding. In addition to fixing the desired traits in the breed, inbreeding also fixes (or increases in frequency) deleterious recessive traits that are genetically linked to the desired traits. ("Genetically linked" simply means that the two genes are located on the same chromosome, close to one another). Many of these deleterious recessive alleles may have only small effects (even when homozygous), but as more of them become fixed or increase in frequency in the population, the fitness of the inbred animals almost always suffers. Thus, fitness usually declines upon inbreeding (and this is particularly true of reproductive traits and traits associated with overall configuration and body size).
All organisms, including the very best bull in any breed, are expected to have some rare, deleterious, recessive alleles at some of their genetic loci. Because these alleles are rare, typically just one of the two alleles at a given genetic locus (= a gene) represents the deleterious trait. Since the alleles are recessive, there is no effect on the phenotype of the animal as long as they remain in the heterozygous state. The problem arises when two of these alleles are present in the same animal (one inherited from each parent). If the frequency of a deleterious allele in a population is only 0.01 (i.e., one out of every 100 alleles), and all the animals mate randomly, then only one out of every 10,000 animals would be expected to inherit the trait from both parents, and thus express the defect. But imagine that this trait is present in a particular bull, and that bull is very popular (because he also has some desirable trait, such as very long horns in the case of Texas Longhorns). As this bull is bred to many females, half of the resultant offspring would now carry any given deleterious recessive trait possessed by that bull. If these individuals are mated to one another, then 25% of their offspring would be homozygous for the deleterious allele, and another 50% would carry the allele in a heterozygous state. In this extreme form of inbreeding, in just two generations the defect went from being expressed in one out of every 10,000 animals to one out of every four animals, with three-quarters of the animals now carrying the defect. The decrease in fitness that results from such inbreeding is known as inbreeding depression.
Fortunately, most breeders are well aware of the negative effects of such severe inbreeding, and few breeders would purposefully breed siblings with one another. However, another strategy, sometimes called linebreeding, is very common, and is practiced to some extent by all Texas Longhorn breeders. Linebreeding involves mating within an historically-defined and related set of individuals (a line). Lines may be formally or informally recognized, and there may be lines within lines. At one level, the Texas Longhorn breed is a line within cattle. Within the Texas Longhorn breed, there are seven commonly recognized historical "families," which represent smaller lines. Within these family lines, some people may maintain or promote even smaller lines (e.g., the Texas Ranger JP line within the Phillips family). Why are these lines maintained? The usual reason is because each line has (or is perceived to have) unique and desirable traits (e.g., disease resistance in Texas Longhorns as a breed, or longer than average horns in the Butler family line, or perhaps overall conformation in the Texas Ranger JP line). However, all linebreeding results in some degree of inbreeding. The degree of inbreeding will be negligible if the number of individuals in the line is very large and care is taken to avoid matings between close relatives, but it will rapidly increase if the size of the line becomes more restricted or if matings between close relatives are common.
Extreme inbreeding may be avoided in any linebreeding program by not mating fathers to daughters or siblings to each other (or other similar matings that involve close relatives). However, the degree of inbreeding is related not just to the immediate relationship between the two mated individuals, but also to the degree to which these individuals were already inbred. For instance, if a particular bull is widely used in a linebreeding program, then many of the cattle in that program will already share the genes of that bull, and the chances of getting two deleterious alleles from that bull in the same offspring are greatly increased. The chances may not be as great as would occur between the matings of siblings, but they are much higher than would occur in a randomly mating population. Even small increases in inbreeding result in some inbreeding depression. For instance, just a 1% increase in inbreeding (that is, a 1% increase in the probability that a calf receives the same gene from both parents) results in a measurable decrease in milk quantity and quality, shortening of productive life, and increase in calving interval in studied breeds of cattle.
How can the effects of inbreeding be avoided? When two lines are crossed, then any deleterious alleles present in one line but not in the other are masked, and there is typically a boost in the fitness of the offspring. This effect is known as heterosis or hybrid vigor, and is essentially the opposite of inbreeding. This effect even extends to crosses between breeds of cattle, which is why crossbreeding programs are popular (for producing cattle for beef operations, for instance). Essentially, one can get many of the benefits of two breeds of cattle in the first generation of a cross.
