Zygosity is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous; the DNA sequence of a gene varies from one individual to another. Those variations are called alleles. While some genes have only one allele because there is low variation, others have only one allele because deviation from that allele can be harmful or fatal, but most genes have two or more alleles. The frequency of different alleles varies throughout the population; some genes may have two alleles with equal distribution.
For other genes, one allele may be common, another allele may be rare. Sometimes, one allele is a disease-causing variation. Sometimes, the different variations in the alleles make no difference at all in the function of the organism. In diploid organisms, one allele is inherited from one from the female parent. Zygosity is a description of whether those two alleles have different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome; the words homozygous and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an otherwise diploid organism, nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing. A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes.
The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits. An individual, homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait; this allele called the "dominant allele", is represented by a capital letter. When an organism is homozygous-dominant for a particular trait, the genotype is represented by a doubling of the symbol for that trait, such as "PP". An individual, homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait; this allele called the "recessive allele", is represented by the lowercase form of the letter used for the corresponding dominant trait. The genotype of an organism, homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp". A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene; the cell or organism is called a heterozygote for the allele in question, therefore, heterozygosity refers to a specific genotype.
Heterozygous genotypes are represented by a capital letter and a lowercase letter, such as "Rr" or "Ss". Alternatively, a heterozygote for gene "R" is assumed to be "Rr"; the capital letter is written first. If the trait in question is determined by simple dominance, a heterozygote will express only the trait coded by the dominant allele, the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex. A heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype - this is called a heterozygote advantage. A chromosome in a diploid organism is hemizygous; the cell or organism is called a hemizygote. Hemizygosity is observed when one copy of a gene is deleted, or, in the heterogametic sex, when a gene is located on a sex chromosome. Hemizygosity must not be confused with haploinsufficiency, which describes a mechanism for producing a phenotype. For organisms in which the male is heterogametic, such as humans all X-linked genes are hemizygous in males with normal chromosomes, because they have only one X chromosome and few of the same genes are on the Y chromosome.
Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are considered to be hemizygous, because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic individual can be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotype of each individual. In cultured mammalian cells, such as the Chinese hamster ovary cell line, a number of genetic loci are present in a functional hemizygous state, due to mutations or deletions in the other alleles. A nullizygous organism carries two mutant alleles for the same gene; the mutant alleles are both complete loss-of-function or'null' alleles, so homozygous null and n
White horses are born white and stay white throughout their lives. White horses may have blue, or hazel eyes. "True white" horses those that carry one of the dominant white genes, are rare. Most horses that are referred to as "white" are "gray" horses whose hair coats are white and may be born of any color and "gray" as time goes on and take on a white appearance. White horses have unpigmented a white hair coat. Many white horses have dark eyes. In contrast to gray horses which are born with pigmented skin they keep for life and pigmented hair that lightens to white with age white horses are born with white hair and pink, unpigmented skin; some white horses are born with partial pigmentation in their skin and hair, which may or may not be retained as they mature, but when a white horse lightens, both skin and hair lose pigmentation. In contrast, grays retain only the hair becomes white. White colorings, whether white markings, white patterns or dominant white are collectively known as depigmentation phenotypes, are all caused by areas of skin that lack pigment cells.
Depigmentation phenotypes have various genetic causes, those that have been studied map to the EDNRB and KIT genes. However, much about the genetics behind various all-white depigmentation phenotypes are still unknown. Dominant white is best known for producing pink-skinned all-white horses with brown eyes, though some dominant white horses have residual pigment along the topline. Dominant white is. At least one parent must be dominant white and it does not "skip" generations because it is not recessive. Nonetheless, new variations or mutations producing dominant white do occur spontaneously from time to time. Dominant white has occurred in many breeds, it has been studied in Thoroughbreds, Arabian horses, the American White horse and the Camarillo White horse. There are 11 identified variants of dominant white, each corresponding to a spontaneously-white foundation animal and a mutation on the KIT gene. No horse has been identified as homozygous dominant white, researchers have suggested that at least some forms of dominant white results in nonviable embryos in the homozygous state.
