Zygosity
Often, the different variations in the genes do not affect the normal functioning of the organism at all.
Zygosity is a description of whether those two alleles have identical or different DNA sequences.
[1] 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.
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".
[citation needed] A chromosome in a diploid organism is hemizygous when only one copy is present.
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.
[citation needed] 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.
[citation needed] 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.
When the two alleles come from different sources (at least to the extent that the descent can be traced), the genotype is called allozygous.
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.
[citation needed] As discussed above, "zygosity" can be used in the context of a specific genetic locus (example[5]).
The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins.
[6] Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos.
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.
If one copy of an essential gene is mutated, the (heterozygous) carrier is usually healthy.
[8] For instance, a single copy of the Kmt5b gene leads to haploinsufficiency and results in a skeletal muscle developmental deficit.
) heterozygosities are compared, defined as follows for diploid individuals in a population:[citation needed] where
