An attempt to determine the pattern of inheritance of coat colors in hair sheep

V R Saldaña-Muñoz, G Torres-Hernández*, J M González-Camacho*, P Díaz-Rivera**, R González-Garduño*** and M Rubio-Rubio

Colegio Superior Agropecuario del Estado de Guerrero. Guerrero No. 81, Iguala, Gro. MÉXICO. *: Colegio de Postgraduados. 56230 Montecillo, Edo. de México. MÉXICO.
**: Colegio de Postgraduados, Campus Veracruz. Tepetates, Ver. MÉXICO.
***: Universidad Autónoma Chapingo. 86800 Teapa, Tabasco. MEXICO
glatohe@colpos.colpos.mx

Abstract

Data from a commercial flock of Pelibuey (70%) and Blackbelly sheep (20%), as well as crosses amomg them (10%) located in tropical Mexico, were utilized in an attempt to investigate the inheritance of coat color. Total numbers of ewes and lambs utilized in this study were 531 and 1410, respectively. The sheep were managed in a plantation of Manila mango (Mangifera indica) and grazed in pastures based on grasses such as Digitaria decumbens, Brachiaria brisanta, Brachiaria decumbens, Panicum maximum and Cynodon plectostachyus. According to their coat color, ewes and lambs were classified as white, brown, spotted, blackbelly and black. Only brown Pelibuey rams were utilized to mate the ewes in the study period. After phenotypic frequencies for color in ewes and after correlations among coat colors were calculated, three hypotheses to study the pattern of inheritance and body distribution of coat color were tested. The first two hypotheses attributed the gene action to two autosomic and independent loci, whereas the third hypothesis based such gene action to three loci with the same characteristics. The hypotheses were tested using the chi-square method.

Results suggest that white and brown are co-dominant and both dominant to the other colors; white and spotted are the same variable; blackbelly behaves as a heterocigote; black is a possible homocigote recessive and independent to white.

The three hypotheses tested were not sufficient to satisfactorily explain the inheritance of coat color in this population.

Key words: coat color, hair sheep, inheritance.

Introduction

There are two hypotheses that attempt to explain how hair sheep were introduced into Mexico. The first hypothesis (Berruecos 1977) says that sheep were introduced by spaniards directly from Africa during their trips while bringing slaves. The second hypothesis (Mason 1980) indicates that they were brought to the Yucatan penninsula from Cuba during the decade 1930-1940.

Most frequent coat colors of hair sheep in Mexico are white, brown and black, as well as certain combinations among them to produce the blackbelly pattern, golondrino (reverse blackbelly pattern: light tan undercolor and facial stripes on dark body), and payaso or harlequin (the blackbelly pattern on a spotted or roan body). Sheep producers in Mexico show a certain preference for the brown and white patterns; however, the literature on coat color of hair sheep is very scarce, even though the knowledge on coat color of hair sheep can be very important to determine their adaptability to varying environmental conditions, since skin and hair of terrestrial mammals, together with hair color, contribute to their climatic adaptation (Gordon 1979). For this reason, it is very important to characterize breeds of hair sheep based on their coat color and body distribution, as indication that they are purebreds. The objective of this investigation was to study the inheritance of coat color in a population of hair sheep, for which three working hypotheses based on two and three autosomic and independent loci were tested.

Materials and methods

Animals and management

Data for this study (1995-1998) were collected from a commercial sheep operation located in Tierra Blanca, Veracruz, Mexico, at 18^o 27' N and 96^o 21' W. The climate has been defined as hot, sub-humid, with summer rains. The average temperature is 27.3 ^oC and the average annual rainfall is 1568 mm (García 1973).

This flock started in 1990 with 294 ewes and 20 rams, mostly Pelibuey, that were acquired from the surrounding region. Since then, and due to the owner´s personal preference, only brown Pelibuey rams are purchased from outside this flock and utilized to mate the ewes. Replacement ewes are chosen from the same breeding flock. Sheep grazed in a plantation of 170 ha of Manila mango (Mangifera indica), in association with grasses such as Digitaria decumbens, Brachiaria brisanta, Brachiaria decumbens, Panicum maximum and Cynodon plestostachyus. There are two mating seasons;  one starts in May and the other in November. Before mating, ewes are supplemented with a concentrate ration with 14% CP and 3,000 Mcal/kg. Water and a mixture of mineral salts are provided ad libitum. Lambs are weaned at 90 days of age.

Statistical analysis

According to their coat color, ewes and lambs were classified as white, brown (from light to dark brown), spotted (mostly dark spots on white surface, in any proportion), blackbelly, and black. Firstly, phenotypic frequencies for coat color in ewes (n=531) and lambs (n=1410) were calculated (Table 1).

Secondly, the correlations among the different color groups of ewes and lambs, based on color frequencies, were also calculated by parametric methods (Steel and Torrie 1980).

Finally, three working hyphotesis based on two and three autosomic and independent loci were tested, utilizing Chi-square tests to prove the concordance between observed and expected results.

Hypothesis 1: the generation of the five color groups is attributed to two autosomic and independent loci; locus A has three alleles A1 (white), A2 (light brown or reddish), and A3 (dark brown or tan), they all being codominants among themselves; locus B has two duplicated alleles B1 and B2 (for color distribution). According to this, the following phenotypic distribution is obtained: white (11.11 %), brown (50.00 %), spotted (22.22 %), blackbelly (11.11 %), and black (5.56 %).