While homologous mutations in mice are linked to anemia and sterility, no such effects have been observed in dominant white horses. Dominant white horses have white noses that can be subject to sunburn. Sabino-white horses are pink-skinned with dark eyes, they are homozygous for the dominant SB1 allele at the Sabino 1 locus, mapped to KIT. Without a DNA test, Sabino-white horses are indistinguishable from dominant white horses; the Sabino1 allele, the associated spotting pattern, is found in Miniature horses, American Quarter Horses, American Paint Horses, Tennessee Walkers, Missouri Fox Trotters, Shetland Ponies, Aztecas. Sabino 1 has not been found in the Arabian horse, Thoroughbred, Standardbred horse, or Shire horse; the Sabino 1 allele is not linked to any health defects, though sabino-whites may need some protection from sunburn. Horses with only one copy of the Sabino1 gene have dramatic spotting, including two or more white legs with white running up the front of the leg, extensive white on the face, spotting on the midsection, jagged or roaned margins to the pattern.
The leopard complex, related to the Leopard gene, characterizes the Appaloosa and Knabstrupper breeds with their spotted coats. Leopard is genetically quite distinct from all other white-spotting patterns; the fewspot leopard pattern, can resemble white. Two factors influence the eventual appearance of a leopard complex coat: whether one copy or two copies Leopard alleles are present, the degree of dense white patterning present at birth. If a foal is homozygous for the LP allele and has extensive dense white patterning, they will appear nearly white at birth, may continue to lighten with age. In other parts of the world, these horses are called "white born." "White born" foals are less common among Appaloosa horses than Knabstruppers or Norikers, as the extensive dense white patterning is favored for producing dramatic full leopards. Homozygous leopards have the LP/LP genotype, may be varnish roan, fewspot leopard, or snowcap patterned. Homozygous leopards are more prone to congenital stationary night blindness.
Congenital stationary night blindness is present at birth and is characterized by impaired vision in dark conditions. Lethal white syndrome is a genetic disorder linked to the Frame overo gene and most studied in the American Paint Horse. Affected foals are carried to term and at birth appear normal, though they have pink-skinned all-white or nearly-white coats and blue eyes. However, the colon of these foals cannot function due to the absence of nerve cells, the condition cannot be treated. Foals with Lethal White Syndrome invariably die of colic within 72 hours, are humanely euthanized. Carriers of the gene, who are healthy and normal, can be identified by a DNA test. While carriers exhibit the "frame overo" pattern, this is not a dispositive trait and testing is necessary, as the pattern can appear in a minimal form as normal white markings or be masked by other white spotting genes. True white horses have unpigmented unpigmented white hair, though eye color varies; the lack of pigment in the skin and hair is caused by the absence of pigment-producing cells called melanocytes.
Some coat colors are characterized by light or white-like coats and pinkish skin, however thes
A foal is an equine up to one year old. More specific terms are colt for a male foal and filly for a female foal, are used until the horse is three or four; when the foal is nursing from its dam, it may be called a "suckling". After it has been weaned from its dam, it may be called a "weanling"; when a mare is pregnant, she is said to be "in foal". When the mare gives birth, she is "foaling", the impending birth is stated as "to foal". A newborn horse is "foaled". After a horse is one year old, it is no longer a foal, is a "yearling". There are no special age-related terms for young horses older than yearlings; when young horses reach breeding maturity, the terms change: a filly over three is called a mare, a colt over three is called a stallion. A castrated male horse is called a gelding regardless of age. Horses that mature at a small stature are called ponies and confused with foals. However, body proportions are different. An adult pony can be ridden and put to work, but a foal, regardless of stature, is too young to be ridden or used as a working animal.
Foals, whether they grow up to be horse or pony-sized, can be distinguished from adult horses by their long legs and small, slim bodies. Their heads and eyes exhibit juvenile characteristics. Although ponies exhibit some neoteny with the wide foreheads and small stature, their body proportions are similar to that of an adult horse. Pony foals are proportionally smaller than adults, but like horse foals, they are slimmer and have proportionally longer legs than their adult parents. Foals are born after a gestation period of 11 months. Birth takes place consistent with the status of a horse as a prey animal, more at night than during the day. Labor lasting over twenty-four hours may be a sign of medical complications. Unlike most predators which are altricial, horses are precocial, meaning they come into the world mature and mobile. Healthy foals can keep up with the rest of the herd only a few hours after birth. If a foal has not eaten within twelve hours, it may require assistance. Healthy foals grow and can put on up to three pounds or over a kilo a day.
