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
Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, a phosphate group; the nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
This information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences; the two strands of DNA are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a
Equine coat color
Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them. While most horses remain the same color throughout life, a few, over the course of several years, will develop a different coat color from that with which they were born. Most white markings are present at birth, the underlying skin color of a horse does not change, absent disease; the basic outline of equine coat color genetics has been resolved, DNA tests to determine the likelihood that a horse will have offspring of a given color have been developed for some colors. Discussion and controversy continues about some of the details those surrounding spotting patterns, color sub-shades such as "sooty" or "flaxen", markings. Genetically, all horses start out as either chestnut, called "red" by geneticists, represented by the absence of the extension gene; therefore and black are the two base colors. The Bay color is expressed when the Agouti gene, works on black; the vast range of all other coat colors are created by additional genes' action upon one of these three coat colors.
Statistically, the most seen horse color phenotypes are identified by the following terms: Bay: Body color ranges from a light reddish-brown to dark brown with "black points". The main color variations are: Dark bay: dark red or brown hair, difficult to distinguish from seal brown. Sometimes called "black bay", "mahogany bay", or "brown". Blood bay: bright red hair. Brown: The word "brown" is used by some breed registries to describe dark bays. There is a distinct allele that darkens a bay coat to seal brown, but it is not the cause of all forms of dark bay. Informally, "brown" is applied to many distinct coat colors. Most horses described by casual observers as "brown" are bay or chestnut. In the absence of DNA testing and bay can be distinguished from each other by looking at the mane and legs for the presence of black points. Chestnut: A reddish body color with no black. Mane and tail are the same lighter than the body coat; the main color variations are: Liver chestnut: dark brown coat. Sometimes a liver chestnut is simply called "brown".
Sorrel: Reddish-tan to red coat, about the color of a new penny. The most common shade of chestnut. Blond or light chestnut: seldom-used term for lighter tan coat with pale mane and tail. Gray: A horse with black skin but white or mixed dark and white hairs. Gray horses can be born any color, lighten as they age. Most will gray out to either a complete white or to a "fleabitten" coat, which retains speckles of the horses original colour. Most "white" horses are grays with a white hair coat. A gray horse is distinguished from a white horse by dark skin noticeable around the eyes, muzzle and other areas of thin or no hair. Variations of gray that a horse may exhibit over its lifetime include: Salt and Pepper or "steel" gray: Usually a younger horse, an animal with white and dark hairs evenly intermixed over most of the body. Dapple gray: a dark-colored horse with lighter rings of graying hairs, called dapples, scattered throughout. Fleabitten gray: an otherwise white-haired horse that develops red hairs flecked throughout the coat.
Rose gray: a gray horse with a reddish or pinkish tinge to its coat. This color occurs with a horse born bay or chestnut while the young horse is "graying out". Black: Black is uncommon, though it is not "rare". There are two types of fading black and non-fading black. Most black horses will fade to a brownish color. Non-fading black is a blue-black shade. Genetically, the two cannot yet be differentiated, some claim the difference occurs due to management rather than genetics, though this claim is hotly disputed. Most black foals are born a mousy grey or dun color; as their foal coat begins to shed out, their black color will show through, though in some breeds black foals are born jet black. For a horse to be considered black, it must be black except for white markings. A sun-bleached black horse is still black though it may appear to be a dark bay or brown. A visible difference between a true black and a dark chestnut or bay is seen in the fine hairs around the eyes and muzzle. Brindle: One of the rarest colors in horses linked to chimerism.