Hypothesis 2: there are at least two autosomic and independent loci involved in the generation of color and distribution patterns; locus C (for color) with three alleles C1 (white), C2 (light brown or reddish), and C3 (dark brown). Intensity of pigmentation (I: more intensity, i: less intensity) is modified by the action of the first locus and its dominance interallelic relation by the action of a second locus D, both being codominants. According to this, the following phenotypic distribution is obtained: white (11.11 %), brown (51.39 %), spotted (22.22 %), blackbelly (11.11 %), and black (4.17 %).

Hypothesis 3: the generation of color and distribution pattern is attributed to three loci (P, C and D); locus P (for pigmentation) has two alleles P1 (light brown or reddish) and P2 (dark brown or tan), conditioned to the presence of a second locus C (for complement) with two alleles C1 (complement of allele P1) and C2 (complement of allele P2). With these two loci four combinations and three phenotypes for color are formed (P1C1: brown, P2C2 :brown, P1C2: white, and P2C1: white). The action of a third locus D (for distribution) allows the different combinations to behave as codominants (spotted) or semidominants (intermediate and uniform coloration: brown). According to this, the following phenotypic distribution is obtained: white (12.50 %), brown (46.88 %), spotted (25.00 %), blackbelly (12.50 %), and black (3.12 %).

Results and discussion

Based on the frequencies of white and brown colors, as well as the high correlation between them (r = -0.71, Table 2), it is postulated that white and brown in this study are co-dominants between themselves and both dominant to the remaining colors, a fact that agrees with hypotheses postulated by Ponzoni (1992) and Ozoje (1998). Ponzoni (1992) has indicated that coat color in hair sheep is influenced by a series of alleles located at the Agouti locus; this way, white and brown behave as dominants to the other colors. In Pelibuey sheep, Berruecos et al (1974) have indicated that brown is dominant to the other colors, white being a recessive allele. Based on field data observations in Mexico, Bradford and Fitzhugh (1983) suggested that white was dominant to brown. In woolled sheep, ordinary white color is produced by the top dominant allele (A^wh) at the A-locus (Adalsteinsson 1983). This author indicated that the primary effect of this allele is to convert all black or moorit eumelanin pigment into tan or red phaeomelanin, and genes at other loci may then take over and dilute, reduce or eliminate the tan pigment, whereby a completely white animal is produced. A deep red type of tan pigment is observed in the French Solognot breed at birth, and the tan color in that breed has been shown to be due to the effect of the A^wh allele (Lauvergne 1975). The dominant brown color in the Karakul sheep is almost certainly a dark tan color caused by the A^wh allele (Adalsteinsson 1970).

The strong correlation between white and spotted (r = 0.94, P<0.01) indicates they are in fact the same variable, a fact that can be explained because the surface of the coat in the majority of the spotted sheep is white, spots being produced by modifier genes, a result that is not in agreement with the observations by Berruecos et al (1974), who indicated that spotted had an intermediate inheritance behaving as a heterocigote. There is evidence in wool sheep that the combination of white or tan at the Agouti locus with spotted at the Spotting locus is more extensively spotted than are other Agouti patterns of the same breed, and this phenomenon can greatly increase the overall whiteness of the animals (Sponenberg 1990).

Since blackbelly represents a coloration where light and dark brown are combined, the allele (or alleles) that produce light brown must be different to the allele (or alleles) that produce dark brown; therefore, it is postulated that blackbelly behaves as a heterocigote, which is in agreement with the hypothesis postulated by Ponzoni (1992). The blackbelly patern is in the Agouti series and appears to be recessive to self-color white or tan, but dominant to black (Lauvergne and Adalsteisson 1976), a hypothesis also mentioned by Ponzoni (1992).

The genetically opposite colors are white and black, with a non-significant negative correlation between them (r = -0.57). Based on this result, as well as the low black frequencies in ewes and lambs, it is postulated that white and black are opposite homocigote genotypes, possibly of different genetic origen; that is, black is a recessive homocigote, a result that has been indicated by Ponzoni (1992).

It was mentioned that only brown Pelibuey rams have been utilized for mating in the experimental population. This has certainly caused a change in the gene and genotypic frequencies of coat colors over the generations; that is, the genetic equilibrium which probably existed before is no longer present. In other words, the Hardy-Weinberg equilibrium (Bourdon 1997) in this population would be feasible only if Pelibuey rams of all possible coat colors were randomly utilized during mating.

Table 3 shows the results after the Chi-square tests for the three postulated hypotheses were performed. According to these results, the three hypotheses utilized did not explain the inheritance of coat color in this population.

Alternate hypotheses to be tested in the future will probably need to consider more loci; for instance, Meldrum (1992) pointed out that coat color in wooled sheep is attributed to four loci. However, the possibility to include more alleles within each locus should not be discarded, but this would require a more refined criteria in the definition and classification of coat colors in hair sheep.

Conclusions

• The three postulated hypotheses were not sufficient to satisfactorily explain the inheritance of coat color in hair sheep. The true mechanism of this inheritance, therefore, remains uncertain. Alternate hypotheses to test in the future will probably need to consider more loci on the one hand and more alleles within each locus on the other hand.
• It is recommended to utilize more refined criteria in the definition of coat colors of hair sheep in order to have uniform standards in the classification of this trait.

Acknowledgements

The authors are very grateful to Dr. Jorge Hubard Ocariz and sons, for allowing us to utilize data of their sheep operation for this research.

Literature Cited

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Received 20 March 2003; Accepted 30 September 2003

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