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Linkage Between Pea Comb and Melanotic Plumage Loci in Chickens
Linkage Between Pea Comb and Melanotic Plumage Loci in Chickens R. D. CRAWFORD Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0 (Received for publication January 13, 1986)
1986 Poultry Science 65:1859-1862
INTRODUCTION
There is continuing interest and activity in mapping the gene loci of chickens. The value of these studies is rapidly increasing as genetic engineering becomes a reality. Investigation of the pea-comb locus on the short arm of Chromosome 1 has been especially productive (Bitgood, 1985). In the present study, briefly reported earlier (Crawford, 1985), the map of Chromosome 1 is further extended by the observation of very close linkage between the pea-comb and melanotic plumage loci.
MATERIALS AND METHODS
The stock under study had been maintained as a closed flock for about 20 generations. It has been traced to a male of unknown origin carrying genes for pea comb and blue egg, mated with Brown Leghorn and Rhode Island Red females. Comb type and plumage were variable in early generations of the population, but the birds were gradually selected for the pea-combed wheaten phenotype. Despite selection, the black phenotype continued to segregate within the population as if it were an autosomal recessive trait, usually appearing in combination with single comb. For several generations prior to the present study, the stock had been held in two separate groups; one of them bred pure for single comb, black plumage, and blue eggshell; the other segregated for pea and single comb, black
and wheaten plumage, and blue and brown eggshell. The two groups were crossed, using 12 sires and 27 dams, to produce F, progeny known to be heterozygous at comb and plumage pattern loci. All progeny were wheaten and pea-combed; certain progeny were also heterozygous at the shell color locus. The F, birds were flock-mated inter se or backcrossed to parental types in three combinations. Eight F[ males were bred to 30 black single comb females, yielding coupling double backcross data for linkage between comb and plumage pattern loci. Thirteen F, males were bred to 30 F, females; this yielded coupling double intercross data for the same loci. Two F, males and 9 F] females, heterozygous at the shell color locus, also yielded coupling double intercross data for comb and plumage pattern, but in addition, they yielded data for a threepoint linkage test of comb, plumage pattern, and shell color loci. Sixteen consecutive weekly hatched were produced from these three matings. All chicks were identified and reared in batteries to 6 weeks of age, when sex, comb type, and juvenile plumage pattern were recorded. Females from the three-point linkage test were kept until sexually mature; they were housed in individual cages and at least two eggs were examined from each to determine whether shells were blue or brown. Data for segregation and linkage were analyzed using the methodology of Green
1859
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
ABSTRACT Segregation for pea vs. single comb and blue vs. brown eggshell has persisted in a closed population for about 20 generations. Most pea-combed birds have wheaten plumage, and single-combed birds are black; recombinants have only rarely been seen indicating close linkage between comb and plumage loci. The population appears to be homozygous for an E locus wheaten allele, probably ey. Wheaten and black phenotypes are caused by segregation of melanotic locus (Ml) alleles. In a coupling double backcross, there were 190 pea wheaten, 192 single black, and 1 single wheaten progeny. In a coupling double intercross, there were 1084 pea wheaten, 370 single black, 1 pea black, and 4 single wheaten progeny. Map distance between loci is estimated as .31 ± .23 units. There was 1 comb and plumage recombinant among 136 females examined for blue vs. brown shell color, giving a tentative locus order on Chromosome 1 of Ml-P-O. (Key words: linkage, pea comb, melanotic, blue egg, chicken)
CRAWFORD
1860
(1963). Her procedures are currently the most used in avian linkage studies, but they have been designed for linkages ranging from 50% to 1%. The linkage reported here is much less than 1% and its precise measurement may be biased by analysis method. RESULTS AND DISCUSSION
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
