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Written by J.E. Held and G. Fitch Spider Lamb Syndrome (SLS) or ovine hereditary chrondrodysplasia is a genetic disorder causing skeletal deformities in young lambs. This defect commonly includes abnormally long, bent limbs, twisted spines, shallow bodies, flattened rib cages, and long necks. The syndrome is inherited as a genetic recessive disorder meaning that affected lambs must inherit the mutation from both of their parents. Because of this inheritance pattern, the identification of genetic carriers of SLS has been difficult without the use of progeny testing. The presence of SLS in several breeds has prompted breeders to divide pedigrees into two categories, "gray pedigreed" animals having ancestors that have produced spider lambs, and "white pedigreed" animals having ancestors that have never produced affected lambs. Recently, researchers at Utah State University and the University of Illinois have identified the genetic mutation responsible for Spider Lamb Syndrome. A simple test using DNA isolated from blood or semen can now be performed to identify genetically clean animals regardless of pedigree. Among sheep in 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. In animals with skeletal structural defects this test can determine whether the spider gene is responsible. Actual spider lambs would be homozygous for the spider lamb gene (SS). Since the development this DNA test has been validated in over 1,000 animals. To date it has been 100% accurate in proper identification of animals that are genetically free or carriers of this genetic defect. The test has been successfully used in Suffolks, Hampshires, Southdowns, Shropshires, and Oxfords. Livestock Molecular Research and Development, Inc. (LMRD), a company owned and operated by the researchers that identified the mutation, performs the molecular genetic test for the spider gene. Information on this company and test procedures can be obtained from LMRD, 44 North Market Street, Montecello, IL 61856-1125, phone: (217) 762-3094, fax (217) 762-7215. Use of The Spider Gene Blood Testing - (As of June 1998) over 2,800 head of sheep have been blood tested for the spider gene since the test was made commercially available in 1997. Those numbers continue to grow by more than 100 head per week. Breeders appear confident in the test, using test information they can now eliminate the spider gene from their flock. The use of the test information in pedigree recording is essential for breeders to make informed selection decisions. It appears the most practical method to standardize the identification of tested clean (NN) animals is to include nomenclature in the animal's name at the time of registration. Identifying Genetically Clean (NN) Animals - Nomenclature has been developed which provides breeders a way to label blood tested clean (NN) animals in their pedigree. Using the letters NNI or NNP in the animal's name at the time of registration would allow breeders to accurately track blood tested clean animals or the parental lines. NNI should be used only where that specific individual (I) was blood tested as clean (NN) prior to registration. NNP should be used when the registration entry has both parents or parental lines (P) supported by NN blood tests. In the future the expected predominant nomenclature in extended pedigrees would be NNP. For this voluntary identification of tested animals to work it must be clear that this nomenclature be reserved for animals completely supported by blood tested NN animals. For example, the entry on a registration form could read: How to Evaluate A Pedigree Using NNI or NNP - When evaluating a pedigree, NNI in the animal name would indicate that it was blood tested clean (NN). If NNI appears as the first tested animal in a family pedigree, the parents status is assumed to be undetermined (Example 1). Example 1 Current (1998)
For animals with NNP it indicates that parents or the parental sire and dam lines are supported by blood tested clean (NN) animals (Example 2, also shown in Example 3). Example 2 Current (1998)
The first NNP appearing in the pedigree for a family line would have NNI parents. However in many cases, NNI is not in the parent's name since no recording application has been used to date. In the future it will be more common for the pedigree to have NNI labeled parents for the first NNP in the pedigree (Example 3). Example 3 Future (2000)
After several generations, some breeders may choose to check the genotype status from NNP labeled parental lines. Individuals tested NN from NNP parents should be named using NNI. For breeders evaluating an extended pedigree, a rechecked animal labeled NNI from NNP parental lines should provide added confidence in selection decisions. Summary - If used correctly, NNI and NNP will help new and veteran breeders identify blood tested clean (NN) animals within pedigrees. This information can be used to make informed selection decisions and reduce the incidence of carrier animals. Using this approach eliminates the need for breeders to rename animal registration papers for tested sires and dams. This model is a way for breeders to set a standard for entering blood tested clean animals into permanent records for the immediate and future benefit of many breeds. For more information on how you can submit an article to the Breeders' World, Click Here. CLICK HERE FOR MORE INFORMATION! The Breeders' World is a subsidiary of Cornerstone Enterprises. |