- Genetic defects and undesirable characteristics in woolled sheep
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Genetic defects and undesirable characteristics in woolled sheep
by J.E. NEL
Agricultural Research Institute of the Karoo Region
Genetic defects in woolled sheep are not of great importance to the practical farmer. The season for this is that most of the genes, which are responsible for these defects, are recessive and relatively scarce. Thus it very seldom occurs that a recessive factor from the father combines with the same type of recessive factor from the mother to bring forth a genetic defect. If such animals do occur. they are immediately culled so that those recessive factors are eliminated.
Genetic defects in a flock are the results of severe inbreeding. However, such inbreeding has so many other disadvantages that it is not used at all in practice.
By a genetic defect is meant a detrimental character or factor transmitted to the progeny from one or both of the parents, as a result of which the animal is malformed or abnormal.
A lethal character is exhibited when a gene essential to a specific vital function is replaced by a gene with the opposite effect. The expression of a lethal gene may be changed by the genotype or the environment and when the lethal action of the gene is incomplete, the gene or the character is semi-lethal. A semi-lethal gene may accordingly be regarded as a lethal gene with a delayed or late action.
Sometimes a dominant lethal mutation occurs, but in the majority of cases this is not observed since the individuals involved usually die before the character - if visible - is expressed.
Although it is alleged that many types of animals are heterozygous for at least one undesirable gene, the incidence of a recessive lethal, semi-lethal or abnormal character will, as a rule, be low. It would appear that the incidence of undesirable genes is raised in a population in which the heterozygote is superior to any of the homozygotes.
In sheep it is difficult to identify lethal factors influencing the early stages of prenatal development. The economic loss caused by such a lethal gene is not necessarily great, since the animal concerned may be served again during the same season. On the other hand, genes causing reduced vigour later in the life of the animal without giving rise to any easily noticeable effect, may be of fairly great economic importance.
The detrimental phenomena observed in sheep up to the present are limited to those of which the action is easily noticed and which may be observed at birth or shortly after birth.
A. LETHAL AND SEMI-LETHAL CHARACTERS
1. Paralysed limbs
All the limbs of the lamb are stiff, and the neck is arched backwards. Lambs affected in this way are usually stillborn. This phenomenon is ascribed to a single recessive gene.
2. Lethal muscular dystrophy
Lambs are born alive but die shortly afterwards because they cannot breathe. The joints are stiff, muscles are undeveloped, the liver is abnormally enlarged and the kidneys are underdeveloped. Test matings have indicated that the condition is the result of a lethal recessive gene.
Here the hind legs are paralysed, and the lambs die shortly after birth. The condition is ascribed to a single recessive gene.
4. Paralysis or nervous inco-ordination at birth
In very serious cases the neck is drawn sideways, and the back is hollow, as a result of muscular contractions. Death usually occurs within 48 hours after birth. It is suspected that this condition is the result of a recessive lethal gene, but that more than one pair of genes are involved. It is also suspected that the degree of seriousness of the abnormality is determined by modifying genes.
5. Stiff foot joints
Such lambs are born alive, but cannot walk or suckle and die within a few days. The foot joints and limbs are stiff and contracted. The body is short and thick and the lamb has an enlarged head with a short upper jaw. The wool of such lambs is usually short, and the skin on the belly - from the breast to the anus - is also often without covering. This condition is apparently the result of the action of a single recessive gene.
Where this semi-lethal character occurs in lambs, the limbs below the foot joints are absent. How exactly this condition is inherited is not yet known.
7. Earlessness and cleft palate
These abnormalities often occur together with tripartite feet and a shortened lower jaw. The lambs are born alive without ears and with a cleft palate, but die shortly afterwards. This phenomenon is ascribed to a single recessive gene.
These lambs have short legs, are broad across the shoulders, and have a protruding forehead. They gain weight rapidly and grow very fat, but die within a few weeks after birth. The phenomenon is ascribed to a single recessive gene. A histological investigation has indicated that the dwarfism is accompanied with a disturbance of the thyroid gland.
9. Sub-lethal grey
The breeding of grey Karakul sheep is hampered by the occurrence of sub-lethal grey, with the result that the individuals genetically pure for the grey colour die some time after birth. Consequently Karakul sheep cannot be purebred for the grey colour.
Post-mortem examination of lambs affected by this sub-lethal action usually reveals disturbances in the alimentary canal particularly affecting the rumen and the abomasum. At birth such lambs are distinguishable with fair certainty by their relatively light grey colour, the absence of pigmentation on the tongue and white spots on the inside of the ears.
Greyish-brown lambs also suffer from the sub-lethal grey factor. Up to the present it has not been possible to distinguish between the heterozygote and the homozygote in respect of this colour.
B. ABNORMALITIES AND UNDESIRABLE CHARACTERS
1. Defects of the jaw in sheep
Undershot and overshot jaws are common phenomena in sheep and seem to have a high heritability. These characters have an adverse effect on the grazing ability of the sheep, which is reflected in the lower body mass and wool production of sheep affected. Such sheep find it particularly difficult to graze short grass.
2. Hairlessness in newborn lambs
This abnormality has already been found in practically all sheep breeds, and is often followed by death after birth.
Hairlessness at birth is probably the result of abnormal development of the skin, and it leads to over-development and consequently thickening of the outer skin layers, and under-development of the hair follicles and their secondary structures.
The lamb is born completely without hair. Only towards the sixth day after birth do hairs appear on the skin surface, and at an age of approximately three weeks the whole body is covered with hair.
This abnormality is probably attributable to a single recessive gene.
3. Inverted eyelids (entropion) in lambs
There are two different types of entropion, namely infantile and Merino entropion. The former condition is found in newborn lambs: affected lambs have sore eyes as a result of the hairs on the inverted eyelid, which rub against the eye. If this irritation is continued for long, the eye may become permanently blind, and later even burst. A large percentage of the affected lambs recover within 10 days. If the eye does not recover by itself, a piece of skin may be cut from the eyelid.
Merino entropion occurs at an age of a few weeks to a few months. Usually only the upper eyelid is affected, but both eyelids may be folded inwards. As the wool of the sheep becomes longer, the condition becomes more noticeable. It is thought that this condition is the result of folds on the skin of the eyelid, but that it has no relation to the skinfolds on the body of the sheep. How exactly the condition is inherited is not yet known. This condition, too, may be remedied by cutting a piece of skin from the eyelid.
This term is used when referring to rams of which one or both of the testicles remain in the abdominal cavity and do not descend into the scrotum. It has been found that this condition is connected with hornlessness in the early-maturing breeds, and it is suspected that the phenomenon may be ascribed to the same gene responsible for hornlessness.
5. The Ancon sheep
This is a well-known mutation, which occurred in the U.S.A. for the first time in 1791. The sheep has short legs, walks with difficulty and the forelegs are lame. This character occurs as a single recessive.
6. Colour faults
Black fleece colour in Merino sheep is a characteristic caused by a recessive gene and in practice all black lambs are culled. It seems, however, as if colour on the non-woolled parts of the woolled sheep is caused by a separate set of heredity factors independent of fleece colour. The inheritance of these factors is more complicated than those of colour in the fleece. In practice sheep with excessive pigmentation are culled as colour in non-woolled parts of the body may increase.
The inheritance of kemp appears to be fairly high. Kemp can also spread to the fleece. Strict selection against kemp must therefore be practised.
Leaflet : Wool production B 2/1975