Pea comb and single comb phenotypes are easily distinguished in day-old chicks and older birds. The gene for pea comb is incompletely dominant autosomal (Hutt, 1949). In adult males and females, the two pea comb genotypes can usually be distinguished by the elevated central blade of the comb in heterozygotes, but genotype identification is not very accurate in day-old or juvenile birds. The blue egg trait is completely dominant autosomal (Hutt, 1949), and there is no difficulty in classifying hens by color of their eggshells. Genetics of the plumage patterns in the study population is less well-understood. At hatching, all chicks are cream-colored, without any regular markings, but primary and secondary pinfeathers in the wings are pigmented. In juveniles and adults of the wheaten phenotype, males are almost identical to classical black-breasted reds. Females are cream-colored, with black in the wings and tail, and they exhibit variable amounts of black in the cream areas of the plumage. In juveniles and adults of the black phenotype, males are glossy black with pale undercolor; they may exhibit some red markings in the neck, saddle, and wings; their beaks and shanks are not melanized. The females are dark charcoalgrey with pale undercolor, and there may be some red in the hackle; beaks and shanks are not melanized. Two genetic kinds of wheaten are recognized (Brumbaugh and Hollander, 1965). Dominant wheaten (eWh) and recessive wheaten (ey) are allelic with almost identical phenotypes. The recessive form is characteristic of Rhode Island Reds (Brumbaugh and Hollander, 1966), and the dominant form is found in New Hampshires (Smyth, 1970). The wheaten phenotype studied here is probably one of these forms. There has been considerable confusion about the genetic basis for black plumages that appear to have an autosomal recessive mode of inheritance. Moore and Smyth (1971) have reported that recessive black plumage is actually caused by an incompletely dominant autosomal gene Ml (melanotic). It appears to have little effect
on the color of the chick down. Its action in juvenile and adult plumages is to extend eumelanin into areas that would normally be red, but its expression can vary depending on which alleles are present at the E locus. Moore and Smyth (1971) studied the effects of Ml in the presence of several E locus alleles, including dominant wheaten (ewh). They report that in dominant wheaten males, melanotic heterozygotes have a dark wild-type pattern, and homozygotes are nearly all black; in dominant wheaten females, melanotic heterozygotes have the classical wheaten female pattern, and melanotic homozygotes are wheaten with dark-brown back and hackle. In the present study, it is thought that the population is uniformly homogygous for an E locus wheaten allele and that wheaten and black phenotypes in the stock are controlled by segregation at the melanotic locus. There are two reasons for believing that the actual E locus allele is recessive wheaten (ey): the birds were derived in part from Rhode Island Reds, which are known to be e>le> (Brumbaugh and Hollander, 1966), and melanotic expression in females is quite different from that described by Moore and Smyth (1971) in females known to be ewh/eWh. If this interpretation is correct, then the findings of Moore and Smyth (1971) can be extended. There is now indication that in the presence of ey/ey, males and females that are homozygous melanotic are black, and heterozygotes and homozygous nonmelanotic birds have the wheaten phenotype. Hence, the black phenotype appears recessive even though it is caused by a dominant gene. Smyth (1985, personal communication) has examined colored photographs of wheaten and black phenotypes from the present study, and agrees that the blacks are probably melanotic (Ml). During the course of this study the black phenotype was thought to be truly autosomal recessive, and no attempt was made to distinguish between presumed homozygotes and heterozygotes among the wheaten birds. If differences actually existed, they were very small and went unnoticed. Subsequently, an attempt was made at phenotypic separation of two melanotic locus genotypes in the wheaten plumage class. Because of the extremely close linkage, to be described later, between the pea comb locus and the melanotic locus, it was assumed that peacombed homozygotes would also be nonmelanotic homozygotes, and pea-combed heterozygotes
PEA COMB AND MELANOTIC LINKAGE
1861
TABLE 1. Linkage between pea comb and melanotic loci Coupling double backcross = pea wheaten
single black
single black
Pea wheaten Pea black Single wheaten Single black
190 0 1 192
single black
X2 pea/single = .0234 X2 wheaten/black = .0026 X2 linkage = 379.0104**
Coupling double intercross = pea wheaten
pea wheaten
Pea wheaten Pea black Single wheaten Single black
1084 1 4 370
single black
X2 pea/single = .3218 X2 wheaten/black = .1428 X2 linkage =1473.7035**
Map distance, comb and plumage = .31 ± .23. P<.01.