Given that some detectable level of inbreeding is present even in the Texas Longhorn breed as a whole, is inbreeding likely to be a problem for Texas Longhorn breeders? The answer to this question depends on the goals and practices of a particular breeding program, and the degree of potential risks and benefits that a particular breeder is willing to assume. The Texas Longhorn breed is almost certainly large and diverse enough at this point in its history that inbreeding no longer needs to be a significant problem (this wasn't necessarily true earlier this century). Most breeders wish to preserve the historical integrity of Texas Longhorns, and the breed clearly possesses many desirable traits that are not found (or else are not common) in other breeds of cattle. Therefore, it makes sense to maintain the breed as a separate line. One could extend this argument to the "seven families" of Texas Longhorns as well: since some people are interested in maintaining families of Texas Longhorns for historical reasons, and since some of the families have traits that are viewed as desirable within the breed, then maintaining "pure" herds of a single family makes sense for those breeders. However, it is important to note that as the linebreeding is restricted to a smaller-and-smaller group, then the effects of inbreeding also become more severe. Crossing between families will result in some degree of heterosis, so Texas Longhorn breeders who select their breeding stock among many different families will have some advantages in the overall health, fitness, and conformation of their animals. On the other hand, if a particular trait is the primary interest of a breeding program, and that trait is best expressed in one family (the very long horns of the Butler family is a good example), then more restrictive linebreeding may be desirable. The important point is to realize that any linebreeding comes with some cost, which must be weighed against any benefits.
I've been asked by several breeders if it is "OK to breed two half-siblings" (usually, a bull and a cow that share the same sire). The short answer is that there is nothing "wrong" with breeding any two animals of any degree of relatedness, as long as one realizes the potential risks and benefits of the mating. Any level of inbreeding does carry some risk (the risk that one or more formerly hidden deleterious recessive traits will be expressed in the homozygous offspring), but there is also the potential for benefits (a beneficial trait in the common sire may be replicated). As the degree of genetic relatedness between a cow and bull increases, more and more of the alleles inherited through the cow and the bull will be the same, so that overall homozygosity will increase. Since homozygosity is negatively correlated with overall fitness and size, close matings are not usually desirable. However, if one wished to replicate a particularly desirable trait in a particular bull, then one of the fastest ways to achieve this would be by mating two half-siblings produced by that bull (or, even more extreme, by mating the bull to his daughters). The danger does exist that undesirable deleterious traits would be exposed in some (perhaps many) of the calves produced from such a mating, so a breeder who is unwilling to cull a large fraction of his or her calves would be best advised to avoid breeding such close relatives. On the other hand, for a breeder who is willing to cull many inferior calves to produce one exceptional calf, then half-sibling matings or other close matings could be a good strategy. The great Texas Longhorn bull Monarch 103, which is generally acknowledged to be one of the best producers of long-horned offspring in the breed (he has sired more daughters with >60" horn than any other sire, according to the Texas Longhorn Journal, Feb/Mar 1997), was the product of a sire (Bevo) mated to one of the sire's own daughters (Lady Butler). Then again, Monarch only weighed about 1300 pounds at maturity, whereas many well-known Texas Longhorn bulls that are the products of outcrossing between families weigh over a ton. [As an aside, we have three Texas Longhorns in our herd that are the result of half-sibling matings by other breeders, and I consider two of these three to be among our very best animals. On the other hand, I've seen plenty of other results of half-sibling matings that I wouldn't want to get near our herd.]
The potential danger of half-sibling matings (as well as other matings among close relatives) also depends on the degree of prior inbreeding of the sire and the two dams involved. For instance, breeding two individuals that have the same sire is less likely to be problematic if the two dams are distantly related (e.g., one from the Yates family and one from the Butler family) than if the two dams are closely related (e.g., both cousins from the Butler family). Therefore, breeders interested in maintaining "pure" herds of a single family need to be much more careful to avoid matings between half-siblings or cousins or other relatives than do breeders who use multiple families. Moreover, the breeder of the "pure" family must accept the likelihood of some level of reduced overall fitness (as a trade-off for the benefits of keeping the historical family herd). There is nothing "wrong" with either outcrossing or linebreeding, but each breeder must weigh the potential costs and benefits and assess which strategy best fits his or her long term goals and interests. These comments about the possibility of reduced fitness also apply, but to an even greater degree, to breeders who wish to linebreed based on a single bull that has particularly desirable traits. In this latter case, the inbreeding depression is likely to be considerable, even if care is taken to avoid matings among the closest relatives. Therefore, I'd advise most smaller breeders (as well as anyone interested in breeding for overall conformation, size, health, and reproductive fitness) to practice outcrossing among the families of Texas Longhorns, and to leave linebreeding to those who wish to specialize on a particular trait or preserve the tradition of a particular family line (and who also can afford a larger fraction of cull heifers).
