In biology, a mutation is the permanent alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA or other genetic elements. Mutations result from errors during DNA replication or other types of damage to DNA, which may undergo error-prone repair, or cause an error during other forms of repair, or else may cause an error during replication. Mutations may result from insertion or deletion of segments of DNA due to mobile genetic elements. Mutations may or may not produce discernible changes in the observable characteristics of an organism. Mutations play a part in both normal and abnormal biological processes including: evolution and the development of the immune system, including junctional diversity; the genomes of RNA viruses are based on RNA rather than DNA. The RNA viral genome can be double single stranded. In some of these viruses replication occurs and there are no mechanisms to check the genome for accuracy; this error-prone process results in mutations.
Mutation can result in many different types of change in sequences. Mutations in genes can either have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. Mutations can occur in nongenic regions. One study on genetic variations between different species of Drosophila suggests that, if a mutation changes a protein produced by a gene, the result is to be harmful, with an estimated 70 percent of amino acid polymorphisms that have damaging effects, the remainder being either neutral or marginally beneficial. Due to the damaging effects that mutations can have on genes, organisms have mechanisms such as DNA repair to prevent or correct mutations by reverting the mutated sequence back to its original state. Mutations can involve the duplication of large sections of DNA through genetic recombination; these duplications are a major source of raw material for evolving new genes, with tens to hundreds of genes duplicated in animal genomes every million years.
Most genes belong to larger gene families of shared ancestry. Novel genes are produced by several methods through the duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions. Here, protein domains act as modules, each with a particular and independent function, that can be mixed together to produce genes encoding new proteins with novel properties. For example, the human eye uses four genes to make structures that sense light: three for cone cell or color vision and one for rod cell or night vision. Another advantage of duplicating a gene is. Other types of mutation create new genes from noncoding DNA. Changes in chromosome number may involve larger mutations, where segments of the DNA within chromosomes break and rearrange. For example, in the Homininae, two chromosomes fused to produce human chromosome 2. In evolution, the most important role of such chromosomal rearrangements may be to accelerate the divergence of a population into new species by making populations less to interbreed, thereby preserving genetic differences between these populations.
Sequences of DNA that can move about the genome, such as transposons, make up a major fraction of the genetic material of plants and animals, may have been important in the evolution of genomes. For example, more than a million copies of the Alu sequence are present in the human genome, these sequences have now been recruited to perform functions such as regulating gene expression. Another effect of these mobile DNA sequences is that when they move within a genome, they can mutate or delete existing genes and thereby produce genetic diversity. Nonlethal mutations increase the amount of genetic variation; the abundance of some genetic changes within the gene pool can be reduced by natural selection, while other "more favorable" mutations may accumulate and result in adaptive changes. For example, a butterfly may produce offspring with new mutations; the majority of these mutations will have no effect. If this color change is advantageous, the chances of this butterfly's surviving and producing its own offspring are a little better, over time the number of butterflies with this mutation may form a larger percentage of the population.
Neutral mutations are defined as mutations whose effects do not influence the fitness of an individual. These can increase in frequency over time due to genetic drift, it is believed that the overwhelming majority of mutations have no significant effect on an organism's fitness. DNA repair mechanisms are able to mend most changes before they become permanent mutations, many organisms have mechanisms for eliminating otherwise-permanently mutated somatic cells. Beneficial mutations can improve reproductive success. Mutationism is one of several alternatives to evolution by natural selection that have existed both before and after the publication of Charles Darwin's 1859 book, On the Origin of Species. In the theory, mutation was the source of novelty
In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis is preceded by the S stage of interphase and is accompanied or followed by cytokinesis, which divides the cytoplasm and cell membrane into two new cells containing equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other; the process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, metaphase and telophase. During mitosis, the chromosomes, which have duplicated and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell; the result is two genetically identical daughter nuclei.
The rest of the cell may continue to divide by cytokinesis to produce two daughter cells. Producing three or more daughter cells instead of the normal two is a mitotic error called tripolar mitosis or multipolar mitosis. Other errors during mitosis can induce apoptosis or cause mutations. Certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Mitosis varies between organisms. For example, animal cells undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Most animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include the gametes -- egg cells -- which are produced by meiosis.
