Zygosity refers to the genetic condition of a zygote. In genetics, zygosity describes the similarity or dissimilarity of DNA between homologous chromosomes at a specific 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 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 twins 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 twins."


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 chromosomes (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").

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.


Allozygosity is when two alleles are alike, but unrelated. The two alleles had different ancestral alleles that through convergent evolution became similar.


Autozygosity is when two alleles are alike by relation, that is to say since they had a common ancestor, and they are similar.


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 parents. For example a heterozygous individual would have the allele combination Pp.


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 heterogametic, such as humans, it refers in particular to X-linked genes, since males normally possess only one X-chromosome. They are hemizygous for (nearly) all genes that are located on the X-chromosome.

In a more extreme example, male honeybees (Drones) are hemizygous organisms since they develop from unfertilized eggs and their entire genome is haploid.

Inheritance of traits

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.

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 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.


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.

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 extrm{if} a_{i1} eq 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.

ee also

*dominance relationship
*Heterozygote advantage
*Nucleotide diversity measures polymorphisms on the level of nucleotides rather than on level of loci.
*Loss of heterozygosity


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • zygosity — noun Etymology: probably from zygous Date: 1946 the makeup or characteristics of a particular zygote; also the genetic relationship between offspring of a single birth especially in regard to being derived from the same or different zygotes …   New Collegiate Dictionary

  • zygosity — /zuy gos i tee, zi /, n. Genetics. 1. the characterization of an individual s hereditary traits in terms of gene pairing in the zygote from which it developed. Cf. homozygous, heterozygous. 2. the characterization of twinning and multiple births… …   Universalium

  • zygosity — noun The quality or characteristics of a zygote See Also: zygose, zygosis, zygospere, zygospore, zygotene, zygotic, zygotically, zygous …   Wiktionary

  • zygosity — The nature of the zygotes from which individuals are derived; e.g., whether by separation of the division of one zygote (monozygotic), in which case they will be genetically identical, or from two separate fertilized ova (dizygotic). * * *… …   Medical dictionary

  • -zygosity — zy·gos·i·ty a word termination denoting relationship to the zygote …   Medical dictionary

  • zygosity —    Twin development from on or two zygotes. If one, the twins are identical (monozygotic); if two, they are fraternal (dizygotic) …   Forensic science glossary

  • zygosity — zy·gos·i·ty …   English syllables

  • zygosity — zy•gos•i•ty [[t]zaɪˈgɒs ɪ ti, zɪ [/t]] n. 1) gen the characterization of a hereditary trait in an individual according to whether the gene pairs for the trait are homozygous or heterozygous 2) dvl the characterization of twins, triplets, etc.,… …   From formal English to slang

  • zygosity — ˈgäsəd.ē noun ( es) Etymology: New Latin zygosis + English ity : zygotic quality or characteristics : specific inheritance …   Useful english dictionary

  • Twin — Monozygotic ( identical ) twins A twin is one of two offspring produced in the same pregnancy.[1] Twins can either be monozygotic (in common parlance, identical ), meaning that they develop from one zygote that splits and forms two embryos, or… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.