CHAPTER 2

QUALITATIVE GENETICS

T. A. OLSON ⁽¹⁾

Qualitative Traits are traits such as coat coloration, genetically controlled defects, polledness,and blood types that are controlled by a relatively small number of genes and are generally not influenced by environmental effects. Most traits of interest to breeders of beef cattle are quantitative traits, such as growth rate, muscling, and milk production, which are influenced by genes at many loci and also by environmental effects such as climate, nutrition, and disease. However, qualitative traits are also of interest and some importance. Before continuing with a discussion of the specific qualitative traits, it is perhaps useful to review some relevant genetic terminology.

Cattle have a total of 60 chromosomes, 29 pairs of autosomal chromosomes and the sex chromosomes, XX in females and XY in males. All the genes to be discussed in this chapter are located on the autosomal (non-sex) chromosomes and thus there will be two genes of each type in normal cattle, one on each of the paired chromosomes. A locus can be defined as the point on a given chromosome at which a gene is located. Different genes which can be found at the same locus are called alleles. Usually the existence of a particular locus is identified when a mutation or change from the normal condition occurs. The mutant gene(s) and the normal gene at a particular locus are said to be alleles. The mutant gene may be dominant, incompletely dominant, or recessive to the normal gene at a particular locus. If the mutant gene is recessive, as is the case for most genetic defects, an animal will not express the defect if it is heterozygous for the mutant gene. That is, an animal with one mutant recessive gene symbolized by d and one normal (or wild-type, signified by +) dominant gene symbolized by D⁺ is said to be heterozygous and will not express the trait. Its genotype can be symbolized as D⁺d. If the gene is dominant, like the polled gene, P, heterozygotes for the mutant gene, Pp⁺, will express the polled condition. A gene is incompletely dominant if the phenotype (visible or measurable expression of the genotype) of the heterozygote is intermediate between those of each of the homozygotes. The most commonly used example of incomplete dominance is the roaning gene of Shorthorn cattle. The roan gene, symbolized by R, is incompletely dominant over its wild-type allele, r⁺, in that in the heterozygote, Rr⁺, the pigmentation is removed from some of the hairs, whereas in the animal homozygous for R, RR, the pigmentation is removed from nearly all hairs, resulting in an essentially white animal.

Inheritance of Coloration and White Spotting

Coloration and spotting patterns of cattle have interested cattle breeders for many centuries. For example, the Lascaux cave drawings of cattle in France indicate white spotting patterns. When breeds were developed, efforts were made to produce a reasonably uniform coloration and spotting pattern within most breeds to aid in breed identity. The white spotting pattern of the Hereford breed is an example. In addition to the primarily aesthetic aspects of coloration, there is evidence that, under tropical conditions with high levels of solar radiation, animals with a lightly colored haircoat and darkly pigmented skin are better adapted. Most Zebu breeds, which are well adapted to tropical conditions, express such coloration.

In recent years, there have been additional reasons for interest in the inheritance of coloration and spotting patterns in cattle. These reasons include the development of composite breeds of cattle in which it may be desired to fix a certain coloration, the establishment of unique coloration and spotting patterns within established breeds which allow upgrading (i.e., solid black Simmental) and the existence of price discounts or premiums for feeder calves of various colorations. For example, solid black or black, white-faced calves may receive a premium regardless of actual breed composition; on the other hand, calves with Zebu breeding that express gray or black and brown colorations may receive a price dock, whereas black or red calves with the same Zebu breeding may not suffer one.

To most effectively discuss variation in coloration and spotting patterns in any species, it is useful to explain the effects of mutants relative to the wild type. For spotting patterns in cattle, the wild type is simply a solidly pigmented animal or lack of any spotting. Choice of a wild type for pigmentation is somewhat more difficult, but the coloration of the Aurochs of Europe, the wild ancestor of most (or all) Bos taurus breeds, seems appropriate. Aurochs were essentially a reddish brown to brownish black with a tan muzzle ring. There apparently was some variation in the degree of darkness and bulls were darker than cows. This coloration or a similar one is occasionally observed in some breeds today. Some Jersey, Brown Swiss, and Longhorn purebreds, as well as crosses of these breeds with red breeds and the Brahman with red breeds, produce the wild-type pattern. Animals with wild-type coloration tend to be darker at their extremities (head and neck, feet, and hindquarters), similar to bay coloration in the horse. Cattle with this type of brownish-black coloration at maturity are born a reddish brown and darken by 7 to 8 months of age as a general rule.

Adult bulls of several wild relatives of cattle, namely Bison and Banteng, have a similar dark brown coloration. In Banteng, adult cows are much lighter than bulls, having more of a tan color, whereas in the Bison, cows are colored like bulls.

