Genetic Defects in Sheep
By: Kris Sweiter
Genetic defects refer to deformities that exist at birth. They occur sporadically and rarely contribute to major losses of lambs in flocks. There are more than 30 known or suspected genetic defects of sheep. Many are lethal. Others are semi lethal, because the effect is crippling but death is not inevitable, although a proportion do die. Some common genetic defects are entropion (inverted eyelids), cleft palate, parrot mouth (undershot jaw, cryptorchidism (one or both testicles retained in the abdomen), hernias, abdominal impaction, and spider lambs, and even prolapses.
Causes of Genetic Defects
Chromosomes inherited from parents determine an animal's genetic make-up. There are many genes in each chromosome. Genetic abnormalities occur when genes are missing, in excess, mutated or in the wrong location (translocation). A few genes can directly cause an abnormality, however, these are rare. Usually, these genes are recessive, meaning two must be present to cause an abnormality. Both parents must be carriers of the gene for a calf to be abnormal. In this case, only one of every four offspring will be abnormal. Two will be carriers and one will be normal.
Certain conditions show that an abnormality is likely to have a genetic origin:
Below are some of the most common genetic defects in sheep.
Jaw defectsJaw defects are present in almost all breeds of sheep and are associated with failure of the incisor teeth to properly meet the dental pad. A jaw is undershot if the incisor teeth extend forward past the dental pad; it is overshot if the teeth hit in back of the dental pad (this condition is known as parrot mouth). Cull sheep with either of these genetic defects. If the sire and dam can be identified, remove them from the flock.
Overbite:(overshot, parrot mouth, class, overjet, mandibular branchygnathism)
Underbite:(undershot, reverse scissors bite, prognathism)
Rectal prolapse:Rectal prolapse is a serious defect most commonly associated with the meat-type sheep. It is most common among lambs fed a high-concentrate ration. It is believed that this weakness is due to inheritance. This condition is sometimes corrected by surgery, but affected animals often continue to prolapse after surgery. Cull from the flock breeding sheep in which this occurs.
Cryptorchidism:Rams with one or both testicles retained in the abdomen, or not descended fully into the scrotum are cryptorchids. Cryptorchidism presents itself in one of two forms: 1) unilateral cryptorchidism - normal descent of only one testicle, 2) bilateral cryptorchidism - retention of both testicles. Unilateral cryptorchid lambs are usually capable of breeding, whereas bilateral cryptorchids are sterile. The condition usually is inherited as a simple recessive trait. There seems to be some association between this condition and the polled characteristic found in some fine-wool rams. Purebred breeders should make every effort to eliminate this condition. In spite of the fact that bilateral cryptorchid lambs are sterile, both bilateral and unilateral cryptorchids should be castrated, to reduce the risk of possible future complications. Unilateral cryptorchids should never be used in a breeding program.
Inverted eyelids:Inverted eyelid (entropion) is widespread among most breeds of sheep. This trait is highly heritable. Inverted eyelids are a "turning in" of the margin of the eyelid and therefore bringing the eyelashes into direct contact with the cornea. This contact creates an irritation, making it necessary for the animal to blink constantly. As the animal blinks, it is compounding the problem by scraping the eyelashes across a more extensive area of the eye. This extreme irritation if left unattended, can eventually cause blindness. The condition may be noted at birth and treated at that time. Entropion should never be left to take care of itself. If left untreated, the condition could cause sore watery eyes, infection, ulcers on the cornea and even blindness. Entropion condition requires surgical correction by a veterinarian. One method of treating this condition is to clip a metal suture to the center of the affected eyelid. Gather enough skin under the clip in a vertical direction to hold the lid away from the eye. The clip can be left in place for several days. Mark the affected lambs and do not allow them to enter the breeding flock.
(Normal, Healthy Eyes)
Spider Lamb Syndrome(SLS) or ovine hereditary chondrodysplasia is a genetic disorder causing skeletal deformities in young lambs. These defects commonly include 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 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.
Since its development, the DNA test has been validated in over 1000 animals. To date, it has been 100% accurate in the proper identification of animals that are genetically free or carriers of this genetic defect. The test has been successfully used in Suffolks, Hamphires, Southdowns, and Oxfords.
What Should You Do if You Think You Have a Defect?
When you suspect that you have a problem lamb, consult your veterinarian. Investigate all symptoms and possible causes before concluding the problem is genetic or environmental. When the cause is genetic, contact the breed association and give them a full report of the findings. Progressive breed associations are working to reduce the frequency of genetic abnormalities within their breed.
To avoid further abnormalities in your flock without culling female carriers, use non-carrier rams unrelated to your herd. Practice no inbreeding within the flock. Crossbreeding to a different breed is another alternative. The occurrence of undesirable traits in the flock requires that a careful review of the breeding records be conducted to identify animals, matings, or familial groups that may be associated with the problem. The detection of carrier animals makes it easier to eliminate genetic disorders.
Genetic abnormalities are not common. When they do occur, they cause economic losses. Genetic and environmental factors cause abnormalities. Environmental causes are quickly corrected while genetic causes require longer-term solutions. If an abnormality occurs on your farm, take immediate action.