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{{BioPsy}}
'''Zygosity''' refers to the [[Genetics|genetic]] condition of a [[zygote]]. In genetics, zygosity describes the similarity or dissimilarity of [[DNA]] between [[homologous]] [[chromosome]]s at a specific [[Allele|allelic]] position or [[gene]].
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'''Zygosity''' refers to the similarity of [[allele]]s for a trait in an organism. If both alleles are the same, the organism is [[Zygosity#Homozygous| homozygous]] for the trait. If both alleles are different, the organism is [[Zygosity#Heterozygous| heterozygous]] for that trait. If one allele is missing, it is [[Zygosity#Hemizygous| hemizygous]], and, if both alleles are missing, it is [[Zygosity#Nullizygous| nullizygous]].
 
The terms '''homozygous''', '''heterozygous''' and '''hemizygous''' are used to simplify the description of the [[genotype]] of a [[diploid]] organism at a single genetic locus. At a given gene or position along a chromosome (a locus), the DNA sequence can vary among individuals in the population. The variable DNA segments are referred to as alleles, and diploid organisms generally have two alleles at each locus, one allele for each of the two [[Homologous chromosome|homologous chromosomes]]. Simply stated, ''homozygous'' describes two identical alleles or DNA sequences at one locus, ''heterozygous'' describes two different alleles at one locus, and ''hemizygous'' describes the presence of only a single copy of the gene in an otherwise diploid organism.
 
   
Zygosity is also used to describe the genetic condition of the zygote(s) from which [[twin]]s emerge, where it refers to the similarity or dissimilarity of the twins' DNA. Identical twins are ''monozygotic'' - they develop from one zygote (one fertilized egg that develops into two embryos). Fraternal twins are ''dizygotic'' - they developed separately from two zygotes (two fertilized eggs). ''For a description of these terms, see [[twin]]s.''
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Most [[eukaryote]]s have two matching sets of [[chromosomes]]; that is, they are [[diploid]]. Diploid organisms have the same genes on each of their two sets of homologous chromosomes, except that the sequences of these genes may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a [[Sex-determination system#Chromosomal determination|sex-determination system]].
   
==Homozygous==
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The DNA sequence of a gene usually 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 only that allele can function properly. Any variation from the DNA sequence of that allele will be fatal in the embryo, and the organism will never survive to be born. 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, and another allele may be rare. Sometimes, one allele is a [[Genetic disorder|disease]] - causing variation while the other allele is healthy. Sometimes, the different variations in the alleles make no difference at all in the function of the organism.
An organism is referred to as being '''homozygous''' (basically meaning of the same alleles) at a specific locus when it carries two identical copies of the gene affecting a given trait on the two corresponding [[homologous chromosome]]s (e.g., the genotype is ''PP'' or ''pp'' when P and p refer to different possible alleles of the same gene). Such a cell or such an organism is called a ''homozygote''.
 
   
A ''homozygous dominant'' genotype occurs when a particular locus has two copies of the dominant allele (e.g. ''PP''). A ''homozygous recessive'' genotype occurs when a particular locus has two copies of the recessive allele (e.g. ''pp'').
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In diploid organisms, one allele is inherited from the male parent and one from the female parent. Zygosity is a description of whether those two alleles have identical or different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome.<ref>{{cite pmid|16780578}}</ref>
   
Pure-bred or [[true breeding]] organisms are homozygous.
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==Types==
For example a homozygous individual could have the allele combinations PP or pp.
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The words homozygous, heterozygous, and hemizygous are used to describe the [[genotype]] of a diploid organism at a single [[locus (genetics)|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, and ''nullizygous'' refers to an otherwise-diploid organism in which both copies of the gene are missing.
All homozygous alleles are either allozygous or autozygous.
 
   
===Allozygous===
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===Homozygous===
'''Allozygosity''' is when two alleles are alike, but unrelated. The two alleles had different ancestral alleles that through [[convergent evolution]] became similar.
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A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both [[homologous chromosomes]].<ref name="Henderson's">Lawrence, Eleanor: ''Henderson's Dictionary of Biology, 14e'', 2008</ref> The cell or organism in question is called a ''homozygote''. [[True breeding organism]]s are always homozygous for the traits that are to be held constant.
   