There are no known linkages between loci influencing coloration and those influencing white spotting patterns in cattle. This indicates that any pattern of white spotting could be combined with any coloration. A common fallacy in genetic textbooks is to discuss a locus where the three possible phenotypes are red, roan, and white, whereas in reality the effects of the roaning gene in its heterozygous or homozygous state can act on any coloration. Similarly, the spotting pattern of the Hereford breed is not linked to the red color. Black animals displaying perfect Hereford markings can easily be produced.

Variations from Wild- Type Coloration

The most commonly observed variants from wild-type coloration in cattle are red and solid black. Other colorations of cattle are simply modifications of three basic colorations: black, wild type (brown-black), and red. Most variations from these basic colors involve lightening or removal of pigmentation. Good examples are the light red coloration of Limousin, the tan coloration of many Jerseys, and the almost complete removal of pigmentation of Chianina and some Brahman and Brown Swiss. Other mutant genes are responsible for the diluted colors of Charolais and Simmental. Genes responsible for the dilute colorations of these breeds dilute pigmentation uniformly over the entire body, whereas those found in Limousin, Jersey, Brown Swiss, Brahman, and Chianina tend to have differential effects on different parts of the body, especially the underline, poll, and along the back. Mutants thought to influence coloration of cattle are shown in table 2.1. These mutants will be discussed by locus and/or mode of action.

The E Locus (Black-Red)

The E locus is probably responsible for most of the variation in cattle coat coloration. Three alleles

present at this locus include: E^d, dominant black; E⁺, the wild-type allele responsible for most combinations of reddish brown and black; and e, recessive red. The order of dominance of these alleles is E^d > E⁺ > e and is complete. The E^d allele is found in animals born solid black or black and white spotted. Angus and Holstein breeds both carry E^d at a high gene frequency. Some Texas Longhorns carry E^d. Animals with E^d do not change coloration with age. Most gray animals are the result of the combined effect of E^d and dilution genes.

The E⁺, or wild-type allele, at this locus produces a reddish brown with varying amounts of black. The black pigmentation may be restricted to the head and neck, feet, hindquarters, and tailor may cover nearly the entire body with only an area of reddish brown over the ribs, a tan dorsal stripe, and a tan muzzle ring and poll. Bulls with wild-type coloration generally are darker than cows. Breeds which possess E⁺ are Jersey, Brown Swiss, Brahman, and Texas Longhorn.

The red color of Hereford, Simmental, Red Angus, and other red breeds is due to homozygosity for the recessive gene, e. There is considerable variation in the intensity of red coloration in cattle, from the dark red of Red Danish, Shorthorn, and Maine-Anjou cattle to the lighter shades of some Herefords and Guernseys. While there may be a major (single) gene influencing the darker red coloration, intensity of red coloration is, in general, quantitative. The desirable aspect of the red coloration, from a genetic standpoint, is that, as a recessive, it will always "breed true." The only exception would be the segregation of very light red or cream-colored animals from some light red parents.

Brindle coloration is observed in the Texas Longhorn and Normande breeds, and is often produced in" crossbreeding programs, especially those including Zebu breeds. The gene responsible for brindle coloration requires the wild-type coloration to be expressed. Nearly all brindle animals are E⁺E⁺Br_ or E⁺eBr_ in genotype. Animals carrying E^d or ee can carry the Br gene but will not express it. Brindling is produced often by crossing Herefords with Jerseys or Brahman because Hereford supplies Br and Brahman or Jersey E⁺, so most calves from such crosses have the genotype E⁺eBrbr⁺. The brindle pattern may vary from slight brindling on the head, neck, and hindquarters of an otherwise red animal to animals that are brindled over the entire body. An explanation for this variation is that brindled areas are apparently restricted to areas that would have been dark brown to black had the animal not possessed Br. Also, E⁺_brbr animals show great variation in the amount of darker pigmentation.

A dark coloration called patterned blackish (symbolized as Bp) is observed in red and white Holsteins and appears to be present in some darker Jerseys, Brown Swiss, and Brahman. Animals carrying Bp and ee are born reddish-colored and turn nearly black after about 6 months. This coloration can be distinguished from true wild-type by increased black pigmentation which does not appear to be sex-influenced (i.e., steers and heifers with Bp are as darkly pigmented as bulls). At maturity, only a tan muzzle ring and some lightening along the back distinguish them from true black (E^d) animals. The brindle gene, Br_, can act upon Bp_ee animals to produce a dark, brindled coloration.

The Dilution Mutants

In cattle, two types of dilution mutants affect the intensity of coloration. One mutant uniformly dilutes pigment over the entire body. These mutants are carried by Charolais, Simmental, and other breeds. Genes responsible for another type of dimunition of color intensity are found in Jersey, Brown Swiss, Brahman, Chianina, and related breeds. Mutants of the second type tend to remove more pigmentation on the underline and seem more effective at removing red than black.

