Cattle Genetics

QUALITATIVE GENETICS - part 2
T. A. OLSON

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.

Table 2.1 Mutants Influencing the Coloration of Cattle
Gene

Symbol

 



Description
Dominant or recessive

Breed distribution
Ed solid black at birth dominant Holstein, Angus, etc.
E+ brown-black with darker extremities, bulls are darker than cows and calves are born a reddish brown (wild type) -- Jersey, Brown Swiss, Brahman
e red without any dark pigmentation recessive to Ed and E+ Hereford, Red Angus, Guernsey, Simmental, etc.
Br brindle, alternating stripes black and red pigmentation dominant to lack of brindling Most solid red and black breeds
Bp blackish coloration similar to wild type pattern but darker and not influenced by sex dominant Holstein, Jersey, Brown Swiss, Brahman
Dc heterozygotes: strong dilution of black to light gray, red to light cream; homozygotes are white or nearly white dominant Charolais
Db heterozygotes: moderate dilution of black to gray, red to light red; homozygotes are lighter incomplete dominant Simmental, Scottish Highland, some Gelbvieh
aw removal of most red pigment and a part of the black pigment while causing more uniform distribution of black pigmentation, especially across the sides of the animal recessive Brown Swiss
ai removal of red and black pigmentation, particularly red along the underline, along the back (dorsal stripe) resulting in tan to fawn coloration recessive Limousin, Jersey, Brahman, Chianina
cch removal of red pigmentation without an effect upon black pigment generally recessive Brown Swiss, Brahman, Chianina

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: Ed, 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 Ed > E+ > e and is complete. The Ed allele is found in animals born solid black or black and white spotted. Angus and Holstein breeds both carry Ed at a high gene frequency. Some Texas Longhorns carry Ed. Animals with Ed do not change coloration with age. Most gray animals are the result of the combined effect of Ed 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 Ed 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 (Ed) 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., Ed_Dbdb+) exhibit varying intensities of gray coloration. Red (ee) animals homozygous for Db are light yellow and black (Ed_), 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, aw, seems more probable. Brown Swiss also seem to be homozygous for cch, 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 cch 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 aw and cch and their wild-type alleles (A+aiC+cchE+e)

The A and C locus mutants, at least when heterozygous, have little or no effect on animals carrying Ed except for a slight lightening to brown on the poll and along the back. Homozygosity for both aw and cch acting upon black produced by Ed may produce a gray coloration similar to animals Ed_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.

Table 2.3. White-Spotting Mutants in Cattle


Symbol


Description
Inheritance relative

to wild-type

Breeds

 

possessing

SH Hereford pattern, white face, belly, feet, and tail, often with white stripe over shoulders when homozygous. Only white face is present in SHS+ incomplete

 

dominant

Hereford, Braford, Beefmaster
SP Sides of body pigmented; variable amounts of white appear along dorsal and ventral areas extending forward from tail and rump incomplete

 

dominant

Pinzgauer, Charolais, Longhorn, Florida Cracker
s Piebald; irregular areas of pigmented and white; feet, belly, and tail usually white recessive Holstein, Guernsey, Jersey, Simmental, Ayrshire, Maine-Anjou and others
R Homozygote: nearly white except of small amounts of pigmentation on the edges of the ears

 



Heterozygote: Interspersed pigmented and white hairs

incomplete

 

dominant

Shorthorn, Belgian Blue
Bt Belt of white of various widths around paunch dominant Dutch Belted, Galloway
Bl White head, often a blaze when heterozygous, without associated white areas on other parts of body produced by Hereford pattern incomplete

 

dominant

Simmental, Holstein (?), Gornigen (European)
Bc Areas of pigmentation within areas of white spotting produced by other mutants dominant nearly all solids colored breeds plus the Shorthorn, Ayrshire, and Normande
Cs Homozygote: white body with pigmented ears, muzzle, and feet (white park pattern)

 



Heterozygote: color-sided pattern, white dorsal stripe with irregular edges (roaned) and white roaning on head; roaning may be confined to head, rump, and tail

incomplete

 

dominant

Texas Longhorn, White Park, British White, Florida Cracker, English Longhorn

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, SP 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 SH = SP>S+>s. Hence, SH and SP are co-dominant. Cattle carrying both SH and SP, such as Pinzgauer x Hereford crossbreds, express both white face due to SH and a white dorsal stripe and white across the underline due to SP. Both SH and SP are incompletely dominant to S+. Animals which are SHS+, such as Angus x Hereford crossbreds, express a restricted Hereford pattern in that they have less white on the head than SHSH animals and have little or no white on other parts of the body. Likewise, animals of genotype SPS+, such as Pinzgauer x Angus crossbreds, have much less white than animals with genotype SPSP. The white on SPS+ 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 SP in low frequency and patterns produced by SP can be seen in animals with Charolais breeding lacking the Dc (dilution) gene. Texas Longhorns and the related Florida Cracker Cattle also possess SP.

Because recessive spotting, s, is completely recessive to SH, SP, 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 SPSP or SPS+ 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 SP. 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

Table 2.4 Mutant Genes Affecting White-Spotting Patterns of U.S. Breeds of Cattle
Breeds S-locus Cs R Bl Bc
Angus

Ayrshire

Belgian Blue

Brahman

Brown Swiss

Charolais

Chianina

Gelbvieh

Guernsey

Hereford/Polled Hereford

Holstein

Jersey

Limousin

Pinzgaur

Santa Gertrudis

Simmental

Shorthorn

Texas Longhorn

+1

 

s

s

+,s

+

SP,+,s?

+

+

s

SH

s

+,s

+

SP

+

s

s

+,SP,s

+

 

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Cs,+

+

 

+

R,+

+

+

+

+

+

+

+

+

+

+

+

+

+

R,+

R,+

+

 

+

Bl,+

+

+

+, Bl(?)

+

+

+

+

+, Bl?

+

+

+

+

Bl,+

+

+, Bl(?)

Bc,+

 

Bc,+

+

Bc,+

+

+,Bc?

+,Bc?

Bc,+

+

+

+

Bc,+

Bc,+

+

Bc,+

+

Bc,+

Bc,+

1Indicates that only wild-type allele is present at locus.

The Simmental breed and a few Holsteins carry a gene which produces a white face that is distinct from SH. 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, Ed_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 SH. 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 SH 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.