Zygosity

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.

The term zygosity is used to describe whether twins are identical or fraternal. 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 these terms, see twins.

Homozygous

An organism is referred to as being homozygous 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.

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 diploid organisms, the two different alleles were inherited from the organism's two parents.

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

Main article: Mendelian inheritance

Main article: Dominance relationship

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 bean plant producing red flowers is crossed to one producing white flowers: the result is a heterozygous plant producing pink flowers.

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 pink 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 pink-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 pink flowers.

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($H_{o}$) 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 frequency of the $i^{th}$ allele at the target locus.

See also

• dominance relationship
• Sickle-cell Anemia as an example of a Lethal Homozygote.