Dilution mutants carried by Charolais and Simmental are clearly understood. Charolais homozygous for the gene Dc are essentially white. Crosses (F₁) of Angus and Charolais are generally light gray in coloration due to the effect of heterozygosity for DC (Dcdc⁺) acting upon black. Many Simmental, Gelbvieh, Scottish Highland, and Texas Longhorn carry a different dilution mutant, Db. This mutant is incompletely dominant to its wild-type allele, db⁺. Thus, genetically red animals heterozygous for Db (i.e., eeDbdb⁺) are light red in coloration and genetically black animals (i.e., E^d_Dbdb⁺) exhibit varying intensities of gray coloration. Red (ee) animals homozygous for Db are light yellow and black (E^d_), while animals homozygous for Db are light gray, and similar to heterozygotes for Dc. If it were desired to select Simmental bulls which would not produce gray progeny when crossed with black breeds, this could be easily accomplished by using only dark red Simmental bulls. Occasionally, the cross of Simmental with a black animal will result in a very dark, charcoal-colored calf. Whether the coloration of such animals is due to a different mutant from Db or is the result of modifier (darkening) genes is not known.

Genes responsible for the colorations of Jersey, Brown Swiss, Brahman, and Chianina are not well understood, due in part to the great variation in coloration within these breeds. It is clear that these breeds carry similar color mutants based on the colorations of crosses between them. Crosses between Jersey and Brahman resemble Jerseys and crosses between Brown Swiss and both Brahman and Chianina resemble Brown Swiss. Also, in Brown Swiss, gray Brahman, and Chianina there is little to no red pigmentation expressed, indicating the presence of a gene which acts similarly to the "chinchilla" mutant, ch, in other mammalian species. A recessive gene, aⁱ, is likely responsible for the lightened underline and overall lightening resulting in tan to light red Limousin, Guernsey, and Jersey. The darker extremities of Jersey are due to E⁺, whereas Limousin is ee, as are most Guernseys. Gray Brahman and Chianina also carry ai in its homozygous state but, in addition, are homozygous for Cch which removes the rest of the red pigment. This results in silver gray in the case of Brahman, which carries E⁺, and white in Chianina, which is ee. Jersey and Limousin which retain some red pigmentation are likely C⁺C⁺ at the C locus. The likely genotypes of many U.S. breeds are shown in table 2.2.

The usual coloration of Brown Swiss differs from that of Brahman (gray) in that its gray pigmentation is more uniformly distributed across the body, except for the underline, and is not usually confined to the extremities. Whether this difference is caused by an independent dominant mutant or a different allele at the A-locus is not known. A different A allele, a^w, seems more probable. Brown Swiss also seem to be homozygous for c^ch, which results in red only on the poll, if at all.

Uniquely colored, silver-brindled cattle seen as results of crossbreeding programs are likely caused by homozygosity for c^ch on a brindle coloration. Cream-colored animals, occasionally seen in crossbreeding programs involving Brahman and Brown Swiss with Red Angus, may be the result of heterozygosity for both aⁱ or a^w and c^ch and their wild-type alleles (A⁺aⁱC⁺c^chE⁺e).

The A and C locus mutants, at least when heterozygous, have little or no effect on animals carrying E^d except for a slight lightening to brown on the poll and along the back. Homozygosity for both a^w and c^ch acting upon black produced by E^d may produce a gray coloration similar to animals E^d_Db_ in genotype.

White-Spotting Mutants

Since the wild type for white spotting is a lack of spotting, any white spotting on cattle is due to a mutant or combination of several mutants. In general, the understanding of the genetic control of white spotting is complete except for a few patterns discussed later. Major mutant genes affecting spotting patterns in cattle are listed in table 2.3.

Much of the variation in spotting among U.S. breeds is due to a multiple allelic series at the S locus. The S locus contains at least three mutants in addition to the wild-type, non-spotting allele, S⁺. These mutants are SH which is responsible for the Hereford pattern when homozygous, S^P which is responsible for the Pinzgauer-type lineback pattern (also referred to as the Gloucester pattern after the rare English breed), and s, recessive spotting responsible for the irregular white spotting of Holstein, Guernsey, Ayrshire, Jersey, and Simmental. It is possible that one or more other spotting mutants (discussed below) are also located at the S locus, but this has not been clearly documented.

The order of dominance at the S locus is S^H = S^P>S⁺>s. Hence, S^H and S^P are co-dominant. Cattle carrying both S^H and S^P, such as Pinzgauer x Hereford crossbreds, express both white face due to S^H and a white dorsal stripe and white across the underline due to S^P. Both S^H and S^P are incompletely dominant to S⁺. Animals which are S^HS⁺, such as Angus x Hereford crossbreds, express a restricted Hereford pattern in that they have less white on the head than S^HS^H animals and have little or no white on other parts of the body. Likewise, animals of genotype S^PS⁺, such as Pinzgauer x Angus crossbreds, have much less white than animals with genotype S^PS^P. The white on S^PS⁺ animals can be restricted to a small amount on the tailor white on the tail head extending along the spine across the rump. Charolais also possess S^P in low frequency and patterns produced by S^P can be seen in animals with Charolais breeding lacking the Dc (dilution) gene. Texas Longhorns and the related Florida Cracker Cattle also possess S^P.

