Description:  The Dwarf Gene is a simple autosomal recessive defect that is characterized by the affected offspring being proportionally reduced in frame size and weight to approximately 80-85% of normal size.  These individuals are often characterized by “increased muscle shape” in the show ring.  It is unclear at this time if the difference in phenotype of the muscle is due to a change in the size of the muscle fibers or due to differences in the distribution of muscle related to the change in frame size.   Carrier animals (and in some cases affected individuals) cannot be easily distinguished from dwarf-free animals due to normal differences in phenotype between different bloodlines and individuals.

Origin:  While a specific origin is unclear at this time, the defect has been seen primarily in black-faced, wether-type sheep.  Reports of sheep exhibiting the phenotype represented by the Dwarf Gene have been reported as early as 2012.

Genetic Testing:  Gene Check, INC can perform testing for the Dwarf Gene at a cost of $13 per test.  Samples can be submitted using EDTA Blood (purple top), blood card or with a tissue sample (Refer to the Gene Check submission form for specific instructions regarding sample submission).   There are 3 possibilities regarding the results of the testing:

FF = Dwarf Gene Free

FD = Dwarf Gene Carrier

DD = Dwarf Gene Affected

Distribution:  The gene appears to be widely distributed within the black-faced breeds.  Initial testing as reported through Gene Check revealed that of the first 3800 tests submitted, that close to half of the individuals were either a Dwarf Carrier or affected individual.    In addition, over the past 3 years (2019-2021), there were 49 individuals that were identified with the Southdown breed that were also carrying at least one copy of the gene.

ASBA Policy:  At the most recent ASBA board meeting (2021) in Louisville this past November, the board determined that the gene was a very undesirable genetic defect for the Southdown breed as well as being evidence that the animal had been cross-bred at some point.  After a lengthy discussion it was determined that for association sponsored sales and shows the gene would be treated in the same manner as the other genetic defects moving forward.

Sales:  Effective January 1, 2022 –  For all sales sponsored by this association, the seller warrants to the buyer that sheep sold are free of Hereditary Chondrodysplasia (spider), Ectodermal Dysplasia (hairy lamb), Ovine Dwarfism and contain a minimum of one “R” at codon 171.

Shows:  Effective January 1, 2023 – Any show may adopt the following policy approved by the American Southdown Breeders’ Association.  Every ram and ewe shown at this show, as well as the following shows; the open and junior shows at the North American International Livestock and Exposition, and the National Southdown open and junior shows, must be a non-carrier of Hereditary Chondrodysplasia (spider syndrome), Ectodermal Dysplasia (“Hairy Lamb Syndrome”) and Ovine Dwarfism.

Spider syndrome is a condition that was first reported in Suffolk and Suffolk crossbred sheep in the United States in the popular press and the scientific literature in the mid-1980’s. Spider lambs have severe bone deformities throughout the entire skeleton that are manifested at birth or within 30 days of age. The most noticeable condition is an outward bending of the front legs from the knees, with many lambs also having a crooked spine in the thoracic area and a marked Roman nose. Spider syndrome is due to a single allele (n) that is completely recessive to the dominant allele (N) for normal skeletal development. Sheep that are homozygous for the spider allele (nn) exhibit spider syndrome and are so deformed that no producer would retain one for breeding, and even if retained, the probability of a successful mating would be very low. Therefore, spider syndrome lambs almost always result from mating two normal appearing, but heterozygous, individuals. The Spider locus is found on Sheep Chromosome 6. There is a DNA test that can identify the presence or absence of the spider gene and this test is used to differentiate between normal appearing animals that are carriers (Nn) or non-carriers (NN) of the spider syndrome gene.

*Excerpt taken from: “Tools Available to Wisconsin Sheep Producers for Genetic Improvement of Their Flocks”. By David L. Thomas, Extension Sheep Specialist, Department of Animal Sciences, University of Wisconsin-Madison.

