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Zygosity

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Zygosity

Zygosity is the degree of similarity of the alleles for a trait in an organism.

Most heterozygous 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. Those variations are called 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. In some cases the term "zygosity" is used in the context of a single chromosome.[1]

Types

The words homozygous, heterozygous, and hemizygous are used to describe the 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 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

An individual that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the

Heterozygous

A

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 chromosome in a diploid organism is hemizygous when only one copy is present.[2] The cell or organism is called a hemizygote. Hemizygosity is also observed when one copy of a gene is deleted, or in the 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 can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotypes of each litter.

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. [4]

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.[2] 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), 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[5]). In addition, 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. Fraternal twins are dizygotic because they develop from two separate eggs that are fertilized by two separate sperm.

Heterozygosity in 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\limits_{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\limits_{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.

See also

References

  1. ^ Carr, M.; Cotton, S.; Rogers, D.; Pomiankowski, A.; Smith, H.; Fowler, K. (2006). "Assigning sex to pre-adult stalk-eyed flies using genital disc morphology and X chromosome zygosity". BMC developmental biology 6: 29.  
  2. ^ a b c Lawrence, Eleanor: Henderson's Dictionary of Biology, 14e, 2008
  3. ^ Molecular Cell Biology, 4th edition by Harvey Lodish, et al Chapter 8. (2000)
  4. ^ Gupta, R.S., Chan, D.H.Y. and Siminovitch, L. (1978). Evidence obtained by segregation analysis for functional hemizygosity at the Emtr locus in CHO cells. Cell 15: 1007-1013
  5. ^ Pujol, C.; Messer, S.; Pfaller, M.; Soll, D. (2003). "Drug resistance is not directly affected by mating type locus zygosity in Candida albicans". Antimicrobial agents and chemotherapy 47 (4): 1207–1212.  
  6. ^ Human Molecular Genetics, 2nd edition by Tom Strachan and Andrew P. Read Chapter 17. (1999).
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