Because recessive spotting, s, is completely recessive to S^H, S^P, and S⁺, matings between animals with perfect Hereford markings have produced spotted (ss) progeny. Similarly, Angus bred to Holstein have produced spotted calves, indicating that Angus were S⁺s but did not show excessive white due to the dominance of S⁺ over s. The amount of white on animals that are ss varies considerably. Some Holstein cattle are 90-95% white, whereas others are 90-95% black. Such differences are due to highly heritable quantitative, modifying factors. These modifiers also influence the degree of expression of all other white-spotting patterns. For example, the amount of white on animals S^PS^P or S^PS⁺ may be increased from that usually observed in the Pinzgauer to cover nearly the entire posterior and part of the anterior half of the body, resulting in pigmentation only on the head, sides of the neck, and shoulders. Some Texas Longhorn and Florida Cracker cattle display such a spotting pattern. Apparently, there has been selection within the Pinzgauer breed for limited expression of S^P. Similarly, the amount of white could be increased or decreased on Herefords, although breed standards of acceptable amounts of white prevent extremes. Mutant spotting genes present at the S and other loci of many U.S. breeds are listed in table 2.4

The Simmental breed and a few Holsteins carry a gene which produces a white face that is distinct from S^H. The symbol used for this gene is Bl, for the blaze pattern it usually produces when heterozygous and in combination with S⁺. Since fullblood Simmentals are all spotted, they must be ss and the white facial spotting must be due to a gene at a locus independent of S. The genotype (for white spotting) of many Simmental x Angus crosses is Blbl⁺ S⁺s. Such animals are solid-colored with a white blaze on their face that usually does not include the eyes. In combination with ss, both BlBl and Blbl⁺ will usually have a solid white face and head.

Shorthorns, Texas Longhorns, and Florida Crackers carry a gene, R, responsible, when heterozygous, for roan color. Roan coloration is a mixture of pigmented and white hairs. When homozygous for R, a nearly entirely white animal is produced with some pigment expressed within the ears. While the most often observed roan is red, the roan gene acts equally effectively in the removal of any pigment. Thus, blue roans, E^d_Rr⁺, can be produced by crossing white or roan Shorthorn with Angus. The expression of the roan gene when heterozygous is highly variable, with some animals being roan over the entire body, while in others, roaning may be restricted to just the center of the forehead.

Texas Longhorn, Florida Cracker, English Longhorns, and some Scandinavian cattle possess what has been called the color-sided pattern. The gene responsible for the color-sided pattern is symbolized as Cs. Animals carrying Cs in the heterozygous state show extreme variation in its expression. The Cs gene is dominant and continues to be expressed in Florida commercial cattle after many generations of crossing with non-spotted breeds. A pattern commonly seen includes a very irregular white strip along the dorsal and ventral parts of the animal with roan areas along the edges and a roan or "dappled" pattern of white on the head. In other heterozygotes, the white stripe may be restricted to the rump and tail along with a little roaning on the head. Homozygotes for Cs often exhibit the "white park" pattern, that is, a nearly solid white animal with pigmented ears, a pigmented muzzle, and often with some pigmentation just above the feet. It has been observed recently that animals which carry both R and Cs but cannot be homozygous for either are white park in phenotype. Allelism between R and Cs has been suggested.

Perhaps the rarest white-spotting mutant, bt, produces the belted pattern of the Dutch Belted and Belted Galloway breeds. Belting is dominant and expresses itself with a white belt of varying widths around the midsection.

A major gene referred to by previous authors as the brockling gene or "pigmented legs" gene, Bc, interacts with apparently any white-spotting mutant, producing areas of pigmentation within areas that would be white if the Bc gene were not present. The most commonly observed expression of the brockling gene is in Hereford x Angus crossbreds where Bc from the Angus produces pigmented spots on the face which other wise would be white due to S^H. In ss animals, legs are usually white, but when an ss animal carries Bc as well, legs are pigmented to varying degrees. Ayrshire cattle with white spotted sides and legs which are pigmented are ssBc_ in genotype. Most non-spotted breeds possess a high frequency of the Bc gene, whereas it has been largely eliminated in spotted breeds with the exception of Ayrshire, Jersey, and Normande. A desirable function of Bc in Hereford crossbreds carrying S^H is that it usually results in pigmented areas surrounding the eyes, which is thought to reduce the likelihood of cancer eye. The so-called red-eyed condition in Hereford and Simmental cattle is very likely due to a different gene(s) which may be dominant, but this has not been well documented.

In some solid-colored breeds, white spotting along the underline, especially in front of the navel, can disqualify an animal from registration. Such spotting may be due to the presence of s. In many cases, however, such spotting is not caused by s and it is unclear as to the genetic mechanism involved. Selection against such animals should reduce the incidence of such spotting, but reduces the selection intensity possible for traits related to productivity.

