Lethal white syndrome ( LWS ), also called overo lethal white syndrome ( OLWS ), white deadly overo ( LWO ), and overo deadly white foal syndrome ( OLWFS ), is a disorder the most common genetic autosomal in American Paint Horse. The affected foal was born after an 11 month full pregnancy and externally looked normal, even though they had all white or almost all white coats and blue eyes. However, internally, this foal has a colon that does not work. Within a few hours, colic signs appear; affected children died within a few days. Because death is often painful, such horses are often euthanized humanely once identified. This disease is very powerful because the foal born seems healthy after being taken to full time.
This disease has a similar cause to Hirschsprung's disease in humans. A mutation in the middle of a B-type endothelin receptor (EDNRB) causes a deadly white syndrome when homozygous. Operators, which are heterozygous - that is, have one copy of a mutated allele, but they are healthy - can now be reliably identified by DNA testing. Both parents must be the bearer of one copy of the LWS allele for the affected foal to be born.
Horses that are heterozygous for genes that cause deadly white syndrome often show color patterns of spots known as "frames" or "overo frames". Coat color alone does not necessarily indicate the presence of LWS or carrier status, however. The frame pattern may be minimally expressed or masked by other spotting patterns. Also, different genetic mechanisms produce healthy white foals and have no relationship to LWS, another reason for genetic testing of potential nursery stocks. Some confusion also occurs because the term overo is used to describe a number of non tobiano spotting patterns other than the frame pattern. Although no treatment or cure for LWS children is known, non-LWS-looking white foals may have treatable conditions.
Video Lethal white syndrome
Alerts
Unlike some color dilution coat colors, which can cause premature births, stillbirths, or weak foals, foals born with deadly white syndrome appear to be fully formed and normal. The coat is entirely or almost wholly clean white with pigmented pink skin. Pigmented areas may be colored, and if present, most common around the muzzle, the bottom of the barrel, and the back or tail. His eyes are blue. Some deadly white foals have proven to be deaf.
Healthy foals pass meconium, the first stool, shortly after birth, although some healthy foals may require enemas to help this process, but meconium LWS children are highly affected in the gut, and never appear, even with enema use. Signs of colic begin to appear on the first day, and all children who suffer from LWS die within the first few days of life. The painful and inevitable deaths that occur usually encourage veterinarians and owners to put to sleep the children who are suspected of having deadly white syndrome.
Death is caused by an undeveloped part of the digestive system. The horse's large intestine is a complex system in which most digestion takes place, and consists of cecum, colon, and rectum. Necropsies in LWS children reveal, colon and intestinal obstruction pale, retarded (impaction). Samples from affected tissues show a lack of nerves that allow the intestine to move material through the digestive system, a condition called bowel agangliosis.
A closer examination of the skin and hair indicates that both are not pigmented, and most hair follicles are inactive and many do not have hair at all. All LWS foals test homozygous for genetic disorders.
Maps Lethal white syndrome
Inheritance and expression
Genetic conditions that affect more than one physical property - in the case of deadly white syndrome, both pigment cells and enteric nerve cells - are called pleiotropic. An unusual example of pleiotropy in LWS children suggests early on that the syndrome is associated with an important part of embryonic tissue called the neural emblem. As the name suggests, stem cells of neural crystals are precursors to nerve cells. Other types of cells that descend from neural crest cells are melanocytes, pigment-producing cells found in hair follicles and skin. Neural migration and melanocytes-precursors from the top of the embryo to the destination are ultimately controlled carefully by the regulator gene.
Such regulatory genes include endothelin receptor type B ( EDNRB ). A mutation in the middle of the EDNRB gene, Ile118Lys, causes deadly white syndrome. In this mutation, a "typo" in isoleucine DNA errors for lysine. The resulting EDNRB protein can not fulfill its role in embryonic development, limiting the migration of melanocyte and enteric neuron precursors.
In the case of LWS, one copy of the EDNRB mutation, the heterozygous state, produces identifiable properties, but with results very different from the homozygous state.
To produce a child with LWS, both parents should be heterozygotes or mutated gene carriers. Without genetic testing, some operators incorrectly identified as having white markings due to other genes, while some are even classified as solids.
