Mendelian inheritance
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2008-07-15T20:28:43Z
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/* Law of Segregation */
{{for|a non-technical introduction to the topic|Introduction to genetics}}
'''Mendelian inheritance''' (or '''Mendelian genetics''' or '''Mendelism''') is a set of primary tenets relating to the transmission of [[heredity|hereditary]] characteristics from parent organisms to their children; it underlies much of [[genetics]]. They were initially derived from the work of [[Gregor Mendel]] published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the [[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 19th century [[Moravia]]n<ref>{{cite book
| last = Henig
| first = Robin Marantz
| title = The Monk in the Garden : The Lost and Found Genius of Gregor Mendel, the Father of Genetics
| publisher = Houghton Mifflin
| date = 2000
| isbn = 0-395-97765-7
| quote = The article, written by an obscure Moravian monk named Gregor Mendel...
}}</ref> monk conducting plant hybridity experiments. Between 1856 and 1863, he cultivated and tested some 28,000 pea plants. His experiments brought forth two generalizations which later became known as ''Mendel's Laws of Heredity'' or ''Mendelian inheritance''. These are described in his essay "[[Experiments on Plant Hybridization]]" that was read to the Natural History Society of [[Brno]] on [[February 8]] and [[March 8]], [[1865]], and 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 results were largely rejected. Though they were not completely unknown to biologists of the time, they were not seen as being crucial. Even Mendel himself did not see their ultimate applicability, and thought they only applied to certain categories of species. In 1900, however, the 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, and Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority, and 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.
Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was [[William Bateson]], who coined the term "[[genetics]]", "[[gene]]", 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. Many biologists also dismissed the theory because they were not sure it would apply to all species, and there seemed to be very few true Mendelian characters in nature. However later work by biologists and statisticians such as [[R.A. Fisher]] showed that if multiple Mendelian factors were involved for individual traits, they could produce the diverse amount of results observed in nature. [[Thomas Hunt Morgan]] and his assistants would later integrate the theoretical model of Mendel with the chromosome theory of inheritance, in which the [[chromosome]]s of [[cell (biology)|cell]]s were thought to hold the actual hereditary particles, and create 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 simplify the emergence of traits to mathematical probability. A large portion of Mendel's findings can be traced to his choice to start his experiments only with true breeding plants. He also only measured absolute characteristics such as color, shape, and position of the offspring. His data was expressed numerically and subjected to statistical analysis. This method of data reporting and the large [[sampling size]] he used gave credibility to his data. He also had the foresight to look through several successive generations of his pea plants and record their variations. Without his careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.
==Mendel's Laws==
=== '''Law of Segregation''' ===
The Law of Segregation, also known as '''Mendel's First Law''', essentially has three parts.
#'''Alternative versions of genes account for variations in inherited characteristics.''' This is the concept of [[allele]]s. Alleles are different versions of genes that impart the same characteristic. For example, each human have genes that control [[eye color]], but there are variations among these genes in accordance with the specific color for which the gene "codes".
#'''For each characteristic, an organism inherits two alleles, one from each parent.''' This means that when [[somatic cell]]s are produced from two alleles, one allele comes from the mother and one from the father. These alleles may be the same (true-breeding organisms/homozygous e.g. ''ww'' and ''rr'' in Fig. 3), or different (hybrids/heterozygous, e.g. ''wr'' in Fig. 3).
#'''The two alleles for each characteristic segregate during gamete production.''' This means that each gamete will contain only one allele for each gene. This allows the maternal and paternal alleles to be combined in the offspring, ensuring variation.
N.B It is often misconstrued that the gene itself is dominant, recessive, codominant, or incompletely dominant. It is, however, the ''trait'' or gene product that the allele encodes that is dominant, etc.
===Law of Independent Assortment===
The Law of Independent Assortment, also known as "Inheritance Law", states that the inheritance pattern of one trait will not affect the inheritance pattern of another. 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 are independently inherited with a 3:1 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 during [[meiosis I]] in [[eukaryotic]] organisms, specifically [[anaphase I]] of ''meiosis'',<ref>{{cite web
| title = Meiosis & Sexual Reproduction
| publisher = [[University of Alabama]]
| author =
| date =
| url = http://bama.ua.edu/~ksuberkp/bsc114/Lectures/B14meios.htm
| accessdate = 2007-02-15 }}</ref> to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with [[chromosomal crossover]], this process aids in increasing 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 end up in a newly-formed gamete 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 2^23 or 8,388,608 possible combinations.<ref>{{cite web
| title = Meiosis
| publisher =
| author =
| 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.
