Misplaced Pages

#426573

28-455: A black horse is an equine coat color. Black Horse and Blackhorse may refer to: Black horse Black horses have dark brown eyes, black skin, and wholly black hair coats without any areas of permanently reddish or brownish hair. They may have pink skin beneath any white markings under the areas of white hair, and if such white markings include one or both eyes, the eyes may be blue. Many black horses "sun bleach" with exposure to

56-416: A black foal will, however, disappear as the black hair coat grows in. Black foals have dark skin and eyes at birth. An adult-like black foal coat often indicates that the foal will gray , if the foal has at least one gray parent. Graying can be confirmed by the presence of white hairs around the eyes and muzzle. Gray Lipizzaner horses are frequently born black. Black adult horses are easier to identify, as

84-431: A great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences. Wild type alleles are often denoted by a superscript plus sign ( i.e. , p for an allele p ). A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles ( polymorphism ) present, or

112-444: A horse that appears visually black is not actually a dark bay or liver chestnut. Horses described as "homozygous black" are simply homozygous for the dominant extension gene (EE); they are homozygous "not-red". Such horses are only "guaranteed" to never produce a red foal. The actual horse may carry additional genetic modifiers that could make it bay, buckskin, gray, bay roan, perlino, silver bay, and so on. A visually black horse that

140-506: A process termed transgenerational epigenetic inheritance . The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence . A specific class of epiallele, the metastable epialleles , has been discovered in mice and in humans which is characterized by stochastic (probabilistic) establishment of epigenetic state that can be mitotically inherited. The term "idiomorph", from Greek 'morphos' (form) and 'idio' (singular, unique),

168-749: A single-gene trait. Recessive genetic disorders include albinism , cystic fibrosis , galactosemia , phenylketonuria (PKU), and Tay–Sachs disease . Other disorders are also due to recessive alleles, but because the gene locus is located on the X chromosome, so that males have only one copy (that is, they are hemizygous ), they are more frequent in males than in females. Examples include red–green color blindness and fragile X syndrome . Other disorders, such as Huntington's disease , occur when an individual inherits only one dominant allele. While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species,

196-424: Is Gregor Mendel 's discovery that the white and purple flower colors in pea plants were the result of a single gene with two alleles. Nearly all multicellular organisms have two sets of chromosomes at some point in their biological life cycle ; that is, they are diploid . For a given locus, if the two chromosomes contain the same allele, they, and the organism, are homozygous with respect to that allele. If

224-490: Is a short form of "allelomorph" ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders ) in the 1900s, which was used in the early days of genetics to describe variant forms of a gene detected in different phenotypes and identified to cause the differences between them. It derives from the Greek prefix ἀλληλο-, allelo- , meaning "mutual", "reciprocal", or "each other", which itself

252-548: Is no reliable way to identify them visually; some smoky black horses are solid black, and some regular black horses are heavily sun bleached. In the study and discussion of equine coat color genetics , black is considered a "base" color, as is red . This designation makes the effects of other coat color genes easier to understand. Coat colors that are designated "black-based" include grullo (also called blue dun ), smoky black , smoky cream , silver black , classic champagne , and blue roan . Sometimes this designation includes

280-480: Is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus. Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.) The frequency of alleles in a diploid population can be used to predict

308-452: Is related to the Greek adjective ἄλλος, allos (cognate with Latin alius ), meaning "other". In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive , according to which of the two homozygous phenotypes the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed

SECTION 10

#1732765608427

336-409: Is tested "homozygous black" is EE and has no other color modifiers. However, it has become popular for individuals owning a horse that is homozygous for the extension gene (EE) to claim that the horse will "throw black." But, generally speaking, one horse cannot be guaranteed to "throw black" with all mates. The mate of a true black horse may contribute the a dominant Agouti allele, which will suppress

364-439: Is the one that leads to the "dominant" phenotype, and the other allele is said to be "recessive". The degree and pattern of dominance varies among loci. This type of interaction was first formally-described by Gregor Mendel . However, many traits defy this simple categorization and the phenotypes are modelled by co-dominance and polygenic inheritance . The term " wild type " allele is sometimes used to describe an allele that

392-403: Is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies ( Drosophila melanogaster ). Such a "wild type" allele was historically regarded as leading to a dominant (overpowering – always expressed), common, and normal phenotype, in contrast to " mutant " alleles that lead to recessive, rare, and frequently deleterious phenotypes. It

420-502: The alleles are different, they, and the organism, are heterozygous with respect to those alleles. Popular definitions of 'allele' typically refer only to different alleles within genes. For example, the ABO blood grouping is controlled by the ABO gene , which has six common alleles (variants). In population genetics , nearly every living human's phenotype for the ABO gene is some combination of just these six alleles. The word "allele"

