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'{{refimprove|date=May 2014}} {{for|a non-technical introduction to the topic|Introduction to genetics}} {{Genetics sidebar}} [[File:Gregor Mendel.png|thumb|250px|[[Gregor Mendel]], the German-speaking Augustinian monk who founded the modern science of genetics.]] '''Mendelian inheritance''' is [[inheritance (biology)|inheritance]] of [[biology|biological]] features that follows the laws proposed by [[Gregor Johann Mendel]] in 1865 and 1866 and re-discovered in 1900. It was initially very controversial . When Mendel's theories were integrated with the [[Boveri–Sutton chromosome theory|chromosome theory of inheritance]] by [[Thomas Hunt Morgan]] in 1915, they became the core of [[classical genetics]]. ==History== {{main|History of genetics}} The laws of inheritance were derived by [[Gregor Mendel]], a nineteenth-century [[Austrians|Austrian]] [[monk]] conducting hybridization experiments in garden peas (''[[Pisum sativum]]'') he planted in the backyard of the church.<ref name=Henig>{{cite book | last = Henig | first = Robin Marantz | title = The Monk in the Garden : The Lost and Found Genius of Gregor Mendel, the Father of Modern Genetics | publisher = Houghton Mifflin | year = 2009 | isbn = 0-395-97765-7 | quote = The article, written by an Austrian monk named Gregor Johann Mendel... }}</ref> Between 1856 and 1863, he cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known as ''Mendel's Principles of Heredity'' or ''Mendelian inheritance''. He described these principles in a two-part paper, ''Versuche über Pflanzen-Hybriden'' (''[[Experiments on Plant Hybridization]]''), that he read to the Natural History Society of [[Brno]] on February 8 and March 8, 1865, and which was published in 1866.<ref>See Mendel's paper in English: {{cite web| title=Experiments in Plant Hybridization| author=Gregor Mendel| year=1865| url= http://www.mendelweb.org/Mendel.html}}</ref> Mendel's conclusions were largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th-century biologists to the apparent [[Quantitative trait locus|blending]] of inherited traits in the overall appearance of the progeny, now known to be due to multigene interactions, in contrast to the organ-specific binary characters studied by Mendel.<ref name=Henig/> In 1900, however, his work was "re-discovered" by three European scientists, [[Hugo de Vries]], [[Carl Correns]], and [[Erich von Tschermak]]. The exact nature of the "re-discovery" has been somewhat debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries' paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.<ref name=Henig/> Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was [[William Bateson]], who coined the terms "[[genetics]]" and "[[allele]]" to describe many of its tenets. The model of heredity was highly contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such as [[R. A. Fisher]] showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed. [[Thomas Hunt Morgan]] and his assistants later integrated the theoretical model of Mendel with the [[chromosome]] theory of inheritance, in which the chromosomes of [[cell (biology)|cell]]s were thought to hold the actual hereditary material, and created what is now known as [[classical genetics]], which was extremely successful and cemented Mendel's place in history. Mendel's findings allowed other scientists to predict the expression of traits on the basis of mathematical probabilities. A large contribution to Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were [[True breeding organism|true-breeding]]. He also only measured absolute (binary) characteristics, such as color, shape, and position of the offspring, rather than quantitative characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large [[sample size]] gave credibility to his data. He also had the foresight to follow several successive generations (f2, f3) of pea plants and record their variations. Finally, he performed "test crosses" (back-crossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportion of recessive characters. <!--Without his diligence and careful attention to procedure and detail, Mendel's work would have had a much smaller impact on the world of genetics.{{cn|date=April 2012}}--> ==Mendel's laws== [[File:Punnett square mendel flowers.svg|thumb|250px|right|A [[Punnett square]] for one of Mendel's pea plant experiments.]] Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the F₁ generation) was purple-flowered. When Mendel [[self-fertilization|self-fertilized]] the F₁ generation pea plants, he obtained a purple flower to white flower ratio in the F₂ generation of 3 to 1. The results of this cross are tabulated in the [[Punnett square]] to the right. He then conceived the idea of heredity units, which he called "factors". Mendel found that there are alternative forms of factors—now called [[gene]]s—that account for variations in inherited characteristics. For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now called [[allele]]s. For each [[trait (biology)|biological trait]], an organism inherits two alleles, one from each parent. These alleles may be the same or different. An organism that has two identical alleles for a gene is said to be [[homozygous]] for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said be [[heterozygous]] for that gene (and is called a heterozygote). Mendel also hypothesized that allele pairs separate randomly, or segregate, from each other during the production of [[gametes]]: egg and sperm. Because allele pairs separate during gamete production, a sperm or egg carries only one allele for each inherited trait. When sperm and egg unite at [[fertilization]], each contributes its allele, restoring the paired condition in the offspring. This is called the '''Law of Segregation'''. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation. This is known as the '''Law of Independent Assortment'''. The [[genotype]] of an individual is made up of the many alleles it possesses. An individual's physical appearance, or [[phenotype]], is determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is called the [[Dominance (genetics)|dominant allele]]; the other has no noticeable effect on the organism’s appearance and is called the [[Dominance (genetics)|recessive allele]]. Thus, in the example above dominant purple flower allele will hide the phenotypic effects of the recessive white flower allele. This is known as the '''Law of Dominance'''. We use upper case letters to represent dominant alleles and lowercase letters to represent recessive alleles. {| border="1" class="wikitable" |+ Mendel's Laws of Inheritance |- ! Law ! Definition |- | Law of Segregation | During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene. |- | Law of Independent Assortment | Genes for different traits can segregate independently during the formation of gametes. |- | Law of Dominance | Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele. |} In the pea plant example above, the capital "P" represents the dominant allele for purple flowers and lowercase "p" represents the recessive allele for white flowers. Both parental plants were true-breeding, and one parental variety had two alleles for purple flowers (''PP'') while the other had two alleles for white flowers (''pp''). As a result of fertilization, the F₁ hybrids each inherited one allele for purple flowers and one for white. All the F₁ hybrids (''Pp'') had purple flowers, because the dominant ''P'' allele has its full effect in the heterozygote, while the recessive ''p'' allele has no effect on flower color. For the F₂ plants, the ratio of plants with purple flowers to those with white flowers (3:1) is called the phenotypic ratio. The genotypic ratio, as seen in the Punnnett square, is 1 ''PP'' : 2 ''Pp'' : 1 ''pp''. ==={{anchor|Law of Segregation}}<!--[[Law of Segregation]] links here-->Law of Segregation (the "First Law")<span id="First Law"></span>=== [[Image:Mendelian inheritance.svg|thumb|left|230px|'''Figure 1 Dominant and recessive phenotypes.'''<br>(1) Parental generation.<br>(2) F₁ generation.<br>(3) F₂ generation. Dominant (<span style="color:#990000;">red</span>) and recessive (white) phenotype look alike in the F₁ (first) generation and show a 3:1 ratio in the F₂ (second) generation.]] The Law of Segregation states that every individual contains a pair of alleles for each particular trait which segregate or separate during cell division (assuming diploidy) for any particular trait and that each parent passes a randomly selected copy (allele) to its offspring. The offspring then receives its own pair of alleles of the gene for that trait by inheriting sets of [[homologous chromosome]]s from the parent organisms. Interactions between alleles at a single locus are termed dominance and these influence how the offspring expresses that trait (e.g. the color and height of a plant, or the color of an animal's fur). Book definition: The Law of Segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.