A sound diet improves growth and leads to a healthier adult animal, although genetics plays a part. In the first weeks of life the foal gets everything. Like a human infant, it receives nourishment and antibodies from the colostrum in milk, produced within the first few hours or days following parturition; the mare needs additional water to help her produce milk for the foal and may benefit from supplementary nutrition. A foal may start to eat solids from ten days of age, after eight to ten weeks it will need more nutrition than the mare's milk can supply, it is important when adding solid food to the foal's diet to not feed the foal excessively or feed an improperly balanced diet. This can trigger one of several possible growth disorders that can cause lifelong soundness problems. On the other hand, insufficient nutrition to mare or foal can cause stunted growth and other health problems for the foal as it gets older, it is typical for foals under human management to be weaned between four and six months of age, though under natural conditions, they may nurse for longer until the following year when the mare foals again.
Some foals can nurse for up to three years in domesticity because the mare is less to conceive another foetus. A foal, weaned but is less than one year old is called a weanling. Mare's milk is not a significant source of nutrients for the foal after about four months, though it does no harm to a healthy mare for a foal to nurse longer and may be of some psychological benefit to the foal. A mare, both nursing and pregnant will have increased nutritional demands made upon her in the last months of pregnancy, therefore most domesticated foals are weaned sometime in the autumn in the Northern Hemisphere if the mare is to be bred again the next season. Weanlings are not capable of reproduction. Puberty occurs in most horses during their yearling year. Therefore, some young horses are capable of reproduction prior to full physical maturity, though it is not common. Two-year-olds sometimes are deliberately bred, though doing so with fillies, puts undesirable stress on their still-growing bodies; as a general rule, breeding young horses prior to the age of three is considered undesirable.
In spite of rapid growth, a foal is too young to be driven. However, foals receive basic horse training in the form of being taught to accept being led by humans, called halter-breaking, they may learn to accept horse grooming, hoof trimming by a farrier, having hair trimmed with electric clippers, to become familiar with things it will have to do throughout life, such as loading into a horse trailer or wearing a horse blanket. Horses in general have excellent memories, so a foal must not be taught anything as a young horse that would be undesirable for it to do as a full-grown animal. There is tremendous debate over the proper age to begin training a foal; some advocate beginning to accustom a foal to human handling from the moment of birth, using a process termed imprinting or "imprint training". Others feel that imprint training of a foal interferes with the mare and foal bond and prefer to wait until the foal is a few days old, but do begin training within the first week to month of life.
Yet other horse breeding operations wai
Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is recessive. For genes on an autosome, the alleles and their associated traits are autosomal dominant or autosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics; the dominant allele codes for a functional protein whereas the recessive allele does not. A classic example of dominance is the inheritance of seed shape in peas. Peas associated with allele r. In this case, three combinations of alleles are possible: RR, Rr, rr; the RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals have round peas. Thus, allele R is dominant to allele r, allele r is recessive to allele R; this use of upper case letters for dominant alleles and lower case ones for recessive alleles is a followed convention.
More where a gene exists in two allelic versions, three combinations of alleles are possible: AA, Aa, aa. If AA and aa individuals show different forms of some trait, Aa individuals show the same phenotype as AA individuals allele A is said to dominate, be dominant to or show dominance to allele a, a is said to be recessive to A. Dominance is not inherent to either its phenotype, it is a relationship between two alleles of their associated phenotypes. An allele may be dominant for a particular aspect of phenotype but not for other aspects influenced by the same gene. Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene; the concept of dominance was introduced by Gregor Johann Mendel. Though Mendel, "The Father of Genetics", first used the term in the 1860s, it was not known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants.
When bred separately, the plants always produced generation after generation. However, when lines with different phenotypes were crossed and only one of the parental phenotypes showed up in the offspring. However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3:1 ratio, the more common phenotype being that of the parental hybrid plants. Mendel reasoned that each parent in the first cross was a homozygote for different alleles, that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes, that one of the two alleles in the hybrid cross dominated expression of the other: A masked a; the final cross between two heterozygotes would produce AA, Aa, aa offspring in a 1:2:1 genotype ratio with the first two classes showing the phenotype, the last showing the phenotype, thereby producing the 3:1 phenotype ratio. Mendel did not use the terms gene, phenotype, genotype and heterozygote, all of which were introduced later.