Numerous descriptions of cell division were made during 18th and 19th centuries, with various degrees of accuracy. In 1835, the German botanist Hugo von Mohl, described cell division in the green alga Cladophora glomerata, stating that multiplication of cells occurs through cell division. In 1838, Schleiden affirmed that the formation of new cells in their interior was a general law for cell multiplication in plants, a view rejected in favour of Mohl model, due to contributions of Robert Remak and others. In animal cells, cell division with mitosis was discovered in frog and cat cornea cells in 1873 and described for the first time by the Polish histologist Wacław Mayzel in 1875. Bütschli and Fol might have claimed the discovery of the process presently known as "mitosis". In 1873, the German zoologist Otto Bütschli published data from observations on nematodes. A few years he discovered and described mitosis based on those observations; the term "mitosis", coined by Walther Flemming in 1882, is derived from the Greek word μίτος.
There are some alternative names for the process, e.g. "karyokinesis", a term introduced by Schleicher in 1878, or "equational division", proposed by Weismann in 1887. However, the term "mitosis" is used in a broad sense by some authors to refer to karyokinesis and cytokinesis together. Presently, "equational division" is more used to refer to meiosis II, the part of meiosis most like mitosis; the primary result of mitosis and cytokinesis is the transfer of a parent cell's genome into two daughter cells. The genome is composed of a number of chromosomes—complexes of coiled DNA that contain genetic information vital for proper cell function; because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. Chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere; when mitosis begins, the chromosomes become visible.
In some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell disappears. Microtubules project from opposite ends of the cell, attach to the centromeres, align the chromosomes centrally within the cell; the microtubules contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes; as the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase. A new nuclear envelope forms around the separated daughter chromosomes, which decondense to form interphase nuclei. During mitotic progression after the anaphase onset, the cell may undergo cytokinesis. In animal cells, a cell membrane pinches inward between the two developing nuclei to produce two new cells. In plant cells, a cell plate forms between the two nuclei. Cytokinesis does not always occur.
The mitotic phase is a short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for the process of cell division. Interphase is divided into three phases: G1, S, G2. During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only durin
An autosome is a chromosome, not an allosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures; the DNA in autosomes is collectively known as atDNA or auDNA. For example, humans have a diploid genome that contains 22 pairs of autosomes and one allosome pair; the autosome pairs are labeled with numbers in order of their sizes in base pairs, while allosomes are labelled with their letters. By contrast, the allosome pair consists of two X chromosomes in females or one X and one Y chromosome in males. Unusual combinations of XYY, XXY, XXX, XXXX, XXXXX or XXYY, among other allosome combinations, are known to occur and cause developmental abnormalities. Autosomes still contain sexual determination genes though they are not sex chromosomes. For example, the SRY gene on the Y chromosome encodes the transcription factor TDF and is vital for male sex determination during development. TDF functions by activating the SOX9 gene on chromosome 17, so mutations of the SOX9 gene can cause humans with an ordinary Y chromosome to develop as females.
All human autosomes have been identified and mapped by extracting the chromosomes from a cell arrested in metaphase or prometaphase and staining them with a type of dye. These chromosomes are viewed as karyograms for easy comparison. Clinical geneticists can compare the karyogram of an individual to a reference karyogram to discover the cytogenetic basis of certain phenotypes. For example, the karyogram of someone with Patau Syndrome would show that they possess three copies of chromosome 13. Karyograms and staining techniques can only detect large-scale disruptions to chromosomes—chromosomal aberrations smaller than a few million base pairs cannot be seen on a karyogram. Autosomal genetic disorders can arise due to a number of causes, some of the most common being nondisjunction in parental germ cells or Mendelian inheritance of deleterious alleles from parents. Autosomal genetic disorders which exhibit Mendelian inheritance can be inherited either in an autosomal dominant or recessive fashion.