The presence of this gene in various horse populations in North America suggests that mutations occur early in American history, perhaps in Spanish type horses.
Heterozygotes
Horse heterozygous for Ile118Lys mutations in malignant equine EDNRB - a deadly white syndrome carrier - usually shows a pattern of white patches called "frames", or "frame overo". The frame is marked with serrated, sharp-edged, horizontal white spots that run along the neck, shoulders, waist, and horse's back legs. The frame pattern by itself does not produce white marks that transverse backward, or affect the feet or the tail. Indeed, however, it often produces bald faces and blue eyes. The term "frame" illustrates the effect of seeing a horse with a frame pattern from the side: white marks appear to be "framed" by dark-colored borders. Until recently, animals that were heterozygous carriers did not show any health problems associated with carrier-only status.
Not all horses with heterozygous mutations actually fit the standard visual description. A horse with an Ile118Lys mutation on EDNRB that is not easily identified as a frame-pattern is called a faint frame. In addition to faint frames, a significant proportion of horses with frame phenotypes are visually misidentified, even in a clinical setting. One study was found from a group of visually inspected Paints, 18% of the stock of dense animals and 35% of the balding horses were actually frames. However, more than a quarter of the paints are listed in the "overo" category instead of the frame, and vice versa, 10% of horses listed as tobiano also carry a genetic framework. The difficulty in accurately identifying the frame has contributed to the unintentional breeding of LWS children.
At a minimum, horses marked heterozygous for the Ile118Lys mutation are not uncommon: a Thoroughbred tested DNA has a white mark restricted to under-weight and two socks below the knee. A Quarter horse tested positive for the gene after him and the Cat horse frame produced the LWS child; horse signs are a thin flame with a white dot cut off in the right nostril, with no other white markings. One major study identified two miniature horses that were completely unmarked, but positive for the Ile118Lys gene.
Many theories are given for this. The variability in the percentage of individuals with specific genotypes expressing the associated phenotype is called penetration, and this may be only evidence of variable penetration. Some research groups have suggested that other "suppressor" genes may limit the expression of white patches of pattern-frames.
At the other end of the spectrum, some horses with white markings are marked so wide that the characters of the frame pattern are obfuscated. In particular, tobiano patterns, dominant genes, are epistatic. Other white spot genes include white splash or "splash", sabino, and "calico". Any combination, or all, of these white spot genes can act together to produce horses with so much white that the existence of a frame can not be determined without DNA testing.
The ambiguous terminology also contributes to the confusion surrounding this disease. Today, the American Paint Horse Association categorizes horses as tobiano, solid, "overo", and tovero. This association breaks the "overo" into three categories: Frame, Splash and Sabino. In the past, "overo" was used more loosely, to refer to the spotted animals that were "Cat, but not tobiano". However, no fewer than four - and possibly more - different genetically different patterns are included under the term "overo". To be categorized as "overo" by the APHA, the horse must conform to the written description: white patches do not pass through the back, at least one solid colored foot, dense tail, facial markings, and irregular white spots, scattered, or spotted. To further complicate the problem, various Sabino patterns also appear in some horse breeds that do not carry genetics for frames or other spotting patterns.
Likewise, the classification of horses officially as unmarked solids is not based on genetic testing, but on the visual description. Horses that carry genetics to frames and other white patch patterns may be very marked for lack of minimum registry requirements for white. This helps explain the horses that are allegedly producing offspring, called cropouts.
The old practice of categorizing Cat horses in this way contributes to the incorporation of the word "overo" into several titles used to describe illness, such as excessive lethal white foal syndrome. However, "overo" refers to some patterns of white spots that are not genetically related, and only the frame patterns indicate the syndrome. Confusion about the nature of LWS is then clarified by statements such as "there are many advantages that do not carry a deadly allele", which is technically true, but only because the term "overo" also includes splash and sabino patterns and frames.
Homozigot
Homozygote for Ile118Lys mutations in the B-type endothelin receptor gene has a deadly white syndrome. In crosses of two mother mains, the statistical probability of producing a solid colored foal is 25%; 50% possibilities exist for children with frame patterns; and 25% chance of having LWS child.