He did introduce the notation of capital and lowercase letters for dominant and recessive alleles still in use today. Most animals and some plants have paired chromosomes, are described as diploid, they have two versions of each chromosome, one contributed by the mother's ovum, the other by the father's sperm, known as gametes, described as haploid, created through meiosis. These gametes fuse during fertilization during sexual reproduction, into a new single cell zygote, which divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each cell as its parents; each chromosome of a matching pair is structurally similar to the other, has a similar DNA sequence. The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene has a corresponding homologue, which may exist in different versions called alleles; the alleles at the same locus on the two homologous chromosomes may be different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father. If two alleles of a given gene are identical, the organism is called a homozygote and is said to be homozygous with respect to that gene; the genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism directly and indirectly affects its molecular and other traits, which individually or collectively are called its phenotype. At heterozygous gene loci, the two alleles interact to produce the phenotype. In complete dominance, the effect of one allele in a heterozygous genotype masks the effect of the other; the allele that mas
The horse is one of two extant subspecies of Equus ferus. It is an odd-toed ungulate mammal belonging to the taxonomic family Equidae; the horse has evolved over the past 45 to 55 million years from a small multi-toed creature, into the large, single-toed animal of today. Humans began domesticating horses around 4000 BC, their domestication is believed to have been widespread by 3000 BC. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses; these feral populations are not true wild horses, as this term is used to describe horses that have never been domesticated, such as the endangered Przewalski's horse, a separate subspecies, the only remaining true wild horse. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, colors, breeds and behavior. Horses' anatomy enables them to make use of speed to escape predators and they have a well-developed sense of balance and a strong fight-or-flight response.
Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down, with younger horses tending to sleep more than adults. Female horses, called mares, carry their young for 11 months, a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training in harness between the ages of two and four, they reach full adult development by age five, have an average lifespan of between 25 and 30 years. Horse breeds are loosely divided into three categories based on general temperament: spirited "hot bloods" with speed and endurance. There are more than 300 breeds of horse in the world today, developed for many different uses. Horses and humans interact in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as police work, agriculture and therapy. Horses were used in warfare, from which a wide variety of riding and driving techniques developed, using many different styles of equipment and methods of control.
Many products are derived from horses, including meat, hide, hair and pharmaceuticals extracted from the urine of pregnant mares. Humans provide domesticated horses with food and shelter, as well as attention from specialists such as veterinarians and farriers. Specific terms and specialized language are used to describe equine anatomy, different life stages and breeds. Depending on breed and environment, the modern domestic horse has a life expectancy of 25 to 30 years. Uncommonly, a few animals live into their 40s and beyond; the oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62. In modern times, Sugar Puff, listed in Guinness World Records as the world's oldest living pony, died in 2007 at age 56. Regardless of a horse or pony's actual birth date, for most competition purposes a year is added to its age each January 1 of each year in the Northern Hemisphere and each August 1 in the Southern Hemisphere; the exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.
The following terminology is used to describe horses of various ages: Foal: A foal of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal, weaned is called a weanling. Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects. Yearling: A horse of either sex, between one and two years old. Colt: A male horse under the age of four. A common terminology error is to call any young horse a "colt", when the term only refers to young male horses. Filly: A female horse under the age of four. Mare: A female horse four years old and older. Stallion: A non-castrated male horse four years old and older; the term "horse" is sometimes used colloquially to refer to a stallion. Gelding: A castrated male horse of any age. In horse racing, these definitions may differ: For example, in the British Isles, Thoroughbred horse racing defines colts and fillies as less than five years old. However, Australian Thoroughbred racing defines fillies as less than four years old.
