Zygosity
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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.
Every gene in a diploid organism has two alleles at the gene's locus. These alleles are defined as dominant or recessive, depending on the phenotype resulting from the two alleles. If a gene's two alleles are both dominant or both recessive, that specific gene is homozygous. If one allele is dominant and the other is recessive, the gene is heterozygous.
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.
Homozygous
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.
Allozygous
Allozygosity is when two alleles are alike, but unrelated. The two alleles had different ancestral alleles that through convergent evolution became similar.
Autozygous
Autozygosity is when two alleles are alike by relation, that is to say since they had a common ancestor, and they are similar.
Heterozygous
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
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
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(<math>H_o</math>) and expected(<math>H_e</math>) heterozygosities are compared, defined as follows for diploid individuals in a population:
- Observed
- <math>H_o = \frac{\sum_{i=1}^{n}{(1\ \textrm{if}\ a_{i1} \neq a_{i2})}}{n}</math>
where <math>n</math> is the number of individuals in the population, and <math>a_{i1},a_{i2}</math> are the alleles of individual <math>i</math> at the target locus.
- Expected
- <math>
H_e = 1 - \sum_{i=1}^{m}{(f_i)^2} </math> where <math>m</math> is the number of alleles at the target locus, and <math>f_i</math> is the frequency of the <math>i^{th}</math> allele at the target locus.
See also
- dominance relationship
- Heterozygote advantage
- Heterosis
- Sickle-cell Anemia as an example of a fatal homozygote.
- Nucleotide diversity measures polymorphisms on the level of nucleotides rather than on level of loci.
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