Scrapie is an infectious disease of sheep that attacks the central nervous system and is always fatal. It is a type of transmissible encephalopathy found in a number of animal species including humans. Upon necropsy, infected animals will have holes or vacuoles in the tissue of the brain. Scrapie has a very long incubation period of several months to a few years so the disease is seldom seen in animals less than 1½ years of age. While scrapie affects relatively few sheep, it is a disease of major concern to Federal animal health officials because it is related to BSE (mad cow disease). Consumption of meat from BSE-infected cattle has been implicated as a cause of an encephalopathy in humans (new variant CJD). Presence of the disease in North America limits the opportunities for breeding sheep exports. Certain alleles at the prion protein locus have an effect on scrapie susceptibility. Differences in amino acids of the prion protein in at least two positions or codons appear to have an effect on susceptibility of sheep to scrapie. At the 136 codon, two amino acids have been identified in sheep populations: alanine (A) decreases susceptibility and valine (V) increases susceptibility to scrapie. At the 171 codon, the amino acid arginine (R) is associated with decreased susceptibility, and the amino acid glutamine (Q) is associated with increased susceptibility to scrapie. Therefore, animals that are homozygous for alanine at codon 136 (AA) and homozygous for arginine at codon 171 (RR) would be the most resistant to scrapie and would produce progeny with the greatest probability of resistance. A list of approved labs for official USDA testing can be found here.

*Excerpt taken from: “Tools Available to Wisconsin Sheep Producers for Genetic Improvement of Their Flocks”. By David L. Thomas, Extension Sheep Specialist, Department of Animal Sciences, University of Wisconsin-Madison.

Definition: Ectodermal Dysplasia is a genetic defect that affects the ectodermal layer of embryologic development. This layer is associated with development of skin, hair, teeth and hooves. Affected individuals have fine, white, straight hair instead of thick, crimped wool. The skin is thinner than normal with abnormal sebaceous and sweat glands. These lambs are quite frequently parrot‐mouthed and often have abnormalities associated with the teeth. While some affected individuals may be vigorous at first, they are very subject to stress (temperature, docking, weaning etc…) and most die within a few days to weeks.

Genetics: ED is a simple autosomal recessive defect with only homozygous recessive individuals being affected. Heterozygous individuals (carriers) have one normal gene and one defective gene. When carriers are mated they will produce affected individuals 25% of the time, carriers 50% of the time and homozygous normal individuals 25% of the time. This gene was mapped using state of the art “SNP chip” technology that was unavailable a few years ago. It can simultaneously screen > 50,000 gene markers per animal. It is a very specific and accurate test that not only identified the gene but allowed for identification of the specific base pair on the DNA strand. This gene mutation has been associated with similar defects in over 20 known species such as: humans, macaques, gibbons pandas dogs horses tilapia and mice just to name a few.

Testing: Tissue samples and blood work may be sent to Gene Check in the same manner as spider or scrapie tests. You may contact Gene Check at 1175 58th Ave, Geeley, CO 80634. Or by phone at (970)472-9951.

Breeding Management: There are 4 levels of management that producers can attempt.

1. Do Nothing – high risk and potential economic loss due to increase of affected lambs

2. Low cost – Use only homozygous normal rams. Will result in no affected individuals but will not eliminate the carrier state.

3. Moderate cost – Keep homozygous normal sires, herd matriarchs and annual replacement ewes. Will eventually lead to an ED free herd over time.

4. High cost – Complete flock management eliminating from the herd all heterozygous individuals. Quickest way to ED free but is costly and may require the loss of quality genetics.

The executive committee has confirmed that the Callipyge mutation is present in some Southdown sheep. Those wishing to protect themselves from this mutation can have a simple DNA test run for ewes and rams that they wish to bring into their flock. Remember, rams or ewes that are carrying the gene can look perfectly normal if they received the gene from the dam or if they have two copies of the gene (one each from the sire and dam). If the animal is showing signs of unnatural muscling of the leg and rack, sloped rump, and short loin then there is a high likelihood that he or she is carrying a copy of the mutation from the ram. A DNA test is now commercially available for $13 per head. Gene Check will conduct a SNP (single nucleotide polymorphisms) test. This is the only reliable test to determine if the mutation exists. Tissue samples and blood work may be sent to Gene Check in the same manner as spider, hairy lamb, or scrapie tests. You may contact Gene Check at 1175 58th Ave, Geeley, CO 80634. Or by phone at (970)472-9951.