would be melanotic heterozygotes; hence, comb genotype should be a good indicator of plumage genotype. Four aged males, shown by both comb morphology and breeding test to be PIP, were compared with four aged males similarly shown to be Plp+. Both had the typical male wheaten plumage pattern, but the heterozygotes had black lacing on the red hackle feathers and black lacing on the saddle feathers. It is assumed that the darker kind were melanotic heterozygotes and the others were nonmelanotic homozygotes, in agreement with Moore and Smyth (1971). Similar comparisons using aged females were inconclusive. Three were no consistent relationships among comb morphology, comb genotype determined by breeding test, and darkness of the plumage. Juvenile males and females were also examined. Comb genotype could not be accurately determined from comb morphology. There was continuous variation in plumage coloring, from near absence of black to very heavy pigmentation in the usual red or wheaten areas of both males and females. Based on these limited observations, it appears that except for adult males, melanotic heterozygotes cannot be distinguished visually from nonmelanotic homozygotes in birds that are homozygous for the reces-
sive wheaten allele. Linkage Observations. Data on the linkage observed between pea comb and melanotic loci are presented in Table 1. They were analyzed using the methods of Green (1963). Map distance between comb and plumage loci was calculated to be .31 ± .23 from combined data for the coupling double backcross and the coupling double intercross. Green's (1963) methodology provides accurate measurement for linkages as close as 1%, but not for any closer than that. Hence, the map distance calculated here may lack precision, although there is no doubt that it is much less than one map unit. Data for a three-point linkage test involving the blue shell locus are presented in Table 2. These data were derived from an intercross, an inefficient way of measuring linkage, as it requires very large numbers of observations to be accurate. In this study, observations were relatively limited (n= 136). However, the data have some value in estimating order of loci on the chromosome, and they relate to earlier reports of linkage in the literature. The map distance between comb and shell color loci was calculated to be 2.39 ± 8.54, and the distance between plumage and shell
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
single black
1862
CRAWFORD TABLE 2. Linkage with blue shell locus
Three-locus intercross =pea wheaten brown shell
pea wheaten brown shell
Zx single black blue shell
67 38 0 1 0 0 30 0
X2 pea/single X2 blue/brown X2 linkage
= = =
.3529 .6274 16.0130**
X2 wheaten/black = X2 blue/brown = X2 linkage =
.6274 .6274 15.1111**
Map distance, comb and shell = 2.39 ± 8.54 Map distance, plumage and shell = 2.50 ± 8.54 P<.01.
color loci was 2.50 ± 8.54. Standard errors in both estimates are very high, reflecting difficulties of analysis caused by limited numbers of observations in some classes and none in others. Using a translocation as a marker, Bitgood (1985) has demonstrated the linear order of pea comb and blue shell loci, known to be on the short (p) arm of Chromosome 1. The order is centromere-pea comb-blue shell. It is of interest to consider the placement of the melanotic locus in relation to these three loci. In the present study there was only one comb and color recombinant among the 136 birds, which caused the estimate of distance between plumage and shell color loci to be larger than that between comb and shell color loci.This result suggests that the order on Chromosome 1 may be: centromeremelanotic-pea comb-blue shell. Confirming evidence is needed. Data from the three-point linkage test are also of interest in relation to earlier reports about map distance between pea comb and blue shell loci. Bitgood et al. (1983) reported a map unit value of 4.28 ± 1.3 between P and O. Earlier research indicated five map units between them (Hutt, 1949). In the present study, map distance based on limited numbers and derived from an intercross was 2.39 ± 8.54. The data are from a normal chromosome background, as are those published by Bitgood et al. (1983), and the two data sets exhibit homogeneity. Those from the present study might eventually contribute to further refinement of map distance.