The height of horses is measured at the highest point of the withers. This point is used because it is a stable point of the anatomy, unlike the head or neck, which move up and down in relation to the body of the horse. In English-speaking countries, the height of horses is stated in units of hands and inches: one hand is equal to 4 inches; the height is expressed as the number of full hands, followed by a point the number of additional inches, ending with the abbreviation "h" or "hh". Thus, a horse described; the size of horses varies by breed, but is influenced by nutrition. Light riding horses range in height from 14 to 16 hands and can weigh from 380 to 550 kilograms. Larger riding horses start at about 15.2 hands and are as tall as 17 hands, weighing from 500 to 600 kilograms. Heavy or draft horses are at least 16 hands (64 inches, 16
Animal euthanasia is the act of putting an animal to death or allowing it to die by withholding extreme medical measures. Reasons for euthanasia include incurable conditions or diseases, lack of resources to continue supporting the animal, or laboratory test procedures. Euthanasia methods are designed to cause minimal distress. Euthanasia is distinct from animal slaughter and pest control although in some cases the procedure is the same. In domesticated animals, this process is referred to by euphemisms such as "put down" or "put to sleep"; the methods of euthanasia can be divided into physical methods. Acceptable pharmacological methods include injected drugs and gases that first depress the central nervous system and cardiovascular activity. Acceptable physical methods must first cause rapid loss of consciousness by disrupting the central nervous system; the most common methods are discussed here, but there are other acceptable methods used in different situations. Upon administration of intravenous anesthetic, respiratory cardiac arrest follow usually within 30 seconds.
Some veterinarians perform a two-stage process: an initial injection that renders the pet unconscious and a second shot that causes death. This allows the owner the chance to say goodbye to a live pet without their emotions stressing the pet, it greatly mitigates any tendency toward spasm and other involuntary movement which tends to increase the emotional upset that the pet's owner experiences. For large animals, the volumes of barbiturates required are considered by some to be impractical, although this is standard practice in the United States. For horses and cattle, other drugs may be available; some specially formulated combination products are available, such as Somulose and Tributame, which cause deep unconsciousness and cardiac arrest independently with a lower volume of injection, thus making the process faster and more effective. A horse injected with these mixtures may display apparent seizure activity before death; this may be due to premature cardiac arrest. However, if normal precautions are taken, this is a problem.
Anecdotal reports that long-term use of phenylbutazone increases the risk of this reaction are unverified. After the animal has died, it is not uncommon for the body to have posthumous body jerks, or for the animal to have a sudden bladder outburst. Gas anesthetics such as isoflurane and sevoflurane can be controlled-atmosphere stunning used for euthanasia of small animals; the animals are placed in sealed chambers. Death may be caused using carbon dioxide once unconsciousness has been achieved by inhaled anaesthetic. Carbon dioxide is used on its own for euthanasia of wild animals. There are mixed opinions on whether it causes distress when used on its own, with human experiments lending support to the evidence that it can cause distress and equivocal results in non-humans. In 2013, the American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a flow rate of 10% to 30% volume/min is optimal for the humane euthanization of small rodents. Carbon monoxide is used, but some states in the US have banned its use in animal shelters: although carbon monoxide poisoning is not painful, the conditions in the gas chamber are not humane.
Nitrogen has been shown to be effective, although some young animals are more resistant to the effects, it is not used. Cervical dislocation, or displacement of the neck, is an older yet less common method of killing small animals such as mice. Performed properly it is intended to cause as painless death as possible and has no cost or equipment involved; the handler must know the proper method of executing the movement which will cause the cervical displacement and without proper training and method education there is a risk of not causing death and can cause severe pain and suffering. It is unknown how long an animal remains conscious, or the level of suffering it goes through after a correct snapping of the neck, why it has become less common and substituted with inhalants; when intravenous injection is not possible, euthanasia drugs such as pentobarbital can be injected directly into a heart chamber or body cavity. While intraperitoneal injection is acceptable, an intracardiac injection may only be performed on an unconscious or sedated animal.
Performing IC injections on a conscious animal in places with humane laws for animal handling is a criminal offense. This can be a means of euthanasia for large animals—such as horses and deer—if performed properly; this may be performed by means of: Firearms Traditionally used in the field for euthanizing horses, deer or other large game animals. The animal is shot in the forehead with the bullet directed down the spine through the medulla oblongata, resulting in instant death; the risks are minimal. Captive bolt gun Commonly used by the meat packing industry to other livestock; the bolt is fired through the forehead causing massive disruption of the cerebral cortex. In cattle, this stuns the animal, though if left for a prolonged period it will die from cerebral oedema. Death should therefore be brought about by pithing or exsanguination. Horses are killed outright by the captive bolt, making exsanguination unnecessary; the reasons fo