Genetic Make-up

Spider Lamb Syndrome(SLS) is an inherited genetic recessive disorder that lambs must inherit the mutation from both parents. In the past the genetic disorder was hard to detect but the use of testing has made it easier to detect SLS.  The presence of SLS has lead breeders to divide pedigrees into three categories, "gray-pedigreed" animals having ancestors that have produced spider lambs, and "white-pedigreed" animals having ancestors that have never produced affected lambs, and "black pedigree" animals that have produced a spider lamb.

The recessive gene that causes Spider Syndrome, which is S, has a normal gene N, which is dominant over the S gene.  Recessive indicates that the S gene will not be seen when an animal has the N gene since the N is dominant.  As genes occur in pairs the three genotype combinations which are possible are NN, NS, SS. Animals with the NN and NS genotypes appear normal.

It is very hard to eliminate SLS from a flock because it is a genetic condition. Lambs born with SLS usually will not survive to breeding age, unless bottle-fed. If these lambs do survive it is not recommened that they are used for breeding.  There is a 50% chance of the lambs being born as carriers for the gene.

A test using DNA from blood or semen can be performed to identify genetically clean animals regardless of pedigree. Among sheep with a normal phenotype, test results will indicate whether an animal is homozygous normal NN and genetically "clean" or heterozygous NS and a carrier of the disorder, capable of producing spider lambs when mated to another carrier.

References

http://www.breedersworld.com/sheep/articles/spidergene.html

http://www.cvmbs.colostate.edu/dlab/webdocs/ext_vet/cleon13.html

http://www.nebraskasheep.com/tipsandtopics/tips_feb00.html