In spite of the fact that inheritance of polledness and scurrying has been studied for over 70 years, certain aspects remain unclear. It is quite clear that a major gene, P, is common in cattle and dominant to its normal allele p⁺ such that Pp⁺ animals are polled. Angus are homozygous or nearly so for the P gene. The p⁺ (horned) gene appears to mutate to P at a fairly high frequency, as indicated by the periodic production of polled animals from horned parents. One recent example was the polled Simmental bull, Polaris, whose parents were both horned, fullblood Simmental. The polled mutant has also occurred in Herefords, resulting in the Polled Hereford breed, and in the Shorthorn, Holstein, and Brahman breeds. The polled gene also has been incorporated into breeds by selecting for it in successive generations of upgrading programs to horned breeds where polled cows were used as foundation dams.

The inheritance of scurs, small growths of horn-like material on the heads of polled animals where horns would have developed, has been difficult to clarify. The traditional explanation, that the expression of scurs is sex-influenced, was based on the fact that scurs tend to be more common on bulls than cows. For example, the cross of Brown Swiss and Angus results in scurred bulls and smooth polled (non-scurred) cows. In theory, polled bulls heterozygous for the scurring gene Sc and its allele sc⁺ would be scurred, whereas heifers of the same genotype would be smooth polled. Evidence from recent years, however, suggests that a bull must carry p⁺, in addition to Sc, to express scurs. Heifers are thought to have to be homozygous for the gene for scurs, ScSc, to express scurs. For the breeder interested in producing polled, non-scurred cattle, the simplest approach would be to never use scurred heifers and to only use scurred bulls if no acceptable non-scurred bulls are available. While bulls of thegenotype PPSc_ are supposed to be scurred, it would be safest to assume that any scurred bull is a likely carrier of the horned gene, p⁺.

Breeders of polled cattle are often interested in determining whether or not a polled bull is homozygous for the polled gene. Polled bulls whose sire and dam were both polled are sometimes referred to as "double-polled." This is unfortunate terminology as it may lead to the assumption that such bulls are homozygous for P when, in fact, if both the bull's sire and his dam were heterozygous polled, a polled bull would have only a 33% probability of being homozygous. There is a relatively easy procedure for testing a polled bull for the presence of p⁺. If a polled bull bred to horned cows produces seven or more polled progeny without a single horned calf, he can be assumed to be homozygous for P with less than a 1% probability of error (see table 2.5). Such a testing procedure would be recommended for all scurred bulls and bulls with one or both parents that were known to be heterozygous for p⁺

One additional complication is that occasionally bulls that are PP will produce a horned calf, particularly from cows with Zebu breeding. An explanation is that the presence of the "African horn gene" from the dam of the calf is epistatic to P, resulting in horned progeny from a PP sire. Support for the existence of the African horn gene was given in an Australian study of inter se mated crossbred populations of Brahman x Polled Shorthorn and Africander x Polled Shorthorn origin (Frisch et al., 1980). In both crossbred populations, horned bulls bred to horned cows produced polled calves; in fact, 23% of heifer calves from such matings in the Africander crossbred population were polled. This indicates that many of the horned parents carried the African horn gene. Thus, a smooth-polled bull that has sired 10 or more polled calves from horned Zebu cows should not be discarded as a carrier of p⁺ simply because he produced a single horned calf. To try to avoid complicating the testing procedure, Brahman crossbred and cows of dairy breeds ( which also have been reported to carry the African horn gene) should not be used as tester animals.

Defective, often dead, calves that are the result of mutant genes are produced occasionally in all breeds of cattle. Such calves may be produced as a result of inbreeding or linebreeding which has allowed the defect to be expressed because the recessive gene responsible for the defect was homozygous in the inbred, affected animal. Genes responsible for the vast majority of all simply inherited defects in cattle are recessive. The explanation for the lack of undesirable genes with a dominant mode of inheritance is simple; those which are dominant always produce an effect on the phenotype and are immediately eliminated, thus eliminating the gene. Not all defective calves are the result of genetic conditions. Some such calves are the result of environmental conditions and others due to unknown causes.

All breeds carry genes which, when homozygous, produce genetic defects. Some of the more commonly seen genetic defects, descriptions of their effects, and the breeds in which they have been observed are listed in table 2.6. Undesirable genes are produced from normal genes through mutation. If the animal carrying the initial undesirable recessive gene or one of its descendants which also carried the gene becomes popular, the frequency of that gene can increase greatly before the gene is detected. If the gene was initially low in frequency or non-existent in the population, the gene may not be detected until linebreeding of the descendants of the original heterozygous (carrier) animal has occurred. The opportunity for widespread distribution of undesirable genes is made greater through the use of artificial insemination and multiple-ovulation/embryo transfer systems. A single sire can produce thousands of progeny through AI and thus increase the frequency of the gene in the breed considerably. Such dangers are reduced in breeds where lists of known heterozygous sires are published.