ACKNOWLEDGMENTS
Marlene G. Ellis, student research assistant, investigated phenotypic distinction between melanotic heterozygotes and nonmelanotic homozygotes; her work is acknowledged with thanks. The counsel and advice provided by J. J. Bitgood, University of Wisconsin-Madison, in analyses and interpretation of data presented here are very greatly appreciated. REFERENCES Bitgood, J. J., 1985. Locating pea comb and blue egg in relation to the centromere of chromosome 1 in the chicken. Poultry Sci. 64:1411-1414. Bitgood, J. J., J. S. Otis, and R. N. Shoffner, 1983. Refined linkage value for pea comb and blue egg: lack of effect of pea comb, blue egg, and naked neck on age at first egg in the domestic fowl. Poultry Sci. 62:235-238. Brumbaugh, J. A., and W. F. Hollander, 1965. A further study of the E pattern locus in the fowl. Iowa State J. Sci. 40:51-64. Brumbaugh, J. A., and W. F. Hollander, 1966. Genetics of buff and related color patterns in the fowl. Poultry Sci. 45:451-457. Crawford, R. D., 1985. Linkage between pea comb and plumage color pattern in domestic fowl. Poultry Sci. 64(Suppl. 1):83. (Abstr.) Green, M. C., 1963. Methods for testing linkage. Pages 56-82 in Methodology in Mammalian Genetics. W. J. Burdette, ed. Holden-Day, Inc., San Francisco, CA. Hutt, F. B., 1949. Genetics of the Fowl. McGraw-Hill Book Co., Inc., New York, NY. Moore, J. W., and J. R. Smyth, Jr., 1971. Melanotic: key to a phenotypic enigma in the fowl. J. Hered. 62:214219. Smyth, J. R., Jr., 1970. Genetic basis for plumage color pattern in the New Hampshire fowl. J. Hered. 61:280284.
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
Pea wheaten blue shell Pea wheaten brown shell Pea black blue shell Single wheaten blue shell Pea black brown shell Single wheaten brown shell Single black blue shell Single black brown shell
single black blue shell
1986 Poultry Science 65:1859-1862
INTRODUCTION
There is continuing interest and activity in mapping the gene loci of chickens. The value of these studies is rapidly increasing as genetic engineering becomes a reality. Investigation of the pea-comb locus on the short arm of Chromosome 1 has been especially productive (Bitgood, 1985). In the present study, briefly reported earlier (Crawford, 1985), the map of Chromosome 1 is further extended by the observation of very close linkage between the pea-comb and melanotic plumage loci.
MATERIALS AND METHODS
The stock under study had been maintained as a closed flock for about 20 generations. It has been traced to a male of unknown origin carrying genes for pea comb and blue egg, mated with Brown Leghorn and Rhode Island Red females. Comb type and plumage were variable in early generations of the population, but the birds were gradually selected for the pea-combed wheaten phenotype. Despite selection, the black phenotype continued to segregate within the population as if it were an autosomal recessive trait, usually appearing in combination with single comb. For several generations prior to the present study, the stock had been held in two separate groups; one of them bred pure for single comb, black plumage, and blue eggshell; the other segregated for pea and single comb, black
and wheaten plumage, and blue and brown eggshell. The two groups were crossed, using 12 sires and 27 dams, to produce F, progeny known to be heterozygous at comb and plumage pattern loci. All progeny were wheaten and pea-combed; certain progeny were also heterozygous at the shell color locus. The F, birds were flock-mated inter se or backcrossed to parental types in three combinations. Eight F[ males were bred to 30 black single comb females, yielding coupling double backcross data for linkage between comb and plumage pattern loci. Thirteen F, males were bred to 30 F, females; this yielded coupling double intercross data for the same loci. Two F, males and 9 F] females, heterozygous at the shell color locus, also yielded coupling double intercross data for comb and plumage pattern, but in addition, they yielded data for a threepoint linkage test of comb, plumage pattern, and shell color loci. Sixteen consecutive weekly hatched were produced from these three matings. All chicks were identified and reared in batteries to 6 weeks of age, when sex, comb type, and juvenile plumage pattern were recorded. Females from the three-point linkage test were kept until sexually mature; they were housed in individual cages and at least two eggs were examined from each to determine whether shells were blue or brown. Data for segregation and linkage were analyzed using the methodology of Green
1859