*{{clarify|date=January 2014}} More precisely, the law states that when any individual produces [[gamete]]s, the copies of a [[gene]] separate so that each gamete receives only one copy (allele). A gamete will receive one allele or the other. The direct proof of this was later found following the observation of [[meiosis]] by two independent scientists, the German botanist [[Oscar Hertwig]] in 1876, and the Belgian zoologist [[Edouard Van Beneden]] in 1883. Paternal and maternal chromosomes get separated in meiosis and the alleles with the traits of a character are segregated into two different gametes. Each parent contributes a single gamete, and thus a single, randomly successful allele copy to their offspring and fertilization. ==={{anchor|Law of Independent Assortment}}<!--[[Law of Independent Assortment]] links here-->Law of Independent Assortment (the "Second Law")=== [[Image:Dihybrid cross.svg|thumb|right|280px|'''Figure 2 Dihybrid cross.''' The phenotypes of two independent traits show a 9:3:3:1 ratio in the F₂ generation. In this example, coat color is indicated by '''B''' (brown, dominant) or '''b''' (white), while tail length is indicated by '''S''' (short, dominant) or '''s''' (long). When parents are homozygous for each trait ('''SSbb''' and '''ssBB'''), their children in the F₁ generation are heterozygous at both loci and only show the dominant phenotypes ('''SsbB'''). If the children mate with each other, in the F₂ generation all combinations of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box).]] The Law of Independent Assortment, also known as "Inheritance Law", states that separate genes for separate traits are passed independently of one another from parents to offspring. That is, the biological selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait. More precisely, the law states that alleles of different genes assort independently of one another during gamete formation. While Mendel's experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios (Fig. 2). But the 9:3:3:1 table shows that each of the two genes is independently inherited with a 3:1 phenotypic ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat's color and tail length. This is actually only true for genes that are not [[Genetic linkage|linked]] to each other. Independent assortment occurs in [[eukaryotic]] organisms during [[metaphase I|meiotic metaphase I]], and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with [[chromosomal crossover|crossing over]], independent assortment increases genetic diversity by producing novel genetic combinations. Of the 46 chromosomes in a normal [[diploid]] human cell, half are maternally derived (from the mother's [[ovum|egg]]) and half are paternally derived (from the father's [[spermatozoon|sperm]]). This occurs as [[sexual reproduction]] involves the fusion of two [[haploid]] gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During [[gametogenesis]]—the production of new gametes by an adult—the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed [[aneuploidy]]. In independent assortment, the chromosomes that result are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 2²³ or 8,388,608 possible combinations.<ref>{{cite web | title = Meiosis | publisher = | author = Perez, Nancy | date = | url = http://www.web-books.com/MoBio/Free/Ch8C.htm | accessdate = 2007-02-15 }}</ref> The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny. ==={{anchor|Law of Dominance}}<!--[[Law of Dominance]] links here-->Law of Dominance (the "Third Law")<span id="Third Law"></span>=== Mendel's Law of Dominance states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. Law of Dominance can be explained easily with the help of a mono hybrid cross experiment:- In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called "dominant" and the one which is suppressed (not expressed) is called "recessive." Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. It is important to note however, that the law of dominance is significant and true but is not universally applicable. According to the latest revisions, only two of these rules are considered to be laws. The third one is considered as a basic principle but not a genetic law of Mendel.{{citation needed|date=October 2014}} ==Mendelian trait== A Mendelian trait is one that is controlled by a single [[locus (genetics)|locus]] in an inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include [[sickle-cell anemia]], [[Tay-Sachs disease]], [[cystic fibrosis]] and [[xeroderma pigmentosa]]. A disease controlled by a single gene contrasts with a multi-factorial disease, like [[arthritis]], which is affected by several loci (and the environment) as well as those diseases inherited in a [[non-Mendelian inheritance|non-Mendelian]] fashion.