===Autozygous===
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An individual that is ''homozygous-dominant'' for a particular trait carries two copies of the allele that codes for the [[Dominance (genetics)|dominant]] trait. This allele, often called the "dominant allele", is normally represented by a capital letter (such as "P" for the dominant allele producing purple flowers in pea plants). 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".
'''Autozygosity''' is when two alleles are alike by relation, that is to say since they had a common ancestor, and they are similar.
 
   
==Heterozygous==<!-- This section is linked from [[Transgenic maize]] -->
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An individual that is ''homozygous-recessive'' for a particular trait carries two copies of the allele that codes for the [[recessive trait]]. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".
An organism is a ''heterozygote'' or is ''heterozygous'' at a locus or gene when it has different alleles occupying the gene's position in each of the homologous chromosomes. In other words, it describes an individual that has 2 different alleles for a trait. In diploid organisms, the two different alleles were inherited from the organism's two [[parent]]s. For example a heterozygous individual would have the allele combination Pp.
 
   
==Hemizygous==
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===Heterozygous===<!-- This section is linked from [[Transgenic maize]] -->
'''Hemizygous''' describes a diploid individual who has only one allele of a gene or chromosome segment rather than the usual two. A '''hemizygote''' refers to a cell or organism whose genome includes only one allele at a given locus. For organisms where the male is [[sex linkage|heterogametic]], such as humans, it refers in particular to [[X-linked gene]]s, since males normally possess only one X-chromosome. They are hemizygous for (nearly) all genes that are located on the X-chromosome.
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A [[Ploidy#Diploid|diploid]] organism is heterozygous at a gene locus when its cells contain two different [[allele]]s of a gene.<ref>[http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb Molecular Cell Biology], 4th edition by Harvey Lodish, et al [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A1876#A1877 Chapter 8]. (2000)</ref> Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele), such as "Rr" or "Ss". The capital letter is usually written first.
   
In a more extreme example, male honeybees ([[Drone (bee)|Drones]]) are hemizygous organisms since they develop from unfertilized eggs and their entire genome is haploid.
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If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele and the trait coded by the recessive allele will not be present. In [[Dominant_allele#Types_of_dominances|more complex dominance schemes]] the results of heterozygosity can be more complex.
   
== Inheritance of traits ==
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===Hemizygous===
{{main|Mendelian inheritance}}
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A chromosome in a diploid organism is hemizygous when only one copy is present.<ref name="Henderson's"/> The cell or organism is called a ''hemizygote''. Hemizygosity is observed when one copy of a gene is deleted, or in the [[sex linkage|heterogametic]] sex when a gene is located on a sex chromosome. For organisms in which the male is heterogametic, such as humans, almost 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]]. In a more extreme example, male honeybees (known as [[Drone (bee)|drones]]) are completely hemizygous organisms. They develop from unfertilized eggs and their entire genome is [[Ploidy#Haploid_and_monoploid|haploid]], unlike female honeybees, which are diploid. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also hemizygous, and can later be bred to homozygosity to reduce the need to confirm genotype of each litter.{{clarification needed|date=September 2011}}
{{main|Dominance relationship}}
 
   
The relationship between different alleles and the phenotypes that they affect is described in [[Dominance relationship]]. Some alleles are neither dominant nor recessive to another allele. In such cases, both alleles affect the phenotype of the heterozygote. Sometimes the result is an intermediate phenotype, such as when a snapdragon plant producing red flowers is crossed to one producing white flowers: the result is a heterozygous plant producing pink flowers. This is called incomplete dominance.
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===Nullizygous===
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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 nullizygous are synonymous.<ref name="Henderson's"/> The mutant cell or organism is called a ''nullizygote''.
   