448-484: The alternative allele. If the first allele is dominant to the second then the fraction of the population that will show the dominant phenotype is p + 2 pq , and the fraction with the recessive phenotype is q . With three alleles: In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) with a number of alleles (a) is given by the expression: A number of genetic disorders are caused when an individual inherits two recessive alleles for

476-423: The base color of a horse, it is important to disregard all pink-skinned white markings . White markings and patterns such as pinto and leopard have no bearing on the underlying base coat color of the animal. Black foals are typically born a mousy gray but can be darker shades. As many foals have primitive markings at birth, some black foals are mistaken for grullo or even bay dun; the primitive markings on

504-464: The bay family: bay , seal brown , buckskin , bay dun , silver bay , perlino , amber champagne , and bay roan . Horses with a black-based coat may also have added spotting patterns including leopard patterns seen on Appaloosas and the pinto coloring known as piebald . The genetics behind the black horse are relatively simple. The color black is primarily controlled by two genes: Extension and Agouti . The functional, dominant allele of

532-399: The black coloring and result in a bay foal. If a black is bred to a gray, the ensuing foal may also be gray. Other modifiers present in the mate may produce additional dilution colors or spotting patterns. Nonetheless certain individual pairings with appropriate DNA testing can, in some cases, be guaranteed to produce black. Allele An allele , or allelomorph , is a variant of

560-487: The coat must be entirely black, even if superficially sun bleached. A sun bleached black may be confused with a dark bay , but a trained eye can distinguish between them, particularly by examining the fine hairs around the eyes and muzzle. When a black horse is sun-bleached, the mane and tail often sun bleach most prominently, and the rest of the coat may have a rusty tinge. A sun-bleached black may also be mistaken as being smoky black , however their phenotypes overlap so there

588-485: The dilution factors) that further modify color. A DNA test , which uses hair with the root intact, has been developed to test for the Extension and Agouti genotypes . However, the terminology can be manipulated. Unfortunately, the extension test is often mislabeled as the "black test", leading to confusion. Neither the extension test nor the agouti test alone can identify a black horse. Together, they can determine that

SECTION 20

#1732765608427

616-605: The elements and sweat, and therefore their coats may lose some of their rich black character and may even resemble bay or seal brown , though examination of the color of hair around the eyes, muzzle and genitals will determine color. Some breeds of horses, such as the Friesian horse , Murgese and Ariegeois (or Merens), are almost exclusively black. Black is also common in the Fell pony , Dales pony , Ostfriesen and Alt-Oldenburger , Kladruber , and Groningen . When identifying

644-418: The extension gene (labeled "E") enables the horse to produce black pigment in the hair. Without this gene ( homozygous recessive condition "ee"), the coat is devoid of black pigment and the horse is some shade of red . The functional, dominant allele (or alleles) of the agouti gene (labeled "A") enable the horse to restrict black pigment to certain parts of the coat, notably the legs, mane and tail, allowing

672-403: The frequencies of the corresponding genotypes (see Hardy–Weinberg principle ). For a simple model, with two alleles; where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, p is the fraction of the population homozygous for the first allele, 2 pq is the fraction of heterozygotes, and q is the fraction homozygous for

700-791: The proportion of heterozygotes in the population. A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive. For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, I , I , and i, which determine compatibility of blood transfusions . Any individual has one of six possible genotypes (I I , I i, I I , I i, I I , and ii) which produce one of four possible phenotypes : "Type A" (produced by I I homozygous and I i heterozygous genotypes), "Type B" (produced by I I homozygous and I i heterozygous genotypes), "Type AB" produced by I I heterozygous genotype, and "Type O" produced by ii homozygous genotype. (It

728-529: The sequence of nucleotides at a particular location, or locus , on a DNA molecule. Alleles can differ at a single position through single nucleotide polymorphisms (SNP), but they can also have insertions and deletions of up to several thousand base pairs . Most alleles observed result in little or no change in the function of the gene product it codes for. However, sometimes different alleles can result in different observable phenotypic traits , such as different pigmentation . A notable example of this

756-511: The underlying red to show through, resulting in bay coloring. Without this gene (homozygous recessive condition "aa"), any black pigment present is unrestricted, resulting in a uniformly black coat. Thus a black horse has at least one copy of the functional, dominant "E" allele and two copies of the non-functional, recessive "a" allele. A mature true black horse can be safely said to possess at least one dominant extension gene (EE or Ee); and has no other dominant genes (such as agouti, gray, or any of

784-489: Was formerly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative "mutant" allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases , and more frequently in heterozygous form in " carriers " for the mutant allele. It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that

#426573