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
ABSTRACT Segregation for pea vs. single comb and blue vs. brown eggshell has persisted in a closed population for about 20 generations. Most pea-combed birds have wheaten plumage, and single-combed birds are black; recombinants have only rarely been seen indicating close linkage between comb and plumage loci. The population appears to be homozygous for an E locus wheaten allele, probably ey. Wheaten and black phenotypes are caused by segregation of melanotic locus (Ml) alleles. In a coupling double backcross, there were 190 pea wheaten, 192 single black, and 1 single wheaten progeny. In a coupling double intercross, there were 1084 pea wheaten, 370 single black, 1 pea black, and 4 single wheaten progeny. Map distance between loci is estimated as .31 ± .23 units. There was 1 comb and plumage recombinant among 136 females examined for blue vs. brown shell color, giving a tentative locus order on Chromosome 1 of Ml-P-O. (Key words: linkage, pea comb, melanotic, blue egg, chicken)
CRAWFORD
1860
(1963). Her procedures are currently the most used in avian linkage studies, but they have been designed for linkages ranging from 50% to 1%. The linkage reported here is much less than 1% and its precise measurement may be biased by analysis method. RESULTS AND DISCUSSION
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
Pea comb and single comb phenotypes are easily distinguished in day-old chicks and older birds. The gene for pea comb is incompletely dominant autosomal (Hutt, 1949). In adult males and females, the two pea comb genotypes can usually be distinguished by the elevated central blade of the comb in heterozygotes, but genotype identification is not very accurate in day-old or juvenile birds. The blue egg trait is completely dominant autosomal (Hutt, 1949), and there is no difficulty in classifying hens by color of their eggshells. Genetics of the plumage patterns in the study population is less well-understood. At hatching, all chicks are cream-colored, without any regular markings, but primary and secondary pinfeathers in the wings are pigmented. In juveniles and adults of the wheaten phenotype, males are almost identical to classical black-breasted reds. Females are cream-colored, with black in the wings and tail, and they exhibit variable amounts of black in the cream areas of the plumage. In juveniles and adults of the black phenotype, males are glossy black with pale undercolor; they may exhibit some red markings in the neck, saddle, and wings; their beaks and shanks are not melanized. The females are dark charcoalgrey with pale undercolor, and there may be some red in the hackle; beaks and shanks are not melanized. Two genetic kinds of wheaten are recognized (Brumbaugh and Hollander, 1965). Dominant wheaten (eWh) and recessive wheaten (ey) are allelic with almost identical phenotypes. The recessive form is characteristic of Rhode Island Reds (Brumbaugh and Hollander, 1966), and the dominant form is found in New Hampshires (Smyth, 1970). The wheaten phenotype studied here is probably one of these forms. There has been considerable confusion about the genetic basis for black plumages that appear to have an autosomal recessive mode of inheritance. Moore and Smyth (1971) have reported that recessive black plumage is actually caused by an incompletely dominant autosomal gene Ml (melanotic). It appears to have little effect
on the color of the chick down. Its action in juvenile and adult plumages is to extend eumelanin into areas that would normally be red, but its expression can vary depending on which alleles are present at the E locus. Moore and Smyth (1971) studied the effects of Ml in the presence of several E locus alleles, including dominant wheaten (ewh). They report that in dominant wheaten males, melanotic heterozygotes have a dark wild-type pattern, and homozygotes are nearly all black; in dominant wheaten females, melanotic heterozygotes have the classical wheaten female pattern, and melanotic homozygotes are wheaten with dark-brown back and hackle. In the present study, it is thought that the population is uniformly homogygous for an E locus wheaten allele and that wheaten and black phenotypes in the stock are controlled by segregation at the melanotic locus. There are two reasons for believing that the actual E locus allele is recessive wheaten (ey): the birds were derived in part from Rhode Island Reds, which are known to be e>le> (Brumbaugh and Hollander, 1966), and melanotic expression in females is quite different from that described by Moore and Smyth (1971) in females known to be ewh/eWh. If