What is Zygosity: zygote (noun, zygote , is from Greek zygotos “, yoked ” from zygon “yoke” ) ( / z aɪ s t i / ) the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of alleles in an organism

Most eukaryotes have two matching sets of chromosomes ; That is, they are diploid . Diploid organisms have the same locus on each of their two sets of homologous chromosomes , except that the sequences of these loci can differ between two chromosomes in a matching pair and that some chromosomes are part of a chromosomal sex-determining system . may be mismatched. If both the alleles of a diploid organism are the same, then 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 null .


The DNA sequence of a gene often varies from person to person. Those variations are called alleles . While some genes have only one allele because there is less variation, others have only one allele because deviation from that allele can be harmful or lethal. But most genes have two or more alleles. The frequency of different alleles varies throughout the population. Some genes may have alleles with a similar distribution. Often, various variations in alleles do not affect the normal functioning of the organism at all. For some genes, one allele may be common, and the other allele may be rare. Sometimes, one allele is a disease-causing variation while the other allele is healthy.

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 the same or different DNA sequences. In some cases the term “zygosity” is used to refer to a single chromosome.


The terms homozygous , heterozygous , and hemizygous are used to describe the genotype on the DNA of a diploid organism in a single locus . 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 one copy of a particular gene in an otherwise diploid organism. Tandem describes the genotype, 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 the same alleles of the gene are present on both homologous chromosomes .

An individual who 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 usually represented by the uppercase form of the letter used for the respective recessive trait (such as “P” for the dominant allele that produces purple flowers in pea plants). Is. When an organism is homozygous-dominant for a particular trait, its genotype is represented by a double of the symbol for that trait, such as “PP”.

An individual who is homozygous for a particular trait carries two copies of the allele that codes for the recessive trait . This allele, often referred to as the “recessive allele”, is usually represented by the lowercase form of the letter used for the respective dominant trait (e.g., in the context of the example above, the “p” white flower in peas). plants producing recessive alleles). The genotype of an organism that is homozygous for a particular trait is represented by a doubling of the appropriate letter, such as “pp”.


A diploid organism is heterozygous at the gene locus when its cells have two different alleles of a gene (a wild-type allele and a mutant allele). [3] The cell or organism is said to be heterozygote for the particular allele in question , and therefore, heterozygosity refers to a specific genotype. The heterozygous genotype is represented by an uppercase letter (representing the dominant/wild-type allele) and a lowercase letter (representing the recessive/mutant allele), as in “Rr” or “Ss”. Alternatively, a heterozygous for the gene “R” is considered “Rr”. Uppercase letter is usually written first.

If the trait in question is determined by simple (absolute) dominance, then a heterozygote will only express 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 heterozygous genotype may have a higher relative fitness than a homozygous dominant or homozygous recessive genotype – this is called heterozygous advantage .


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 heterozygous sex , when a gene is located on one sex chromosome. Hemizygosity is not the same as haploid insufficiency , which describes a mechanism for producing a phenotype. For organisms in which the male is heterozygous, such as humans, almost all X-linked genes are hemizygous in males with normal chromosomes, as they have only one X chromosome and some identical genes are on the Y chromosome ., Transgenic mice generated via exogenous DNA microinjection are considered hemizygous, because the pronucleus of an embryo is also expected to contain only one copy of any locus. A transgenic individual can subsequently be bred for homozygosity and maintained as an inbred line to reduce the need to confirm each individual’s genotype.

In cultured mammalian cells, such as the Chinese hamster ovary cell line, many genetic loci exist in a functional hemizygous state, due to mutations or deletions in other alleles.


A null organism carries two mutant alleles for the same gene. The mutated alleles are both complete loss-of-function or ‘null’ alleles, so the homozygous null and the null allele are synonymous. [2] The mutated cell or organism is called a nullzygote .

Autozygous and Allozygous

Zygosity can also refer to the origin(s) of alleles in a genotype. When two alleles at a locus arise from a common ancestor through nonrandom mating ( inbreeding ), the genotype is said to be autozygous . It is also known as “similar by descent”, or IBD. When two alleles come from different sources (at least to the extent that ancestry can be traced), the genotype is said to be allogeneic . This is known as “uniform by state,” or IBS. [ clarification needed ]

Since the alleles of an autozygous genotype come from the same source, they are always homozygous, but alleles of an allogeneic genotype can also be homozygous. Heterozygous genotypes are often, but not necessarily, allogeneous because different alleles can be produced by mutation some time after a common origin. Hemizygous and nullizygous genotypes do not have enough alleles to allow comparison of sources, so this classification is irrelevant to them.

Monozygotic and dizygotic twins

As discussed above, “zygosity” can be used to refer to a specific genetic locus (eg [5] ). The term zygosity can also be used to describe the genetic similarity or dissimilarity of twins. [6] Identical twins are monozygotic , meaning they develop from a single zygote that divides to form two embryos. Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two different sperm . sesquizygoticTwins are halfway between resembling and dizygotic and two spermatozoa are thought to be born after fertilizing a single oocyte which later splits into two morula.

Heterozygosity in 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 is heterozygous.

Typically, the observed ( ) and expected ( ) heterozygotes are compared, defined as follows for the diploid individuals in the population:} Ho He


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

where is the number of individuals in the population, and is the individual’s alleles at the target locus.


Be expected

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

where is the number of alleles at the target locus, and is the allele frequency of the allele at the target locus.