Produce a frame color pattern without producing deadly white color
Color coats, including frames, are popular and are sought after by breeders. Whilst many boys of deadly white syndrome are inadvertently generated when breeders cross two untested cryptic frames, or known frames and faint frames, some are produced by the deliberate breeding of two known frames, either due to ignorance or indifference. Producing foals with LWS is now completely avoidable, as most animal genetic laboratories now offer DNA tests for it. Whether a horse visually appears to have a frame pattern or not, testing the frame horses or "overo" lineages is strongly recommended.
The statistical possibility for generating life-mates, framed by crossing two frames is 50%, the same chance of generating frameless livelihoods from frame-to-non frame frames that carry no deadly risk. white foal syndrome. Therefore, breeding two overos frames does not provide benefits for breeders hoping to produce another frame overo.
Dominant or recessive?
White lethal syndrome has been described by researchers as dominant and recessive inherited. White lethal syndrome is described as recessive because heterozygotes (written Oo or N/O ) are not affected by intestinal aganglionosis. However, if the characteristic frame pattern is included, inheritance properties follow an incomplete dominant pattern. The concept of antedate molecular biology is "recessive" and "dominant" and technically applies only to properties, not to the genes themselves. In pleiotropic conditions, such as deadly white syndrome, the application of "recessive" or "dominant" can be ambiguous.
A separate problem is the nomenclature applied to the frame pattern itself. Although following the dominant pattern of inheritance, deviations occur. The majority of horses with the Ile118Lys mutation show a recognizable frame pattern, but a small percentage too simple is marked to be classified as "tainted" by the breed registrars. Such a "solid" horse, bred into a solid pair, can produce a classically marked frame. The phenomenon of "crop-out" can make the frame appear to follow the recessive mode of inheritance.
Prevalence
Genes for LWS are the most common in American Paint Horse, but occur in every type that can carry frame genetics, including American Quarter Horses, Appaloosas, Horse Racing, Horse Horses, Miniature Horses, Tennessee Walking Horses, and Mustangs, as well as horses. which are derived from these breeds. Only two Morgan horses were identified as frame overos. Breeds that do not carry genes for frame patterns also do not carry LWS.
Turn off deadly white
Not all blue-eyed white foals are affected by LWS. Other genes can produce red-skinned horses with healthy blue eyes with white or very light skin. For a while, some really white horses are called "living lethal", but this is a misnomer. Before reliable information and DNA testing is available to breeders, very healthy, white, blue-eyed children are sometimes euthanized for fear they are deadly white skin, a result that can be avoided today by testing and understanding which is better about the color genetics of the mantle or even waiting 12 hours or more for the children to develop clinical signs. The availability of testing also allows breeders to determine whether blue-eyed and blue-eyed sons who become ill are LWS children who require euthanasia or non-LWS children with simple diseases that may be successfully treated.
- Multiple diluted creams such as cremello, perlinos, and smoky cream, have a creamy coat, blue eyes, and pink skin. The faint pigmentation of their coat creams can be distinguished from unpigmented white marks and pigmented pink skin. A similarly-looking "pseudo double encute" can be produced with the help of the pearl genes or "bar-link factor" or the champagne genes.
- The combination of tobiano with other white patches can result in a white or almost white horse, which may have blue eyes.
- Sabino horses homozygous for the sabino-1 gene ( Sb-1 ) are often called "sabino-white", and all-or almost all-white. Not all sabino horses carry Sb-1 .
- The dominant white genetics is not fully understood, but is characterized by all or almost all white coats.
Analog conditions
From the start of genetic research, LWS has been compared with Hirschsprung's disease in humans, which is also caused by mutations in the EDNRB gene. Various polymorphisms in this gene produce intestinal agangliosis, in some cases followed by unusual skin and eye pigmentation, and deafness. Sometimes the condition of officer pigmentation in humans is called Waardenburg-Shah syndrome.
The terms "piebald-lethal" and "deadly spotting" apply to the same conditions in rats and mice, respectively, both caused by mutations in the EDNRB gene. Only deadly in homozygous state, mutations related to whites, deafness, and megacolon caused by intestinal agangliosis.
See also
- dilution genes
- Combine the mantle color genetics
References
Source of the article : Wikipedia
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