this interpretation is correct, then the findings of Moore and Smyth (1971) can be extended. There is now indication that in the presence of ey/ey, males and females that are homozygous melanotic are black, and heterozygotes and homozygous nonmelanotic birds have the wheaten phenotype. Hence, the black phenotype appears recessive even though it is caused by a dominant gene. Smyth (1985, personal communication) has examined colored photographs of wheaten and black phenotypes from the present study, and agrees that the blacks are probably melanotic (Ml). During the course of this study the black phenotype was thought to be truly autosomal recessive, and no attempt was made to distinguish between presumed homozygotes and heterozygotes among the wheaten birds. If differences actually existed, they were very small and went unnoticed. Subsequently, an attempt was made at phenotypic separation of two melanotic locus genotypes in the wheaten plumage class. Because of the extremely close linkage, to be described later, between the pea comb locus and the melanotic locus, it was assumed that peacombed homozygotes would also be nonmelanotic homozygotes, and pea-combed heterozygotes
PEA COMB AND MELANOTIC LINKAGE
1861
TABLE 1. Linkage between pea comb and melanotic loci Coupling double backcross = pea wheaten
single black
single black
Pea wheaten Pea black Single wheaten Single black
190 0 1 192
single black
X2 pea/single = .0234 X2 wheaten/black = .0026 X2 linkage = 379.0104**
Coupling double intercross = pea wheaten
pea wheaten
Pea wheaten Pea black Single wheaten Single black
1084 1 4 370
single black
X2 pea/single = .3218 X2 wheaten/black = .1428 X2 linkage =1473.7035**
Map distance, comb and plumage = .31 ± .23. P<.01.
would be melanotic heterozygotes; hence, comb genotype should be a good indicator of plumage genotype. Four aged males, shown by both comb morphology and breeding test to be PIP, were compared with four aged males similarly shown to be Plp+. Both had the typical male wheaten plumage pattern, but the heterozygotes had black lacing on the red hackle feathers and black lacing on the saddle feathers. It is assumed that the darker kind were melanotic heterozygotes and the others were nonmelanotic homozygotes, in agreement with Moore and Smyth (1971). Similar comparisons using aged females were inconclusive. Three were no consistent relationships among comb morphology, comb genotype determined by breeding test, and darkness of the plumage. Juvenile males and females were also examined. Comb genotype could not be accurately determined from comb morphology. There was continuous variation in plumage coloring, from near absence of black to very heavy pigmentation in the usual red or wheaten areas of both males and females. Based on these limited observations, it appears that except for adult males, melanotic heterozygotes cannot be distinguished visually from nonmelanotic homozygotes in birds that are homozygous for the reces-
sive wheaten allele. Linkage Observations. Data on the linkage observed between pea comb and melanotic loci are presented in Table 1. They were analyzed using the methods of Green (1963). Map distance between comb and plumage loci was calculated to be .31 ± .23 from combined data for the coupling double backcross and the coupling double intercross. Green's (1963) methodology provides accurate measurement for linkages as close as 1%, but not for any closer than that. Hence, the map distance calculated here may lack precision, although there is no doubt that it is much less than one map unit. Data for a three-point linkage test involving the blue shell locus are presented in Table 2. These data were derived from an intercross, an inefficient way of measuring linkage, as it requires very large numbers of observations to be accurate. In this study, observations were relatively limited (n= 136). However, the data have some value in estimating order of loci on the chromosome, and they relate to earlier reports of linkage in the literature. The map distance between comb and shell color loci was calculated to be 2.39 ± 8.54, and the distance between plumage and shell
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
single black
1862
CRAWFORD TABLE 2. Linkage with blue shell locus
Three-locus intercross =pea wheaten brown shell
pea wheaten brown shell
Zx single black blue shell
67 38 0 1 0 0 30 0
X2 pea/single X2 blue/brown X2 linkage
= = =
.3529 .6274 16.0130**
X2 wheaten/black = X2 blue/brown = X2 linkage =
.6274 .6274 15.1111**
Map distance, comb and shell = 2.39 ± 8.54 Map distance, plumage and shell = 2.50 ± 8.54 P<.01.