The first genetic defects studied intensively were various types of dwarfism. Cooperation between university researchers and breeders of Hereford, Angus, and Shorthorn cattle and their respective breed associations in the study of the causes and control of dwarfism is credited by some as being responsible for "breaking the ice" that had existed between the two groups prior to that time. Today, university researchers and beef breed associations work very closely in breed improvement programs. The initial work with the pedigrees of "snorter" dwarf cattle indicated a recessive mode of inheritance for the defect. In spite of considerable effort by animal scientists, detection of heterozygotes for snorter dwarfism based on phenotype (physical measurements or x-rays) was not possible and thus it was necessary to detect carriers though progeny testing programs. A sire is a suspected carrier if his sire or maternal grandsire were known carriers of a particular recessive gene. Procedures used for testing a bull for a particular recessive gene are: (1) breed the bull to females homozygous recessive for the defective gene (if such females are viable and fertile); (2) breed the bull to known heterozygous females; and (3) use a combination of homozygous and heterozygous females. Heterozygotes may be identified on the basis of having produced affected (homozygous recessive) progeny or by having an affected parent. If a sire is being tested for the presence of recessive spotting, s, or the gene allowing normal horn growth, p⁺, homozygous recessive females are fully fertile and readily available; simply use spotted or horned cows, respectively, as tester females. The production of seven solid-colored (S⁺_) progeny sired by a Simmental bull being selected for non-spotting and from spotted cows (ss) strongly indicates that the bull in question is free of s. This statement can be made as the probability that a bull that was heterozygous for s (S⁺s) would produce seven solid-colored (S⁺_) calves from spotted cows due to chance is only .0078, less than one chance in 100. Similarly, the production of seven polled calves from horned cows and no horned calves is very strong evidence that the polled bull in question is homozygous (PP). Degrees of accuracy for various numbers of progeny produced by different types of tester females are shown in table 2.5.

When heterozygous females must be utilized due to lethality, sterility, or unavailability of homozygous females, more test matings are required. Production of 16 normal progeny from a suspect bull and known carrier cows indicates that the bull is homozygous with only a 1% chance of falsely declaring a heterozygous bull clear of the undesirable gene. As an example, an Angus bull whose sire was a carrier of osteopetrosis would have a 50% chance of being heterozygous for the gene. Such a bull could be tested by mating him to at least 16 cows which had previously given birth to calves with osteopetrosis, thus proving them to be carriers. If at least 16 calves were born and all were normal, there would be only a 1% chance that the sire was a carrier. Often, it may be difficult to locate 16 known carrier cows. In such case, those cows available could be superovulated and their embryos implanted into recipient females. It may not even be necessary to allow the resulting fetuses to go to term to determine their status. With the stage of gestation dependent on the defect in question, recipient females could be slaughtered and their fetuses recovered and evaluated to speed the process of evaluation.

In certain circumstances, it might be useful to use a combination of homozygous and heterozygous females as testers due to availability within a herd. For example, one is able to test a bull with similar accuracy with four progeny from homozygous females and seven progeny from heterozygous females as one could with seven matings to homozygous recessive matings (see table 2.5). The probability of detecting a carrier bull can be determined for any combination of homozygous and heterozygous tester females by calculating [(1-(.5)^n1*(.75)ⁿ²)], where n1 is the number of progeny from homozygous tester females and n² is the number of progeny from heterozygous tester females.

A problem with both the homozygous recessive and heterozygous female testing procedures is that they test only for the specific recessive gene carried by the tester females. Thus, it is possible (although unlikely) that a bull could be tested free of the osteopetrosis gene and then be found to carry the gene responsible for syndactylism (mulefoot) years later, after siring many heterozygous progeny. Matings of a bull to his daughters offers the opportunity to declare a bull free of all recessive genetic defects with a similar degree of accuracy as the previous tests, a probability of detection of 99% if 35 sire-daughter matings are made. The level of accuracy with fewer matings is shown in table 2.5. This procedure, of course, has certain limitations including the delay in evaluating the bull until his daughters are of breeding age and the resulting inbred progeny which may be less productive and less valuable than outcross animals even if all normal progeny are produced. Also, it should be emphasized that 35 different daughters must be utilized to test the sire. If individual daughters produce more than one progeny, more than 35 total progeny of the sire-daughter matings are required. If two progeny are produced from each daughter, a total of 19 daughters and 38 total progeny would be required to retain the same accuracy. Comparable numbers of daughters and total progeny produced would be 14 and 42 for 3 progeny per daughter and 11 and 44 for 4 progeny per daughter.