To symbolize how a gene is inherited, the dominant allele is indicated with an upper case character and the recessive with a lower case character. The colour of flowers in [[Gregor Mendel|Mendel's]] inheritance experiments are often indicated as ''PP'' for the dominant homozygote, which produces a red flower, and ''pp'' for the recessive homozygote, which produces a white flower. When these two are crossed, the F1 or first filial generation receives one chromosome with the ''P'' allele from the red-flowered parent and a corresponding chromosome with the ''p'' allele from the white-flowered parent.All of the F1 generation are heterozygous, and this genotype is indicated with ''Pp''. All of the F1 plants produce red flowers, as this is the dominant allele.
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==Autozygous and allozygous==
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Zygosity may also refer to the origin(s) of the alleles in a genotype. When the two alleles at a locus originate from a common ancestor by way of nonrandom mating (inbreeding), the genotype is said to be ''autozygous''. This is also known as being "identical by descent", or IBD. When the two alleles come (at least to the extent that the descent can be traced) from completely different sources, as is the case in most normal, random mating, the genotype is called ''allozygous''. This is known as being "identical by state", or IBS.
   
==Heterozygosity==
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Because the alleles of autozygous genotypes come from the same source, they are always homozygous, but allozygous genotypes may be homozygous too. All heterozygous genotypes are, by definition, allozygous because they contain two completely different alleles. Hemizygous and nullizygous genotypes do not contain enough alleles to allow for comparison of sources, so this classification is irrelevant for them.
   
Heterozygosity refers to the state of being a heterozygote. Heterozygosity can also refer to the fraction of loci within an individual that are heterozygous. In [[population genetics]], it is commonly extended to refer to the population as a whole, i.e. the fraction of individuals in a population that are heterozygous for a particular locus.
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==Monozygotic and dizygotic twins==
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{{Details|Twin|the terms "monozygotic" and "dizygotic"}}
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As discussed above, "zygosity" can be used in the context of a specific genetic locus (example<ref>{{cite pmid|12654648}}</ref>). In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins.<ref>[http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=hmg Human Molecular Genetics], 2nd edition by Tom Strachan and Andrew P. Read [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=hmg&part=A2178#A2306 Chapter 17]. (1999).</ref> Identical twins are '''monozygotic''', meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are '''dizygotic''' because they develop from two separate eggs that are fertilized by two separate [[sperm]].
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==Heterozygosity in population genetics==
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In [[population genetics]], the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus. It can also refer to the fraction of loci within an individual that are heterozygous.
   
 
Typically, the observed(<math>H_o</math>) and expected(<math>H_e</math>) heterozygosities are compared, defined as follows for diploid individuals in a population:
 
Typically, the observed(<math>H_o</math>) and expected(<math>H_e</math>) heterozygosities are compared, defined as follows for diploid individuals in a population:
   
 
;Observed
 
;Observed
:<math>H_o = \frac{\sum_{i=1}^{n}{(1\ \textrm{if}\ a_{i1} \neq a_{i2})}}{n}</math>
+
:<math>H_o = \frac{\sum\limits_{i=1}^{n}{(1\ \textrm{if}\ a_{i1} \neq a_{i2})}}{n}</math>
 
where <math>n</math> is the number of individuals in the population, and <math>a_{i1},a_{i2}</math> are the alleles of individual <math>i</math> at the target locus.
 
where <math>n</math> is the number of individuals in the population, and <math>a_{i1},a_{i2}</math> are the alleles of individual <math>i</math> at the target locus.
   
 
;Expected
 
;Expected
 
:<math>
 
:<math>
H_e = 1 - \sum_{i=1}^{m}{(f_i)^2}
+
H_e = 1 - \sum\limits_{i=1}^{m}{(f_i)^2}
 
</math>
 
</math>
 
where <math>m</math> is the number of alleles at the target locus, and <math>f_i</math> is the [[allele frequency]] of the <math>i^{th}</math> allele at the target locus.
 
where <math>m</math> is the number of alleles at the target locus, and <math>f_i</math> is the [[allele frequency]] of the <math>i^{th}</math> allele at the target locus.
   
 
==See also==
 
==See also==
*[[dominance relationship]]
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*[[Dominance relationship]]
*[[Heterozygote advantage]]
 
 
*[[Heterosis]]
 
*[[Heterosis]]
  +
*[[Heterozygote advantage]]
 
*[[Nucleotide diversity]] measures polymorphisms on the level of nucleotides rather than on level of loci.
 
*[[Nucleotide diversity]] measures polymorphisms on the level of nucleotides rather than on level of loci.
   
