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In genetics, zygosity refers to the similarity or dissimilarity of the DNA sequences in specific coding segments, or genes, on the homologous chromosomes of a zygote, or fertilized egg. The DNA sequence of any gene can vary among individuals in the population. The various forms of a gene are called alleles, and diploid organisms generally have two alleles for each gene, one on each of the two homologous chromosomes on which the gene is present. In diploid organisms, the alleles are inherited from the individual's parents, one from the male parent and one from the female. Zygosity in general is a description of whether those two alleles code for the same trait expression or different ones, but there are several specific applications of the term.

Homozygous, heterozygous, hemizygous, and nullizygous

The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Simply stated, 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.

Homozygous

An organism is homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. 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 purple flowers, which are dominant 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 white flowers, which are recessive 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".

Heterozygous

An organism is heterozygous for a particular gene when two different alleles occupy the gene's position on the homologous chromosomes. The cell or organism is called a heterozygote. Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele), such as "Rr". The capital letter is usually written first.

If the trait in question is determined by simple (AKA 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.

Hemizygous

A diploid organism is hemizygous for a particular gene when only one allele for the gene is present and the second homologous chromosome does not contain an allele for that gene. The cell or organism is called a hemizygote. 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.

Nullizygous

Nullizygosity is very rare and usually either fatal or extremely harmful to the individual since it prevents an entire trait from being expressed. The cell or organism is called a nullizygote. Researchers sometimes breed organisms to be nullizygous in a particular trait so that they can study how the organism is affected by the loss of the trait.

Autozygous and allozygous

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 heterzygous 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 twins

In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins. 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.

Population genetics

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: $H_{o}={\frac {\sum _{i=1}^{n}{(1\ {\textrm {if}}\ a_{i1}\neq a_{i2})}}{n}}$

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: $H_{e}=1-\sum _{i=1}^{m}{(f_{i})^{2}}$

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.

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 {{Split|date=August 2008}}
+In [[genetics]], '''zygosity''' refers to the similarity or dissimilarity of the DNA sequences in specific coding segments, or [[gene]]s, on the [[homologous chromosomes|homologous]] [[chromosome]]s of a [[zygote]], or [[fertilisation|fertilized]] [[ovum|egg]].  The DNA sequence of any gene can vary among individuals in the population. The various forms of a gene are called [[allele]]s, and [[Ploidy#diploid|diploid]] organisms generally have two alleles for each gene, one on each of the two homologous chromosomes on which the gene is present.  In diploid organisms, the alleles are inherited from the individual's [[parent]]s, one from the male parent and one from the female.  Zygosity in general is a description of whether those two alleles code for the same trait expression or different ones, but there are several specific applications of the term.
-'''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]].
-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]]s. 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.
+==Homozygous, heterozygous, hemizygous, and nullizygous==
-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.''
+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.  Simply stated, ''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.
-==Homozygous==
+===Homozygous===
-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''.
+An organism is homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes.  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.
-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'').
+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 purple flowers, which are dominant 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|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 white flowers, which are recessive 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".
-Pure-bred or [[true breeding]] organisms are homozygous.
-For example a homozygous individual could have the allele combinations PP or pp.
-All homozygous alleles are either allozygous or autozygous.
+===Heterozygous===<!-- This section is linked from [[Transgenic maize]] -->
-===Allozygous===
+An organism is ''heterozygous'' for a particular gene when two different alleles occupy the gene's position on the homologous chromosomes.  The cell or organism is called a ''heterozygote''.  Heterozygous genotypes are represented by a capital letter (representing the dominant allele) and a lowercase letter (representing the recessive allele), such as "Rr".  The capital letter is usually written first.
-'''Allozygosity''' is when two alleles are alike, but unrelated.  The two alleles had different ancestral alleles that through [[convergent evolution]] became similar.
+If the trait in question is determined by simple (AKA 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.
-===Autozygous===
-'''Autozygosity''' is when two alleles are alike by relation, that is to say since they had a common ancestor, and they are similar.
+===Hemizygous===
-==Heterozygous==<!-- This section is linked from [[Transgenic maize]] -->
+A diploid organism is hemizygous for a particular gene when only one allele for the gene is present and the second homologous chromosome does not contain an allele for that gene.  The cell or organism is called a ''hemizygote''.  For organisms in which the male is [[sex linkage|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.
-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==
+===Nullizygous===
+{{Expand-section|date=Jan 2009}}
-'''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.
+Nullizygosity is very rare and usually either fatal or extremely harmful to the individual since it prevents an entire trait from being expressed.  The cell or organism is called a ''nullizygote''.  Researchers sometimes breed organisms to be nullizygous in a particular trait so that they can study how the organism is affected by the loss of the trait.
+==Autozygous and allozygous==
-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.
+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 heterzygous 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.
-==Inheritance of traits==
-{{main|Mendelian inheritance}}
-{{main|Dominance relationship}}
+==Monozygotic and dizygotic twins==
-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.
+In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins.  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]].
+{{Details|Twin|the terms "monozygotic" and "dizygotic"}}
-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.
+==Population genetics==
-==Heterozygosity==
-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.
+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:
@@ Line 55: / Line 52: @@
 ==See also==
-*[[dominance relationship]]
 *[[Heterosis]]
 *[[Heterozygote advantage]]

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