{| class="wikitable" style="text-align:center; width:100%"
|-
|[[Image:Mendelian inheritance 3 1.png|thumb|center|230px|'''Figure 1:''' Dominant and recessive phenotypes.<br>(1) Parental generation. (2) F<sub>1</sub> generation. (3) F<sub>2</sub> generation. Dominant (<font color="#990000">red</font>) and recessive (white) phenotype look alike in the F<sub>1</sub> (first) generation and show a 3:1 ratio in the F<sub>2</sub> (second) generation]]
|[[Image:Dihybrid cross.png|thumb|center|280px|'''Figure 2:''' The genotypes of two independent traits show a 9:3:3:1 ratio in the F<sub>2</sub> 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 F<sub>1</sub> generation are heterozygous at both loci and only show the dominant phenotypes. If the children mate with each other, in the F<sub>2</sub> generation all combination 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).]]
|[[Image:Mendelian inheritance 1 2 1.png|thumb|center|230px|'''Figure 3:''' The color alleles of [[Mirabilis jalapa]] are not dominant or recessive.<br>(1) Parental generation. (2) F<sub>1</sub> generation. (3) F<sub>2</sub> generation. The "<font color="#990000">red</font>" and "white" allele together make a "<font color="#FF00FF">pink</font>" phenotype, resulting in a 1:2:1 ratio of <font color="#990000">red</font>:<font color="#FF00FF">pink</font>:<font color="#000000">white</font> in the F<sub>2</sub> generation.]]
|}
==Background==
The reason for these laws is found in the nature of the [[cell nucleus]]. It is made up of several [[chromosome]]s carrying the genetic [[trait]]s. In a normal cell, each of these chromosomes has two parts, the [[chromatid]]s. A reproductive cell, which is created in a process called [[meiosis]], usually contains only one of those chromatids of each chromosome. By merging two of these cells (usually one male and one female), the full set is restored and the genes are mixed. The resulting cell becomes a new [[embryo]]. The fact that this new life has half the genes of each parent (23 from mother, 23 from father for total of 46) is one reason for the Mendelian laws. The second most important reason is the varying [[dominant gene|dominance]] of different genes, causing some traits to appear unevenly instead of averaging out (whereby dominant doesn't mean more likely to reproduce - [[recessive gene]]s can become the most common, too).
There are several advantages of this method (sexual reproduction) over reproduction without genetic exchange:
# Instead of nearly identical copies of an organism, a broad range of offspring develops, allowing more different abilities and [[evolution]]ary strategies.
# There are usually some errors in every cell nucleus. Copying the genes usually adds more of them. By distributing them randomly over different chromosomes and mixing the genes, such errors will be distributed unevenly over the different children. Some of them will therefore have only very few such problems. This helps reduce problems with copying errors somewhat.
# Genes can spread faster from one part of a population to another. This is for instance useful if there's a temporary isolation of two groups. New genes developing in each of the populations don't get reduced to half when one side replaces the other, they mix and form a population with the advantages of both sides.
# Sometimes, a mutation (e. g. [[sickle cell anemia]]) can have positive side effects (in this case [[malaria]] resistance). The mechanism behind the Mendelian laws can make it possible for some offspring to carry the advantages without the disadvantages until further [[mutation]]s solve the problems.
==Mendelian trait==
A '''Mendelian trait''' is one that is controlled by a single [[locus (genetics)|locus]] and shows a simple Mendelian 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. The [[Mendelian Inheritance in Man]] database is a catalog of, among other things, genes in which Mendelian mutants causes disease.
==See also==
*[[non-Mendelian inheritance]]
*[[Dominance relationship]]
*[[Mendelian error]]
*[[List of Mendelian traits in humans]]
==References==
<references/>
*{{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 = Baltimore
Genetics:The life of DNA
Publisher = ANDRNA press
Author = Jean Atics
}}
[[Category:Classical genetics]]
[[af:Wette van Mendel]]
[[bg:Закони на Мендел]]
[[ca:Lleis de Mendel]]
[[cs:Mendelovy zákony dědičnosti]]
[[de:Mendelsche Regeln]]
[[et:Mendeli seadused]]
[[el:Μεντελική κληρονομικότητα]]
[[es:Leyes de Mendel]]
[[fr:Lois de Mendel]]
[[ko:멘델의 유전법칙]]
[[id:Hukum Pewarisan Mendel]]
[[it:Gregor Mendel#Le_leggi_di_Mendel]]
[[he:חוקי התורשה של מנדל]]
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[[nl:Wetten van Mendel]]
[[ja:メンデルの法則]]
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[[ru:Законы Менделя]]
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[[sr:Менделови закони]]
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[[uk:Закони Менделя]]
[[zh:孟德尔定律]]