 
----
 
----
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==References==
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{{reflist}}
   
  +
[[Category:Breeding]
 
[[Category:Classical genetics]]
 
[[Category:Classical genetics]]
   

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Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait. If one allele is missing, it is hemizygous, and, if both alleles are missing, it is nullizygous.

Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same genes on each of their two sets of homologous chromosomes, except that the sequences of these genes may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a sex-determination system.

The DNA sequence of a gene usually 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 only that allele can function properly. Any variation from the DNA sequence of that allele will be fatal in the embryo, and the organism will never survive to be born. 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, and another allele may be rare. Sometimes, one allele is a disease - causing variation while the other allele is healthy. 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 the male parent and one from the female parent. Zygosity is a description of whether those two alleles have identical or different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome.[1]

TypesEdit

The words homozygous, heterozygous, 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, and nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing.

HomozygousEdit

A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes.[2] The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.

An individual that is homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait. This allele, often called the "dominant allele", is normally represented by a capital letter (such as "P" for the dominant allele producing purple flowers in pea plants). 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 that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".

HeterozygousEdit

A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene.[3] Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele), such as "Rr" or "Ss". The capital letter is usually written first.

If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele and the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex.

HemizygousEdit

A chromosome in a diploid organism is hemizygous when only one copy is present.[2] 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. For organisms in which the male is heterogametic, such as humans, almost 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. In a more extreme example, male honeybees (known as drones) are completely hemizygous organisms. They develop from unfertilized eggs and their entire genome is haploid, unlike female honeybees, which are diploid. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also hemizygous, and can later be bred to homozygosity to reduce the need to confirm genotype of each litter.Template:Clarification needed

NullizygousEdit

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 nullizygous are synonymous.[2] The mutant cell or organism is called a nullizygote.

Autozygous and allozygousEdit

Zygosity may also refer to the origin(s) of the alleles in a genotype. When the two alleles at a locus originate from a common ancestor by way of nonrandom mating (inbreeding), the genotype is said to be autozygous. This is also known as being "identical by descent", or IBD. When the two alleles come (at least to the extent that the descent can be traced) from completely different sources, as is the case in most normal, random mating, the genotype is called allozygous. This is known as being "identical by state", or IBS.

Because the alleles of autozygous genotypes come from the same source, they are always homozygous, but allozygous genotypes may be homozygous too. All heterozygous genotypes are, by definition, allozygous because they contain two completely different alleles. Hemizygous and nullizygous genotypes do not contain enough alleles to allow for comparison of sources, so this classification is irrelevant for them.

Monozygotic and dizygotic twinsEdit

For more details on this topic, see Twin.

As discussed above, "zygosity" can be used in the context of a specific genetic locus (example[4]). In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins.[5] Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic because they develop from two separate eggs that are fertilized by two separate sperm.

Heterozygosity in population geneticsEdit

In population genetics, the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus. It can also refer to the fraction of loci within an individual that are heterozygous.

Typically, the observed(H_o) and expected(H_e) heterozygosities are compared, defined as follows for diploid individuals in a population:

Observed
H_o = \frac{\sum\limits_{i=1}^{n}{(1\ \textrm{if}\ a_{i1} \neq a_{i2})}}{n}

where n is the number of individuals in the population, and a_{i1},a_{i2} are the alleles of individual i at the target locus.

Expected

H_e = 1 - \sum\limits_{i=1}^{m}{(f_i)^2}

where m is the number of alleles at the target locus, and f_i is the allele frequency of the i^{th} allele at the target locus.

See alsoEdit


ReferencesEdit

  1. PMID 16780578 (PMID 16780578)
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  2. 2.0 2.1 2.2 Lawrence, Eleanor: Henderson's Dictionary of Biology, 14e, 2008
  3. Molecular Cell Biology, 4th edition by Harvey Lodish, et al Chapter 8. (2000)
  4. PMID 12654648 (PMID 12654648)
    Citation will be completed automatically in a few minutes. Jump the queue or expand by hand
  5. Human Molecular Genetics, 2nd edition by Tom Strachan and Andrew P. Read Chapter 17. (1999).

[[Category:Breeding]

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