Characteristics are any color with "zebra-like" stripes, but most common is a brown horse with faint yellowish markings. A heritable brindle pattern in a family of American Quarter Horses, named Brindle1 was identified and announced in late 2016. Buckskin: A bay horse with one copy of the cream gene, a dilution gene that "dilutes" or fades the coat color to a yellow, cream, or gold while keeping the black points. Champagne: Produced by a different dilution gene than the cream gene, it lightens both skin and hair, but creates a metallic gold coat color with mottled skin and light colored eyes. Champagne horses are confused with palomino, dun, or buckskins. Cream dilution, an incomplete dominant gene that produces a diluted coat color with one copy of the allele and a full dilution with two copies. Colors produced include Palomino, Perlino and Smoky Cream or Smoky black. Cremello: A horse with a chestnut base coat and two cream genes that wash out all color until
A pinto horse has a coat color that consists of large patches of white and any other color. The distinction between "pinto" and "solid" can be tenuous, as so-called "solid" horses have areas of white hair. Various cultures throughout history appear to have selectively bred for pinto patterns. Many breeds of horses carry pinto patterns. Pinto coloring, known as "coloured" in nations using British English, is the most popular in the United States. While pinto colored horses are not considered as a "breed," several competing color breed registries have formed to encourage the breeding of pinto colored horses; the word "paint" is sometimes used to describe pinto horses but today, it is used for the American Paint Horse. Pinto patterns are visually and genetically distinct from the leopard complex spotting patterns characteristic of horse breeds such as the Appaloosa. Breeders who select for color are careful not to cross the two patterns, registries that include spotting color preferences will refuse registration to horses that exhibit characteristics of the "wrong" pattern.
The word "pinto" is Spanish for'painted', also'dappled' or'spotted'. While pinto coloration has yet to be identified as a wildtype by DNA studies or seen in cave art predating horse domestication, images from pottery and other art of ancient antiquity show horses with flashy spotted patterns, indicating that they may have been desirable traits and selectively bred for. Images of spotted horses appear in the art of Ancient Egypt, archaeologists have found evidence of horses with spotted coat patterns on the Russian steppes before the rise of the Roman Empire. Spotted horses were among those brought to the Americas by the Conquistadors. By the 17th century in Europe, spotted horses were quite fashionable, though when the fad ended, large numbers of newly-unsellable horses were shipped to the Americas, some of which were sold while others were turned loose to run wild; the color became popular among Native Americans, was bred for in the United States, which now has the greatest number of pinto horses in the world.
There are a number of words which describe the various color and spotting patterns of pinto horses. A pinto horse is genetically created when an allele for a spotting pattern is present; the genes that create the underlying base coat color are not related to the genes that create white spotting. The precise mechanisms that create spotting are not all understood, but those that are known have human parallels, such as piebaldism. What horse terminology describes as "pinto" or "coloured" has been called leucism or "partial albinism" by pigment researchers. Common terms for describing different types of pinto horses include: Piebald:. Any pinto pattern on a black base coat, thus a black-and-white spotted horse. Skewbald:. Any pinto pattern on any base coat other than black; as chestnut and bay are the most common base coat colors, skewbalds are most chestnut and white or bay and white. At one time, the term may have applied more to brown-looking pinto horses, but today it encompasses any color other than black.
Coloured: The term for pinto coloration in nations using British English, including both piebald and skewbald. Tricolored or Tricoloured: a term for horse with three colors, in nations using British English, it is incorporated into the term skewbald. Tobiano: The most common type of pinto, tobiano is a spotting pattern characterized by rounded markings with white legs and white across the back between the withers and the dock of the tail arranged in a vertical pattern and more white than dark, though the ideal is a 50-50 distribution, with the head dark, having markings seen on a non-pinto horse. I.e. star, strip, or blaze. Tobiano is a simple dominant trait caused by a single gene and therefore all tobiano horses have at least one tobiano parent. A DNA test exists for tobiano. Tobiano is not associated with any health concerns. Overo: A collective term used by the American Paint Horse Association, overo means "pinto, but not tobiano." It denotes patterns produced by at least three different genetic mechanisms: frame, splashed white or sabino, described below.
These patterns are characterized by irregular markings with more jagged edges than tobiano markings. The white crosses the back. While some currently-identified overo patterns appear to be dominant or incomplete dominant traits, overo-patterned foals are produced from two solid-colored parents. Frame or frame overo: Frame is a popular and recognized type of non-tobiano pinto; this spotting pattern, in the absence of genes for other patterns, is characterized by horizontally-oriented white patches with jagged, crisp edges. White patches include the head and lateral aspects of the neck and body, the eyes can be blue. Frame overos may have modest markings that are not "pinto." This quality allows the pattern to "hide" for generations, is thought to be responsible for some cases of "cropouts." Frame is an incomplete dominant trait. However, foals born with two copies die shortly after birth. N/O frame horses do not have any known health defe
The Haflinger known as the Avelignese, is a breed of horse developed in Austria and northern Italy during the late nineteenth century. Haflinger horses are small, are always chestnut with flaxen mane and tail, have distinctive gaits described as energetic but smooth, are well-muscled yet elegant; the breed traces its ancestry to the Middle Ages. Haflingers, developed for use in mountainous terrain, are known for their hardiness, their current conformation and appearance are the result of infusions of bloodlines from Arabian and various European breeds into the original native Tyrolean ponies. The foundation sire, 249 Folie, was born in 1874. All Haflingers can trace their lineage back to Folie through one of seven bloodlines. World Wars I and II, as well as the Great Depression, had a detrimental effect on the breed, lower-quality animals were used at times to save the breed from extinction. During World War II, breeders focused on horses that were shorter and more draft-like, favored by the military for use as packhorses.