At this point, you may be asking "what are these supposed deleterious traits?" One answer is that many quantitative traits, such as total body weight, are affected by many different genes, and these traits tend to show obvious effects of inbreeding and outcrossing. This is one reason why most competitive Texas Longhorn show cattle are produced from blends across families. The heterosis produced from outbreeding will almost always produce larger-bodied, more muscular offspring with superior conformation compared to offspring produced from linebreeding a single family. Reproductive fitness also tends to be tightly linked to heterozygosity, so outbred cattle are likely to show higher fertility than are inbred cattle. This higher fitness in outbred compared to inbred cattle is largely the result of many homozygous deleterious genes (each with a relatively small negative effect) in the inbred cattle. On the other hand, linebreeding in the Butler family has produced some of the longest-horned animals in the breed (again, consider the pedigree of Monarch), so pure Butler cattle are highly competitive in the Horn Showcase competition. This is an example of how linebreeding has been used to develop a desirable trait. In other words, both linebreeding and outcrossing have advantages under different sets of circumstances and criteria. (Note, however, that many of the competitive cattle in the Horn Showcase also come from programs that blend cattle among the families, so I do not mean to suggest that inbreeding is the only way to breed for very long horns).
In addition to the many genes of small effect, inbreeding sometimes uncovers single alleles that have relatively large deleterious effects. Shortly before writing this article, I was contacted by a Texas Longhorn breeder in Nebraska who had discovered exactly such a genetic defect. The genetic defect produces a phenotype that is very similar to a trait called "rat-tailed" syndrome that has been reported in Simmental cattle (Schalles and Cundiff, 1999: J. Anim. Sci. 77:1144–1147). However, the genetic basis and expression of the trait that occurs in Texas Longhorns appears to be somewhat different than in Simmental cattle. In Texas Longhorns, there are two primary consequences of the trait. First, the tail switch is greatly reduced or even absent, unless the tail hair color is white (see photo below). Second, the colored hairs (or at least wild-type colored hairs; see the page on coloration in Texas Longhorns) on the body are shorter than the white hairs. This latter condition is much more obvious in the winter coat, and may be missed in areas with mild winters. This produces an unsightly animal (see photograph at the beginning of this article), but it also probably has a large fitness effect. The very similar rat-tail syndrome in Simmental cattle is known to result in calves that show significantly lower weight gains in the winter months, presumably because they are expending more energy just to maintain body heat. Moreover, a missing or reduced tail switch is considered undesirable in Texas Longhorns (see TLBAA or ITLA breed standards), is considered to detract from their appearance, and has a direct effect on fitness as well (the switch is important for keeping blood-sucking flies off the animal). A cow with a long switch will be bothered by flies and other insects much less than will a cow with a rat-tail.
At this point, the genetic basis of reduced-switch syndrome has not been definitively determined. It appears to be the result of a recessive allele (unlike rat-tail syndrome), and even homozygous animals may show variable degrees of expression. It does not appear to affect white hair, so white-haired animals may be homozygous for the allele and not show any signs of reduced switch or hair length (also true for the rat-tail syndrome). To date, reduced-switch syndrome has been observed mostly in herds in areas with cold winters, and so it may be expressed more strongly or simply be more obvious in areas with cold weather. In addition, limited pedigree analysis suggests that the trait appears in herds that trace back multiple times to a particular and popular Texas Longhorn bull that has been used widely for artificial insemination. (Or, possibly, to two such bulls that are themselves related). Therefore, the frequency of this trait appears to be related to inbreeding (and in particular, to linebreeding these popular bulls), although the trait may also appear in herds that have no significant inbreeding history.
I would like to receive information from any breeders who have observed this trait in their herds, together with the pedigree information for the affected animals. Responsible breeders will want to identify the source and genetic cause of this defect, so that all breeders can more easily select against it and maintain the standards of quality for the breed. I will keep all information confidential, but will use the pedigree information (with names of the animals and owners removed) to report on the genetic inheritance of this trait. Once the genetic basis is documented, then the breed associations can use this information to establish guidelines for assuring that bulls certified for artificial insemination are free of the defect.
Send information on this or any other genetic defects in Texas Longhorns to: DoubleHelix@att,net. By studying and selecting against defects, we can maintain the health and vitality of this historic breed.
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