The Board of Directors of the American Southdown Breeders Association will be taking an official action on the presence of this genetic defect in the Southdown breed and all individuals with opinions or interest are encouraged to contact any or all of the Board of Directors.

In recent years a trend has been identified by a number of concerned Southdown producers regarding the sale of Southdown breeding animals and wethers that carry a genetic mutation that is characterized by muscle hypertrophy (double muscling). Breeding of this particular mutation, which was discovered over 30 years ago in Oklahoma, has far reaching implications for not only the Southdown breed but the whole sheep industry as well. In 1983 a Dorset ram lamb was born that when bred produced excessive muscle development. The ram, who was later named “Solid Gold,” was bred extensively and his offspring were scattered throughout the country. In subsequent years extensive research involving his descendants determined that the excess muscling was due to a mutated gene that was named the “Callipyge Gene”. Callipyge comes from the Greek word meaning “Beautiful Buttocks”. In addition, the gene was found to exhibit a unique inheritance pattern termed “Polar Over-dominance”. The gene is only expressed when the offspring acquire a copy of the mutation from the sire. If the offspring acquires two copies of the gene (one from the dam and one from the sire), the gene will not express itself. If the offspring receives a copy of the mutation from the dam, the gene will not express itself. A ewe purchased with the double muscling trait will not produce a double muscled offspring. When the mutation was first studied, it was thought to be a great breakthrough for sheep producers because it caused a great increase in the production of lean red meat. However, subsequent research revealed that the proteins and muscle fibers in affected animals were dramatically different from normal sheep muscle. The muscle (particularly the loin muscle) in affected animals was found to be significantly less tender than normal. As a result, packers discriminate against the meat from animals that have the mutation due to concerns that the consumer will be less likely to buy lamb if this mutation is widely distributed. In addition, while lambs born with the gene do not initially exhibit excessive muscling, reports from some breeders who have had experience with these animals feel that they are more difficult to lamb out due to the excessive muscle of the dam decreasing the diameter of the birth canal.

The ASBA Board of Directors is currently in the process of establishing a comprehensive policy regarding the Callipyge gene because the implications of allowing this gene to be distributed widely in the Southdown population are enormous. In addition, from the standpoint of registration, carrying the gene is evidence that the affected animal is not a purebred Southdown because the gene first arose in Dorsets. Identification of the mutation at this time must be done through the appearance of the animal which is very characteristic. Below you will find a list of the implications of breeding for the mutation as well as al list of characteristics that will help you identify animals that are affected by the Callipyge mutation. Implications of allowing the gene to proliferate within the Southdown breed:

1. Discrimination against Southdowns as a breed regarding marketability to the consumer.

2. Discrimination against Southdowns for those who do not want the gene in their flocks.

3. Disqualification of Southdowns at stock shows that exhibit the gene (Currently many stock shows across the nation disqualify individuals that exhibit the mutation).

4. Fraudulent representation of these animals by some breeders as normal, well-muscled sheep… duping uneducated buyers into purchasing a Callipyge carrier.

5. Registration issues for affected animals that are not pure Southdowns

6. Increased disputes between buyer and seller from sale of animals carrying the gene.

7. Promotion of a gene that could have negative industry wide implications

8. Potential for more difficulty during lambing.

9. Increased costs to all producers if we have to begin genetic testing for the gene in order to protect producers that do not want the gene in their flock.

10.Extreme difficulty in eliminating the gene from your herd once it has been introduced due to the irregular inheritance pattern and lack of a commercial test.

Red flags that may indicate that the gene may be present in an individual:

1. Extreme, well-defined muscling of the leg muscles, forearm, loin and rack.

2. Tendency to have a steep hip (although this cannot always be appreciated in Southdowns)

3. Be wary of flocks that tend to have some lambs that are extreme in their muscle design with the other sheep in the same flock having average to below average amounts of muscle mass.

4. These sheep tend to be marketed in the wool and when they are less than 90 days old in order to mask some of the more extreme aspects of their muscle mass. Let the buyer beware!

5. If you view a lamb and the muscle mass appears almost too much to be true, then be very suspicious that the lamb may be carrying the gene.