Zygosity (the noun,
zygote
A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.
In multicell ...
, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
or
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
have the same genetic sequence. In other words, it is the degree of similarity of the
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chrom ...
s in an organism.
Most
eukaryotes have two matching sets of
chromosomes
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
; 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
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is 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 the alleles in an organism.
...
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. These gene variants are called
alleles. While some genes have only one allele because there is low variation, others have only one allele because deviation from that allele can be harmful or fatal. But most genes have two or more alleles. The frequency of different alleles varies throughout the population. Some genes may have alleles with equal distributions. Often, the different variations in the genes do not affect the normal functioning of the organism at all. For some genes, one allele may be common, and another allele may be rare. Sometimes, one allele is a
disease-causing variation while another 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 identical or different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome.
Types
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.
Homozygous
A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both
homologous chromosomes.
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 the uppercase form of the letter used for the corresponding recessive trait (such as "P" for the dominant allele producing purple flowers in pea plants). When an organism is homozygous-dominant for a particular trait, its 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".
Heterozygous
A
diploid organism is heterozygous at a gene locus when its cells contain two different
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chrom ...
s (one wild-type allele and one mutant allele) of a gene. The cell or organism is called a heterozygote ''specifically'' for the allele in question, and therefore, heterozygosity refers to a specific genotype. Heterozygous genotypes are 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 heterozygote for gene "R" is assumed to be "Rr". The uppercase 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 heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype – this is called a
heterozygote advantage.
Hemizygous
A chromosome in a diploid organism is hemizygous when only one copy is present.
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. Hemizygosity is not the same as
haploinsufficiency, which describes a mechanism for producing a phenotype. 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 individual can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotype of each individual.
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.
Nullizygous
A nullizygous organism carries two mutant alleles for the same gene. The mutant alleles are both complete
loss-of-function
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...
or 'null' alleles, so homozygous null and nullizygous are synonymous.
The mutant cell or organism is called a ''nullizygote''.
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
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders a ...
), the genotype is said to be ''autozygous''. This is also known as being "identical by descent", or IBD. When the two alleles come from different sources (at least to the extent that the descent can be traced), 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. Heterozygous genotypes are often, but not necessarily, allozygous because different alleles may have arisen by mutation some time after a common origin. 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
As discussed above, "zygosity" can be used in the context of a specific genetic locus (example). 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
Oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
s (egg cells) that are fertilized by two separate
sperm.
Sesquizygotic twins are halfway between monozygotic and dizygotic and are believed to arise after two sperm fertilize a single oocyte which subsequently splits into two
morula.
Medicine and disease
Zygosity is an important factor in human medicine. If one copy of an essential gene is mutated, the (heterozygous) carrier is usually healthy. However, more than 1,000 human genes appear to require both copies, that is, a single copy is insufficient for health. This is called
haploinsufficiency. For instance, a single copy of the
Kmt5b
Histone-lysine N-methyltransferase KMT5B is an enzyme that in humans is encoded by the ''KMT5B'' gene. The enzyme along with WHSC1 is responsible for dimethylation of lysine 20 on histone H4 in mouse and humans.
This gene encodes a protein that ...
gene leads to haploinsufficiency and results in a
skeletal muscle developmental deficit.
Heterozygosity in population genetics
In
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and po ...
, 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 (
) and expected (
) heterozygosities are compared, defined as follows for diploid individuals in a population:
;Observed
:
where
is the number of individuals in the population, and
are the alleles of individual
at the target locus.
;Expected
:
where
is the number of alleles at the target locus, and
is the
allele frequency
Allele frequency, or gene frequency, is the relative frequency of an allele (variant of a gene) at a particular locus in a population, expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in the population tha ...
of the
allele at the target locus.
See also
*
Heterosis
Heterosis, hybrid vigor, or outbreeding enhancement is the improved or increased function of any biological quality in a hybrid offspring. An offspring is heterotic if its traits are enhanced as a result of mixing the genetic contributions o ...
*
Heterozygote advantage
*
Loss of heterozygosity
*
Nucleotide diversity measures polymorphisms on the level of nucleotides rather than on level of loci.
*
Pseudolinkage
*
Runs of Homozygosity (ROH)
References
External links
*
{{Authority control
Classical genetics