Zygosity

Zygosity refers to the similarity of genes for a trait (inherited characteristic) in an organism. If both genes are the same, the organism is homozygous for the trait. If both genes are different, the organism is heterozygous for that trait. If one gene is missing, it is hemizygous, and if both genes are missing, it is nullizygous.

Most organisms have two sets of chromosomes, that is, they are diploid. Diploid organisms have one copy of each gene on each chromosome.

The DNA sequence of a gene usually varies from one individual to another. Those variations are called alleles. Some genes have only one allele. 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.

1 Types
2 Autozygous and allozygous
3 Monozygotic and dizygotic twins
4 Population genetics
5 See also
6 References

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.^[1] 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".

Heterozygous

A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene^[2]. 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". 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.

Hemizygous

A diploid organism is hemizygous for a particular gene when only one allele for the gene is present.^[1] The cell or organism is called a hemizygote. Hemizygosity is 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. 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. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also hemizygous, and can later be bred to homozygosity to reduce the need to confirm genotype of each litter.

Nullizygous

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.^[1] The mutant 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. Natural nullizygosity is very rare and can be fatal or extremely harmful to the individual since it prevents an entire trait from being expressed.

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 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

As discussed above, "zygosity" can be used in the context of a specific genetic locus (example^[3]). In addition, the word "zygosity" may also be used to describe the genetic similarity or dissimilarity of twins.^[4] 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.

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

In some cases the term "zygosity" is used in the context of a single chromosome.^[5]

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.

References

↑ ^1.0 ^1.1 ^1.2 Lawrence, Eleanor: Henderson's Dictionary of Biology, 14e, 2008
↑ Molecular Cell Biology, 4th edition by Harvey Lodish, et al Chapter 8. (2000)
↑ PMID 12654648 (PubMed)
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↑ Human Molecular Genetics, 2nd edition by Tom Strachan and Andrew P. Read Chapter 17. (1999).
↑ PMID 16780578 (PubMed)
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