The emphasis after the war shifted toward animals of increased height. In the postwar era, the Haflinger was indiscriminately crossed with other breeds and some observers feared the breed was in renewed danger of extinction. However, starting in 1946, breeders focused on producing purebred Haflingers and a closed stud book was created. Interest in the breed increased in other countries and between 1950 and 1974 the population grew while the overall European horse population decreased. Population numbers continued to increase and as of 2005 250,000 Haflingers existed worldwide. There are breeding farms in several countries, although most of the breeding stock still comes from Austria. In 2003, a Haflinger became the first horse to be cloned. Haflingers have many uses including light draft, harness work and various under-saddle disciplines such as endurance riding, equestrian vaulting and therapeutic riding, they are still used by the Austrian and German armies for work in rough terrain. The World Haflinger Federation, the international governing body that controls breed standards for the Haflinger, is made up of a confederation of 22 national registries, helps set breeding objectives and rules for its member organizations.
The name "Haflinger" comes from the village of Hafling. The breed is called the Avelignese, from the Italian name for Hafling, Avelengo or Aveligna. Haflingers are always chestnut in color and come in shades ranging from a light gold to a rich golden chestnut or liver hue; the mane and tail are flaxen. The height of the breed has increased since the end of World War II, when it stood an average of 13.3 hands. The desired height today is between 15 hands. Breeders are discouraged from breeding horses under the minimum size, but taller individuals may pass inspection if they otherwise meet requirements of the breed registry; the breed has a refined light poll. The neck is of medium length, the withers are pronounced, the chest deep; the back is medium-long and muscular, the croup is long sloping and well-muscled. The legs are clean, with broad, flat knees and powerful hocks showing clear definition of tendons and ligaments; the Haflinger has ground-covering gaits. The walk is energetic; the trot and canter are elastic and athletic with a natural tendency to be light on the forehand and balanced.
There is some knee action, the canter has a distinct motion forwards and upwards. One important consideration in breeding during the second half of the 20th century was temperament. A requirement for a quiet, kind nature has become part of official breed standards and is checked during official inspections; some sources recognize two types of Haflinger, a shorter, heavier type used for draft work and a taller, lighter type used for pleasure riding, light driving and under-saddle competition. The Food and Agriculture Organization recognizes both an "Avelignese" and an "Avelignese Tradizionale" as existing in Italy, although, as of 2007, only 13 of the latter existed, including only one breeding stallion. However, all breed organizations register only one type. All Haflingers today trace their lineage through one of seven stallion lines to Folie, the foundation stallion of the breed. Colts are given a name beginning with the letter or letters denoting their stallion line, fillies are given a name beginning with the first letter of their dam's name.
The exceptions are France, where foals are given a name beginning with a letter of the alphabet designated to be used for that year. The seven stallion lines are: A-line. Founded by Anselmo, born 1926. One of the most prevalent lines today, descendants include the second-largest number of stallions at stud. Anselmo was brought back to stud at the age of 21, when a lack of stallions after World War II led to concerns that the line would not survive, produced several stallions now represented in all Haflinger breeding populations worldwide. B-line. Founded by Bolzano, born 1915. Bolzano's less common line, although strong in Austria, is not prevalent elsewhere; the line is spreading nevertheless. S. and several European countries including Great Britain are establishing Bolzano lines. M-line. Founded by Massimo, born 1927. An Italian
The cream gene is responsible for a number of horse coat colors. Horses that have the cream gene in addition to a base coat color, chestnut will become palomino if they are heterozygous, having one copy of the cream gene, or cremello, if they are homozygous. Horses with a bay base coat and the cream gene will be buckskin or perlino. A black base coat with the cream gene becomes the not-always-recognized smoky black or a smoky cream. Cream horses those with blue eyes, are not white horses. Dilution coloring is not related to any of the white spotting patterns; the cream gene is an incomplete dominant allele with a distinct dosage effect. The DNA sequence responsible for the cream colors is the cream allele, at a specific locus on the MATP gene, its general effect is to lighten the coat and eye colors. When one copy of the allele is present, it dilutes "red" pigment to yellow or gold, with a stronger effect on the mane and tail, but does not dilute black color to any significant degree; when two copies of the allele are present, both red and black pigments are affected.