Not all genetic conditions are as simply inherited. The double-muscled condition has been described as being recessive and, indeed, seems to be inherited in this manner in some breeds. In Europe, however , double-muscled bulls are maintained in AI studs for the purpose of producing veal calves from dairy cows. Obviously, this effort would be futile if muscling were not increased in heterozygous calves. A recent Canadian study (Arthur et al., 1989) has shown that heterozygotes for double muscling had higher cutability and muscle:bone ratios than homozygous normal animals. This finding confirms the partial dominance of the gene(s). The expression of the homozygous double-muscled condition seems also to be conditioned by the genetic composition of the animal. For example, homozygous double-muscled Angus calves are usually more stress susceptible and less vigorous than normal calves, whereas in the Piedmontese breed, the modifying genes of this breed's genetic composition seem to allow for much more viable homozygotes, perhaps by delaying the onset of the extreme muscling and reducing muscling in cows relative to bulls. Belgian Blue cattle which also appear to be homozygous for double muscling have recently been imported into the United States. It is unclear whether the Belgian Blue breed possesses the modifying genes present in the Piedmontese.

Other commonly observed congenital (present at birth) defects in cattle include umbilical hernias and cryptorchidism (retained testicles). While in some studies hernias seem to be inherited as a dominant, other studies have not supported this conclusion. Perhaps it is most appropriate to conclude that there is genetic control over hernias and cryptorchidism, but that they are not simply inherited. Fortunately, umbilical hernias are fairly rare in cattle, and when they do appear, affected animals usually are not surgically corrected and then used for breeding purposes. Monorchid (unilateral cyptorchid) bulls are almost always slaughtered. Perhaps this is sufficient selection pressure against these conditions until their mode of inheritance is better understood or their incidence increases.

Ideally it would be desirable to eliminate all genes which produce genetic defects in cattle. Unfortunately, this is not feasible because, as was previously discussed, these genes are recessive in mode of inheritance and thus it is usually impossible to detect heterozygotes. The only time that an animal is proven to be a carrier (heterozygous) is by the production of affected. (homozygous recessive) progeny. Unfortunately, when an undesirable gene is at low frequency in a population, it is unlikely that two heterozygotes will be mated and even then only 25% of the progeny will be homozygous recessive. Thus, it is highly probable that most animals that are heterozygous for rare undesirable genes will never produce undesirable progeny and, therefore, never be culled. The existence of rare undesirable genes in a breed often is determined only when inbreeding is practiced. Once a genetic defect has been discovered in a breed, the question then is how to proceed. The dwarfism problem in Hereford and Angus cattle was brought under control by the elimination of entire bloodlines which carried the dwarf gene. This approach, while effective, results in the culling of homozygous normal animals as well as heterozygotes. If the bloodline in which the genetic defect occurs is superior for important economic traits, such a culling program could do considerable harm to the breed in question. If the bloodline (for example, a particular sire that is heterozygous for a genetic defect and his progeny) is sufficiently superior for traits of economic merit, a program of progeny testing could be established to identify descendants of the bull that are probably free of the genetic defect.

Because complete elimination of recessive defects in breeds of cattle is not feasible, it is necessary to develop mechanisms to prevent large increases in the frequencies of undesirable genes. This can be accomplished through prompt reporting of cases of suspected genetic defects which can allow culling or at least more restricted use of the affected animal's sire and dam once it has been established that the affected calf is indeed homozygous for an undesirable gene and that it is the progeny of the parents of record. The latter can be done through blood typing procedures (discussed later), while the former will require pedigree analysis of the affected calf and its clinical examination. Many types of congenital defects may be simply due to developmental abnormalities caused by environmental pollutants or other usually unknown factors, even if the same defect may be caused by homozygosity for a recessive gene. Arthrogryposis associated with cleft palate is one example. Due to possible environmental causes of arthrogryposis, a bull should not be culled for producing a single calf with arthrogryposis unless evaluation of the pedigree indicates that linebreeding or inbreeding was involved and/or the bull is of a breed in which the undesirable gene is commonly found.

Once a sire has been proven to carry an undesirable gene, this fact should be made known to all breeders who might consider using the bull. Dissemination of this information is best handled through breed associations. While many bulls are sent to slaughter soon after identification of their status as heterozygotes, others, if they are sufficiently genetically superior, continue to be used. Breeders who use such bulls must be particularly careful how they mate the progeny as 50% would be expected to be carriers.

The primary use of blood typing in beef cattle breeding programs today is parentage determination. Calves born as a result of embryo transfer are generally required to be blood typed, along with their sire and dam, to be registered. In most breeds, bulls to be used by artificial insemination must be blood typed to allow later verification of parentage where the wrong unit of semen may have mistakenly been used. With regard to natural service sires, occasionally two bulls will breed a cow during the same estrus. When such an event happens, it is necessary to blood type both bulls, the cow, and the resulting calf to determine the likely sire of the calf. It is even possible for the calves of two cows to have been switched if cows, especially young ones, calved at the same time in close proximity. If there is reason to suspect such an event, blood typing can often resolve the question. In recent years, there also have been questions about breed purity. The possibility of Holstein and Chianina influence in some Angus and Holstein and Simmental influence in Polled Hereford has been raised in recent years. In some of these cases, blood typing can give evidence to support or refute such accusations.