{{Citation needed|date=December 2012}} ==Non-Mendelian inheritance== [[Image:Mendelian inheritance 1 2 1.png|thumb|right|230px|In four o'clock plants, the alleles for red and white flowers show incomplete dominance. As seen in the F₁ generation, heterozygous (''wr'') plants have "<span style="color:magenta;">pink</span>" flowers—a mix of "<span style="color:#990000;">red</span>" (rr) and "white" (ww) coloring. The F₂ generation shows a 1:2:1 ratio of <span style="color:#990000;">red</span>:<span style="color:magenta;">pink</span>:<span style="color:black;">white</span>]] {{Main|Non-Mendelian inheritance}} Mendel explained inheritance in terms of discrete factors—genes—that are passed along from generation to generation according to the rules of probability. Mendel's laws are valid for all sexually reproducing organisms, including garden peas and human beings. However, Mendel's laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel's laws can strictly account for the patterns of inheritance are relatively rare. Often, the inheritance patterns are more complex. The F₁ offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance," the dominant allele had the same phenotypic effect whether present in one or two copies. But for some characteristics, the F₁ hybrids have an appearance ''in between'' the phenotypes of the two parental varieties. A cross between two four o'clock (''[[Mirabilis jalapa]]'') plants shows this common exception to Mendel's principles. Some alleles are neither dominant nor recessive. The F₁ generation produced by a cross between red-flowered (RR) and white flowered (WW) ''Mirabilis jalapa'' plants consists of pink-colored flowers (RW). Which allele is dominant in this case? Neither one. This third phenotype results from flowers of the heterzygote having less red pigment than the red homozygotes. Cases in which one allele is not completely dominant over another are called '''[[incomplete dominance]]'''. In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes. A similar situation arises from '''[[codominance]]''', in which the phenotypes produced by both alleles are clearly expressed. For example, in certain varieties of [[chicken]], the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as "erminette," speckled with black and white feathers. Unlike the blending of red and white colors in heterozygous four o'clocks, black and white colors appear separately in chickens. Many human genes, including one for a protein that controls cholesterol levels in the blood, show codominance, too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels. In Mendelian inheritance, genes have only two alleles, such as ''a'' and ''A''. In nature, such genes exist in several different forms and are therefore said to have '''[[multiple alleles]]'''. A gene with more than two alleles is said to have multiple alleles. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. One of the best-known examples is coat color in rabbits. A rabbit's coat color is determined by a single gene that has at least four different alleles. The four known alleles display a pattern of simple dominance that can produce four coat colors. Many other genes have multiple alleles, including the human genes for [[ABO blood type]]. Furthermore, many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be '''[[polygene|polygenic traits]]'''. ''Polygenic'' means "many genes." For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The variety of skin color in humans comes about partly because more than four different genes probably control this trait. ==See also== * [[List of Mendelian traits in humans]] * [[Mendelian disease]]s (monogenic disease) * [[Mendelian error]] * [[Particulate inheritance]] * [[Punnett square]] * [[Introduction to genetics]] ==References== {{Reflist}} ==Notes== * {{cite book | author = Peter J. Bowler | year = 1989 | title = The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society | publisher = Johns Hopkins University Press | location = }} * {{cite book|last=Atics|first=Jean|title=Genetics: The life of DNA|publisher=ANDRNA press}} *Reece, Jane B., and Neil A. Campbell. "Mendel and the Gene Idea." Campbell Biology. 9th ed. Boston: Benjamin Cummings / Pearson Education, 2011. 265. Print. ==External links== * [http://www.mendeliangenetics.net/ Mendelian Genetics Laws] * [http://www.khanacademy.org/video/introduction-to-heredity?playlist=Biology Khan Academy, video lecture] * [http://anthro.palomar.edu/mendel/mendel_2.htm Probability of Inheritance] * [http://www.biotechlearn.org.nz/themes/mendel_and_inheritance/mendel_s_principles_of_inheritance=Mendel's principles of Inheritance] * [http://embryo.asu.edu/handle/10776/6240= ""Experiments in Plant Hybridization" (1866), by Johann Gregor Mendel" by A. Andrei at the Embryo Project Encyclopedia] {{Lysenkoism}} {{DEFAULTSORT:Mendelian Inheritance}} [[Category:Genetics]] [[Category:Gregor Mendel|Genetics]] [[it:Gregor Mendel#Le leggi di Mendel]]'