A single copy of the allele has minimal impact on eye color, but when two copies are present, a horse will be blue-eyed in addition to a light coat color. The cream gene is one of several dilution genes identified in horses. Therefore, it is not always possible to tell by color alone whether the CCr allele is present without a DNA test. Other dilution genes that may mimic some of the effects of the cream gene in either single or double copies include the pearl gene, silver dapple gene, the champagne gene. Horses with the dun gene may mimic a single copy of the cream gene. To complicate matters further, it is possible for a horse to carry more than one type of dilution gene, sometimes giving rise to coloring that researchers call a pseudo double dilute; the discovery of the cream gene had a significant effect on breeding, allowing homozygous blue-eyed creams to be recognized by many breed registries that had registered palominos but banned cremellos, under the mistaken notion that homozygous cream was a form of Albinism.
Cream coat colors are described by their relationship to the three "base" coat colors: chestnut and black. All horses obtain two copies of the MATP gene. A horse may have the non-cream allele on each gene; those with two non-cream alleles will not exhibit true cream traits. Horses with one cream allele and one non-cream allele, popularly called "single dilutes," exhibit specific traits: all red pigment in the coat is gold, while the black pigment is either unaffected or only subtly affected; these horses are palomino, buckskin, or smoky black. These horses have light brown eyes. Horses with 2 copies of the cream allele exhibit specific traits: cream-colored coats, pale blue eyes, rosy-pink skin; these horses are called cremello, perlino, or smoky cream. Horses that are heterozygous creams, that is, have only one copy of the cream gene, have a lightened hair coat; the precise cream dilute coat color produced depends on the underlying base coat color. Unless affected by other, unrelated genes, they maintain dark skin and brown eyes, though some heterozygous dilutes may be born with pink skin that darkens with age.
Some have lighter, amber eyes. However, the heterozygous cream dilute must not be confused with a horse carrying champagne dilution. Champagne dilutes are born with pumpkin-pink skin and blue eyes, which darken within days to amber, green or light brown, their skin acquires a darker mottled complexion around the eyes and genitalia as the animal matures, it is possible for a heterozygous cream horse to carry more than one of the other dilution alleles. In such cases, they may exhibit some characteristics more typical of a homozygous dilute. Palomino is the best known cream-based coat color, is produced by the action of one cream allele on a chestnut coat, it is characterized by a cream or white tail and yellow or gold coat. The classic golden shade akin to that of a newly minted gold coin is common, but there are other variations: the darkest shades are called sooty palominos, unusual but most seen in Morgans, can include a mane and tail with darker hairs and heavy dappling in the coat; the palest varieties can be nearly white, retaining darker skin and eyes, are sometimes mistakenly confused with cremello, are called isabellas in some places.
Buckskin is a well-known color, produced by the action of one cream gene on a bay coat. All red hairs in the base coat are diluted to gold; the black areas, such as the mane and legs, are unaffected. The cream gene acting on a "blood bay" coat, the reddest shade, are pale gold with black points, they are sometimes called buttermilk buckskins. The cream gene acting on the darkest bays, may dilute to a sooty buckskin. True seal brown buckskins can be difficult to identify owing to their all-black coats, it is only the reddish markings around the eyes, muzzle and groin, which are turned gold, that may give them away. Smoky black, a horse with a black base coat and one copy of the cream allele, is less well-known than the two golden shades. Since a single copy of the cream gene affects red pigment, with only a subtle effect on black, smoky blacks can be quite difficult to identify. Smoky blacks may have reddish guard hairs inside their ears, experienced horse persons may detect something "off" about the coat of a smoky black, though the burnished look is chalked up to sun bleaching, which can be s
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