digplanet beta 1: Athena
Share digplanet:


Applied sciences






















Zygosity is the degree of similarity of the alleles for a trait in an organism.

Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes, except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous, and, if both alleles are missing, it is nullizygous.

The DNA sequence of a gene often 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]


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.


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".


A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene.[3] The cell or organism is called a heterozygote specifically for the allele in question, therefore, heterozygosity refers to a specific genotype. 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". Alternatively, a heterozygote for gene "R" is assumed to be "Rr". 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.


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 also 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. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also considered to be hemizygous because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotypes of each litter.

In cultured mammalian cells, such as the Chinese hamster ovary cell line, a number of genetic loci are present in a functional hemizygous state, due to mutations or deletions in the other alleles. [4]


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 allozygous[edit]

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 twins[edit]

For more details on the terms "monozygotic" and "dizygotic", see Twin.

As discussed above, "zygosity" can be used in the context of a specific genetic locus (example[5]). In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins.[6] 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 genetics[edit]

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:

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.


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 also[edit]


  1. ^ Carr, M.; Cotton, S.; Rogers, D.; Pomiankowski, A.; Smith, H.; Fowler, K. (2006). "Assigning sex to pre-adult stalk-eyed flies using genital disc morphology and X chromosome zygosity". BMC developmental biology 6: 29. doi:10.1186/1471-213X-6-29. PMC 1524940. PMID 16780578.  edit
  2. ^ a b c Lawrence, Eleanor: Henderson's Dictionary of Biology, 14e, 2008
  3. ^ Molecular Cell Biology, 4th edition by Harvey Lodish, et al Chapter 8. (2000)
  4. ^ Gupta, R.S., Chan, D.H.Y. and Siminovitch, L. (1978). Evidence obtained by segregation analysis for functional hemizygosity at the Emtr locus in CHO cells. Cell 15: 1007-1013
  5. ^ Pujol, C.; Messer, S.; Pfaller, M.; Soll, D. (2003). "Drug resistance is not directly affected by mating type locus zygosity in Candida albicans". Antimicrobial agents and chemotherapy 47 (4): 1207–1212. doi:10.1128/AAC.47.4.1207-1212.2003. PMC 152535. PMID 12654648.  edit
  6. ^ Human Molecular Genetics, 2nd edition by Tom Strachan and Andrew P. Read Chapter 17. (1999).

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Zygosity — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
207 videos foundNext > 


Zygosity is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the...


Copyright (c) 2010 - Z4Films A short film about what makes us different ... and what makes us the same.


Identical or Fraternal? Twin Zygosity/DNA Test and Results

http://www.theconsciousdoer.com Our experience finding out about the zygosity of our twins.

Testing Zygosity -- Are They Identical or Not?

Joe and Jason are twins, but they were never sure if they were fraternal or identical. There were a lot of differences between them. We test them to find out if they were fraternal twins or...

Zygosity (Director's Cut)

Copyright (c) 2010 - Tyler Nicholas & Z4Films.com A short film about what makes us different ... and what makes us the same.

How to Pronounce Zygosity

Learn how to say words in English correctly with Emma Saying free pronunciation tutorials. Over 140000 words were already uploaded... Check them out! Visit ...

Heterozygosity, Homozygosity problem and solution

Zygosity is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the...


Finding out if our twins are identical or fraternal through a twin zygosity test!

Testcross and gene linkage

In genetics, a test cross, first introduced by Gregor Mendel, involves the breeding of a dominant trait individual with a recessive individual, in order to determine the zygosity of the former...

207 videos foundNext > 

48 news items

Slate Magazine
Wed, 24 Aug 2011 10:33:24 -0700

One wore football pads and a helmet, the other my best approximation of hipster chic. A sealed envelope lay beside the bed containing the outcome of a genetic zygosity test. This was at the height of a Pinter phase, and the ensuing pages were rife with ...

The Guardian

The Guardian
Wed, 11 Feb 2015 16:45:00 -0800

In 2014, the Royal Croquet Club felt like a nice idea that failed on the execution; the great design and location in Adelaide's newly renovated Victoria Square was let down by some lacklustre programming. This year, with a new curatorial team and a ...

Jewish Journal

Jewish Journal
Wed, 04 Feb 2015 12:31:41 -0800

I'm sitting with three other people in a narrow booth in a bustling cafe in Manhattan. It's the week of snowstorms and icy weather that most Angelenos dread but that I like so much, even if it does hurt to breathe outdoors, the cold air stabbing the ...


Tue, 10 Mar 2015 12:30:00 -0700

The authors write that they then “performed subsequent matched within-pair twin case-control analyses with all complete twin pairs discordant for LBP regardless of zygosity, and separately for dizygotic and monozygotic pairs. This sequential analysis ...

Mad In America

Mad In America
Mon, 12 Jan 2015 08:44:45 -0800

Zygosity has been determined using a standard zygosity questionnaire which has shown 95% accuracy (Price et al. 2000). TEDS families were invited to take part if they fulfilled the following SAIL project inclusion criteria: twins' birthdates between ...
EurekAlert (press release)
Mon, 05 Jan 2015 07:22:30 -0800

The study group comprised 1,412 twin pairs who were between the ages of 8 and 18 years: 739 monozygotic pairs, 672 dizygotic pairs and one pair with unknown zygosity. Participants were followed up at three additional time points. Average ages at each ...

Bury Free Press

Bury Free Press
Sat, 27 Sep 2014 04:03:45 -0700

The proud parents of three of Mildenhall's smallest residents recently made a big discovery – their 10-month-old triplet sons are identical. Mum Clare Sansom, gave birth to her three youngest sons two months early last November and recently applied to ...


Fri, 25 Jul 2014 07:38:23 -0700

Genetic analysis of Antarctic fur seals, alongside decades of in-depth monitoring,* has provided unique insights into the effect of climate change on a population of top-predators. Published in Nature this week, the findings show that the seals have ...

Oops, we seem to be having trouble contacting Twitter

Support Wikipedia

A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia. Please add your support for Wikipedia!

Searchlight Group

Digplanet also receives support from Searchlight Group. Visit Searchlight