Color Genetics of Gotland Sheep in North America

by Franna Pitt, PhD

Copyright, 2007

Genetics 101 for Gotland Sheep in North America

This section introduces the genetics terms and the color genes referenced in the following pages. It is not necessary to understand all these terms to read the paper, however, it will increase your undertstanding of color genetics, and help to explain the examples and text to come. Feel free to refer back to this page as you read the paper.

Genetics terminology:

Gene -

Single trait controlled at one location (locus) in the DNA strands - i.e. eye color

Allele -

One of several options available for each gene - i.e. blue eyes, brown eyes, green eyes

Gene Pair -

Genes always come in pairs, no more, no less. One from the sire, one from the dam.

Homozygous -

Gene pair with two identical alleles

Heterozygous -

Gene pair with two different alleles

Genotype -

The actual alleles present at each gene locus

Phenotype -

The appearance of the sheep, may or may not represent the complete genotype

Dominant -

An allele that appears preferentially in the phenotype and hides the effect of other alleles

Recessive -

An allele that doesn't express itself unless it has two copies at the locus (homozygous)

Co-dominant -

The effects of both alleles can be seen in the phenotype

Eumelanin -

Type of pigment responsible for the black and brown coloration of sheep. Eumelanin can be found in both hair and wool fibers.

Phaeomelanin -

Type of pigment responsible for white/tan coloration in sheep. Phaeomelanin is usually found in the hair fibers of the face and legs and in the birth coat.

Color Loci relevant to the North American Gotland sheep:

• B - Known as the Black/Brown locus, there are only two alleles associated with B - black and brown. B determines which type of eumelanin is expressed in the sheep, black or brown. All sheep are either black or brown at this locus no matter what the phenotype of the sheep actually is.
  • BB - black, dominant to brown. Gotland sheep are presumed to be homozygous BB.
  • Bb - brown, recessive to black, must be homozygous to be seen in the phenotype. Common in Icelandic and Shetland sheep. Present but not common in Finnsheep. Probably uncommon in the Longwool breeds.

• A - Known as the "pattern" locus, it has many alleles associated with it. Many sheep color geneticists believe there are more as yet unidentified patterns and variations to come. The A alleles control the extent and location of the two pigments, black/brown eumelanin and tan phaeomelanin
  • AWt - white/tan, dominant to all the other alleles at A. Phenotype is uniform white with or without tan areas. Tan usually appears in the hairy areas of the head and legs, and if seen in the wool, it usually fades with maturity. Common in all of the Foundation breeds.
  • Ab - badgerface, recessive to AWt, co-dominant with At, Abl, Ag, dominant to Aa. Phenotype is dark throat, chest, underbelly to tail with distinctive eumelanin and phaeomelanin on the head and legs. The upper body is light in color. Common in Shetlands and Icelandics. Reported but rare in Finnsheep.
  • At - mouflon or reverse badgerface. Also known as black and tan. Recessive to AWt, co-dominant with Ab, Abl, Ag, dominant to Aa. Phenotype is light throat, chest, underbelly to tail with distinctive eumelanin and phaeomelanin on the head and legs. The upper body is dark in color, sometimes with lighter areas on the flanks. Common in Shetlands and Icelandics.
  • Abl - English Blue. Recessive to AWt, co-dominant with Ab, At, Ag, dominant to Aa. Phenotype is similar to mouflon with light teardrops under each eye, light underbelly, lighter areas on the flanks. Abl sheep can be very light in color and all degrees to quite dark. Common in Leicester Longwools, Wensleydales, Cotswolds, and Lincolns. Blue Faced Leicesters exhibit a pattern similar to Abl, which might be Abl or a variation.
  • Ag - Grey. Recessive to AWt, co-dominant with Ab, Abl, At, dominant to Aa. Phenotype is expressed as a solid color in lambs with telltale white wisps in the ears, the groin, and sometimes in a necklace, along with "sugar lips". Occasionally seen as wildly swirled black and white areas especially on Shetland lambs. By 8 weeks of age, the fleece can be seen coming in as auniform grey color at the skin..Common in Icelandics, Shetlands. Reported but not common in Finnsheep.
  • Agg - Gotland Grey. Recessive to AWt, co-dominant with Ab, Abl, At, dominant to Aa. As yet, indistinguishable from Ag. The existance of this allele is unproven, however, there is evidence that it is distinct from Ag, and that other color and wool characteristics are inherited along with the Gotland Grey. The phenotype is very similar to Ag, with a very distinct change to grey at 1 to 2 months of age. Usually darker in color than the Ag Icelandics and Shetlands. Homozygous in Gotlands.
  • Aa - Solid. Recessive to all the other A alleles. Phenotype is solid color with no distinct pattern of eumelanin and phaeomelanin. The lamb usually keeps the solid body color through its first year. Thereafter it may "silver" - developing white fibers throughout the wool. This silvering increases with age, and is different from Ag or Agg. Common in all the Foundation Breeds.