There are over 10 different blood-group systems used in attempting to resolve these questions. Each system represents a different locus at which blood-group factors or groups of blood-group factors, called phenogroups, are inherited like alleles. The number of phenogroups present varies from two for several systems to over 600 for the B system. Blood factors tested represent antigens found on the surface of red blood cells. When red blood cells which carry a particular blood factor are injected into an animal not carrying that factor, the animal can produce antibodies specific for the blood factor (antigen) involved. When red blood cells carrying a particular blood factor are mixed with a serum containing antibodies for that factor (a reagent), the antibodies in the reagent attach themselves to the antigens on the red blood cells, resulting in hemolysis (breakage of the red blood cells). Usually, hemolysis requires the presence of complement (fresh rabbit serum) to stimulate the reaction. The B system is the most useful in parentage and other tests due to the extreme number of phenogroups (over 600) found at this locus. As many as 200 different phenogroups have been found in a given breed. Examples of B system phenogroups found within the Brahman breed are listed in table 2.7. Phenogroups, such as BI₁O₁QD'I' and PY₂A' of the Brahman breed, act as alleles in that these combinations of six and three blood factors are each inherited as a group. By this it is meant that if a bull carries BI₁O₁QD'I'(B340) and PY₂A'(B287), half of the progeny of this bull will receive B340 and half B287. All progeny of this bull must carry either B340 or B287; if potential progeny do not carry either, they are excluded as progeny of this sire. This is the principle of parentage testing using blood-grouping systems; a bull is excluded as the sire of a particular animal if the possible offspring of that sire does not carry one of the potential sire's phenogroups at all loci tested. This principle can be clarified using an example. In this hypothetical example, it is assumed that both Bull A and Bull B had bred Cow C and Calf X was the result.

In this case, Bull B is excluded as the sire of Calf X based on the B, C, and R'-S' systems. In the B system, Calf X has phenogroups B340 and B287, neither of which is possessed by Bull B, thus excluding him as a potential sire. Similarly, calf X is homozygous for C2 which excludes Bull B based on the C system as he does not carry C2 and since Cow C carries only S' at the R'-S' locus, she must have provided the S' allele of Calf X which forces the sire to have provided the R' allele, which only Bull A possesses. It should be emphasized, however, that the exclusion of Bull B does not prove that Bull A is the sire of the calf; it does show that Bull A could have been the sire and that Bull B could not have been the sire.

Because each breed has its own particular frequencies of phenogroups within each system, it is sometimes possible to identify the breed of an animal simply from its blood type. This is particularly true for the B system where some phenogroups are very common in one breed and nonexistent or extremely rare in another breed. For example, the most common phenogroup in the Texas Longhorn breed, BG₂ KA'O'A"B"(B506), is not found in most other breeds and should it be observed in a Hereford animal, it would strongly indicate that the Hereford with B506 contained some Longhorn breeding. In recent years, there have been some Polled Hereford cattle questioned as to being part red and white Holstein. Since the s gene of the red Holstein is recessive to S^H of the Polled Hereford, as is the p⁺ allele of the Holstein, it would not be possible to determine conclusively by visual observation whether the animal was part red and white Holstein. The presence of one of the common Holstein blood groups B3, B84, B202, etc., however, would lead one to strongly suspect that the animal was indeed part Holstein. It could not, however, be proven to be part Holstein as most of the common Holstein B system phenogroups have also been found at low frequencies in Herefords. It is because of such overlap of phenogroups from one breed to another that it may be difficult to document clearly the introduction of another breed unless the reported parents and perhaps grandparents of the animal with an unusual phenogroup combination can be examined.

In addition to red blood cell antigens, blood samples from cattle may be tested for other genetic markers by use of starch-gel electrophoresis. These markers include hemoglobin types, transferrin (-globulin) types, and carbonic anhydrases (red blood cell enzymes). These markers can be useful in parentage testing and are currently being used to test for bison markers in Beefalo cattle. Cattle have two alleles for hemoglobin type, A and B, and three associated phenotypes, A, B, and AB. The symbol bi has been used to symbolize the allele responsible for the hemoglobin type of bison. The hemoglobin produced by bi is electrophoretically distinct from that of cattle and thus animals typed as A//bi or B//bi can be said to be composed of both cattle and bison germ plasm. Bison also have been shown to have a transferrin type which is distinct from the four types in cattle and to have three electrophoretic forms of carbonic anhydrases, two of which are distinguishable from the two types in cattle. In addition to these three genetic markers, reagents are available which can differentiate between the red blood cell antigens of bison and cattle and classify animals as B, C, or BC where B indicates a reaction with bison-specific markers, C indicates a reaction with cattle specific markers, and BC indicates that the red blood reacted with both the cattle and bison specific reagents.

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1. Department of Animal Science, University of Florida, Gainesville.