COLOUR GENETICS FOR SHETLANDS
By Linda Wendelboe

Click on the button for a Glossary of Genetic Terms  

Adalsteinsson (see below) did the early work in figuring out Northern European Short Tail Sheep colour genetics.  His work is used as a basis for Shetland as well as Icelandic and other breed colour genetics. At each gene, a sheep will have two alleles; one inherited from its mother and one inherited from its father.  Its genotype is made up of all of these pairs of alleles.

B Gene, Brown or Black:  The B gene controls whether the sheep will be black toned (black or grey in phenotype) or brown toned (moorit or musket in phenotype).  Black (BB) is dominant over brown (Bb).

All sheep have a "base" colour that is either black (BBBB or BBBb) or brown (BbBb).  This "base" colour is then modified or covered up by the pattern created by the Agouti gene.  

A Gene, Agouti Patterns:  The Agouti gene controls the "pattern" of our Shetlands.  It has a number of known patterns with more being revealed by test breeding by the researchers.  In general terms, "no colour" or "not pigmented" patterns are dominant to "colour" or "pigmented" patterns. The Agouti gene alleles or patterns identified in Shetlands, so far, in order of dominance are:

White (Awt) - most dominant pattern, results in a white sheep and will cover up whatever pattern is represented by the sheep's second Agouti allele;

Grey (Ag) - usually salt and pepper grey or musket phenotype, with both light and dark fibres in the fleece; will be "covered up" by an Awt allele but will "cover up" an Aa allele;

Katmoget (Ab) and Gulmoget(At) - these two alleles or patterns seem to be co dominant.  Both are recessive to (will be covered up by) the Awt allele and can be "washed out" if coupled with an allele for the Ag (grey) pattern.  Both can "cover up" an Aa allele; and

Non Agouti (Aa) - solid phenotype, recessive to the other patterns so will be "covered up" or masked if it is paired with any other allele except Aa.

So a sheep that has the dominant Awt allele at the Agouti gene will look white, even though its base colour is either black or brown and even if it has a different second Agouti allele.

Another Agouti allele, Light Badger Face, is known to exist in other sheep breeds.  Initial test breeding shows that it is likely to exist in Shetlands as well.  See:  Katmoget Genetics

Shetlands also seem to have some additional modifying genetic factors, producing the pewters, fawns and miogets.  These colours are not a mixture of light and dark fibres (as seen in the salt and pepper grey or the musket of the Ag pattern) but seem to have each fibre the same, lightened colour.  Test breedings are being done to gather more information on this mechanism.  See:  Modified Colours

E Gene, Extension:  In addition to the Agouti gene and the B gene, there is also an Extension gene.  It has the effect of cancelling out the patterns set by Agouti to let the base colour show through.  This is sometimes called dominant black but is really dominant colour as this gene will allow the base colour of the sheep, either black or brown, to show through the Agouti patterns.  If you get a white lamb out of two black or moorit parents, you look at the possibility of mistaken paternity or the existence of the Extension gene in your flock.  There may be some Extension coming into North American Shetlands through the semen imports from the UK but it was not thought to be present here before those.  

S Gene, Spotting:  White spots are controlled at a different gene - S.  Non spotted is dominate to spotted so in order to show spots, the sheep has to be homozygous for spots (have two copies of the spotting allele) .

Head, Socks and Tail white (HST) may be controlled by S but different genes may also be involved.  This gives the krunet and sokket type markings.

Lanette Scapillatto, a Shetland breeder, has a nice basic genetics piece on her web site: www2.localaccess.com/primolana/default.htm

Susan Mongold breeds Icelandic sheep which have similar colour genetics.  Her genetics site is:  http://www.icelandicsheep.com/genetics.html

Probably the best source for technical colour genetics information is the Mendelian Inheritance in Sheep site:      www.angis.org.au/bin/Databases/BIRX/birx_doc?mis+1

You change the+1 at the end of the URL to +2 or +3 or +4, etc. to see more of the genes. this may
 be updated in the future as researchers are finding quite a few new patterns at Agouti , etc.

Another source for the technical stuff is the Stefan Adalsteinsson paper:
Colour Inheritance in Icelandic Sheep and Relation Between Colour, Fertility and Fertilization, published by The Journal of Agricultural Research in Iceland. You may be able to get it from:  www.isbona.com  . Alternately, you can order it from Iceland at a nominal cost or perhaps borrow it from the NASSA Lending Library:   kgbaker@allstream.net   .

There is also a Sheep Colour Genetics internet list group at Yahoo that gets
into genetics discussions  in-depth:    http://groups.yahoo.com/group/sheep-color-genetics/  .

©  Linda Wendelboe, 01/03/05