• E - Known as the Extension locus, it has two alleles of importance to the Gotland upbreeding program. E controls the action of the A (pattern) locus and presence or absence of the two pigments, eumelanin or phaeomelanin.
  • E+ - The "wild" allele. This allele allows expression of both pigments and turns the pattern locus "on", allowing expression of all the A locus variations. Present in all of the Foundation breeds. Gotland sheep are presumed to be homozygous for E+.
  • ED - Known as Extension dominant. This allele hides the effects of the A (pattern) locus and inhibits phaeomelanin. White spotting is not affected. Common in the longwool breeds, ED is also present in North American Finnsheep and Shetland populations.

   

• S - the spotting locus. Two alleles are important. S controls the presence or absence of white spots. It is postulated that other, as yet unidentified, loci determine the location and extent of the white spotting. It is also possible that multiple alleles exist at S, and that those unidentified alleles control the extent and location of the spotting.
  • SS - Dominant. Phenotype is the absense of white spots.
  • Ss - Recessive. Phenotype is the presence of white spots. There is some question as to whether small white spots indicate the homozygous Ss/Ss, or heterozygous SS/Ss state. Common in Finnsheep, present in Icelandic and Shetland sheep. Also present in UK Gotland lines. Seems to be uncommon in the Longwool breeds.

Notation

Gene pairs are indicated with their locus (capitalized) and allele typed together - BB/BB is a pair of B alleles at the B locus. AWt/Aa is one each of the white/tan and solid pattern locus alleles. In general, the dominant allele is capitalized - "W" in AWt, and the recessive alleles are lower case - "a" in Aa. A non-spotted Gotland sheep's color genotype is indicated by:

BB/BB Agg/Agg E+/E+ SS/SS, black pigment, grey pattern, wild extension (allowing the pattern to be expressed), and no spots. Note that no spots could also be SS/Ss.

A spotted Gotland sheep's color genotype is indicated by:

BB/BB Agg/Agg E+/E+ Ss/Ss, black pigment, grey pattern, wild extension, and white spots present.

Punnett Squares

A handy tool to use in determining the possible genotypes of offspring is known as a Punnett Square. The individual alleles of the sire and dam are listed on the top and left of the square and combined within the square grid to yield the possible genotypes of the offspring. Below is an example of a Punnett Square of the B locus of a Gotland sire (BB/BB)and a black Shetland dam (BB/Bb) whose sire is moorit.

The four squares in the lower right are created by taking the allele from the corresponding row and column and combining them. In this case, all of the offspring are phenotypically black, and half of them carry brown. Next, if we take the BB/Bb offspring and mate them, the following Punnett square shows the result.

This time, the four possibilities for offspring include one (25%) homozygous black, two (50%) blacks carrying brown, and one (25% - homozygous) brown.

This technique can be used anytime the genotype of the sire and dam are known or can be guessed at. You can also use it to determine genotypes at multiple loci. Two loci would yield a 4 x 4 grid of offspring; three loci would yield an 8 x 8 grid of offspring.

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