color loci was 2.50 ± 8.54. Standard errors in both estimates are very high, reflecting difficulties of analysis caused by limited numbers of observations in some classes and none in others. Using a translocation as a marker, Bitgood (1985) has demonstrated the linear order of pea comb and blue shell loci, known to be on the short (p) arm of Chromosome 1. The order is centromere-pea comb-blue shell. It is of interest to consider the placement of the melanotic locus in relation to these three loci. In the present study there was only one comb and color recombinant among the 136 birds, which caused the estimate of distance between plumage and shell color loci to be larger than that between comb and shell color loci.This result suggests that the order on Chromosome 1 may be: centromeremelanotic-pea comb-blue shell. Confirming evidence is needed. Data from the three-point linkage test are also of interest in relation to earlier reports about map distance between pea comb and blue shell loci. Bitgood et al. (1983) reported a map unit value of 4.28 ± 1.3 between P and O. Earlier research indicated five map units between them (Hutt, 1949). In the present study, map distance based on limited numbers and derived from an intercross was 2.39 ± 8.54. The data are from a normal chromosome background, as are those published by Bitgood et al. (1983), and the two data sets exhibit homogeneity. Those from the present study might eventually contribute to further refinement of map distance.
ACKNOWLEDGMENTS
Marlene G. Ellis, student research assistant, investigated phenotypic distinction between melanotic heterozygotes and nonmelanotic homozygotes; her work is acknowledged with thanks. The counsel and advice provided by J. J. Bitgood, University of Wisconsin-Madison, in analyses and interpretation of data presented here are very greatly appreciated. REFERENCES Bitgood, J. J., 1985. Locating pea comb and blue egg in relation to the centromere of chromosome 1 in the chicken. Poultry Sci. 64:1411-1414. Bitgood, J. J., J. S. Otis, and R. N. Shoffner, 1983. Refined linkage value for pea comb and blue egg: lack of effect of pea comb, blue egg, and naked neck on age at first egg in the domestic fowl. Poultry Sci. 62:235-238. Brumbaugh, J. A., and W. F. Hollander, 1965. A further study of the E pattern locus in the fowl. Iowa State J. Sci. 40:51-64. Brumbaugh, J. A., and W. F. Hollander, 1966. Genetics of buff and related color patterns in the fowl. Poultry Sci. 45:451-457. Crawford, R. D., 1985. Linkage between pea comb and plumage color pattern in domestic fowl. Poultry Sci. 64(Suppl. 1):83. (Abstr.) Green, M. C., 1963. Methods for testing linkage. Pages 56-82 in Methodology in Mammalian Genetics. W. J. Burdette, ed. Holden-Day, Inc., San Francisco, CA. Hutt, F. B., 1949. Genetics of the Fowl. McGraw-Hill Book Co., Inc., New York, NY. Moore, J. W., and J. R. Smyth, Jr., 1971. Melanotic: key to a phenotypic enigma in the fowl. J. Hered. 62:214219. Smyth, J. R., Jr., 1970. Genetic basis for plumage color pattern in the New Hampshire fowl. J. Hered. 61:280284.
Downloaded from http://ps.oxfordjournals.org/ at Oakland University on June 3, 2015
Pea wheaten blue shell Pea wheaten brown shell Pea black blue shell Single wheaten blue shell Pea black brown shell Single wheaten brown shell Single black blue shell Single black brown shell
single black blue shell