These disorders are passed on by either sex with equal frequency. Autosomal dominant disorders are present in both parent and child, as the child needs to inherit only one copy of the deleterious allele to manifest the disease. Autosomal recessive diseases, require two copies of the deleterious allele for the disease to manifest; because it is possible to possess one copy of a deleterious allele without presenting a disease phenotype, two phenotypically normal parents can have a child with the disease if both parents are carriers for the condition. Autosomal aneuploidy can result in disease conditions. Aneuploidy of autosomes is not well tolerated and results in miscarriage of the developing fetus. Fetuses with aneuploidy of gene-rich chromosomes—such as chromosome 1—never survive to term, fetuses with aneuploidy of gene-poor chromosomes—such as chromosome 21— are still miscarried over 23% of the time. Possessing a single copy of an autosome is nearly always incompatible with life, though rarely some monosomies can survive past birth.
Having three copies of an autosome is far more compatible with life, however. A common example is Down syndrome, caused by possessing three copies of chromosome 21 instead of the usual two. Partial aneuploidy can occur as a result of unbalanced translocations during meiosis. Deletions of part of a chromosome cause partial monosomies, while duplications can cause partial trisomies. If the duplication or deletion is large enough, it can be discovered by analyzing a karyogram of the individual. Autosomal translocations can be responsible for a number of diseases, ranging from cancer to schizophrenia. Unlike single gene disorders, diseases caused by aneuploidy are the result of improper gene dosage, not nonfunctional gene product. Aneuploidy Autosomal dominant Autosomal recessive Homologous chromosome Pseudoautosomal region XY sex-determination system Genetic disorder
Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, a phosphate group; the nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
This information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences; the two strands of DNA are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a
Chromosomal crossover is the exchange of genetic material between two homologous chromosomes non-sister chromatids that results in recombinant chromosomes during sexual reproduction. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover occurs when matching regions on matching chromosomes break and reconnect to the other chromosome. Crossing over was described, in theory, by Thomas Hunt Morgan, he relied on the discovery of Frans Alfons Janssens who described the phenomenon in 1909 and had called it "chiasmatypie". The term chiasma is linked, to chromosomal crossover. Morgan saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of his research on the heredity of Drosophila; the physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931.
The linked frequency of crossing over between two gene loci is the crossing-over value. For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure tends to be constant and the same is true for the crossing-over value, used in the production of genetic maps. There are two popular and overlapping theories that explain the origins of crossing-over, coming from the different theories on the origin of meiosis; the first theory rests upon the idea that meiosis evolved as another method of DNA repair, thus crossing-over is a novel way to replace damaged sections of DNA. The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity. In 1931, Barbara McClintock discovered a triploid maize plant, she made key findings regarding corn's karyotype, including the shape of the chromosomes. McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, showed the first cytological demonstration of crossing over in meiosis.
Working with student Harriet Creighton, McClintock made significant contributions to the early understanding of codependency of linked genes. Crossing over and DNA repair are similar processes, which utilize many of the same protein complexes. In her report, “The Significance of Responses of the Genome to Challenge”, McClintock studied corn to show how corn's genome would change itself to overcome threats to its survival, she used 450 self-pollinated plants. She used modified patterns of gene expression on different sectors of leaves of her corn plants show that transposable elements hide in the genome, their mobility allows them to alter the action of genes at different loci; these elements can restructure the genome, anywhere from a few nucleotides to whole segments of chromosome. Recombinases and primases lay a foundation of nucleotides along the DNA sequence. One such particular protein complex, conserved between processes is RAD51, a well conserved recombinase protein, shown to be crucial in DNA repair as well as cross over.
Several other genes in D. melanogaster have been linked as well to both processes, by showing that mutants at these specific loci cannot undergo DNA repair or crossing over. Such genes include mei-41, mei-9, spnA, brca2; this large group of conserved genes between processes supports the theory of a close evolutionary relationship. Furthermore, DNA repair and crossover have been found to favor similar regions on chromosomes. In an experiment using radiation hybrid mapping on wheat's 3B chromosome, crossing over and DNA repair were found to occur predominantly in the same regions. Furthermore, crossing over has been correlated to occur in response to stressful, DNA damaging, conditions The process of bacterial transformation shares many similarities with chromosomal cross over in the formation of overhangs on the sides of the broken DNA strand, allowing for the annealing of a new strand. Bacterial transformation itself has been linked to DNA repair many times; the second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating genetic diversity..
Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive. It is that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes. Meiotic recombination may be initiated by double-stranded breaks that are introduced into the DNA by exposure to DNA damaging agents or the Spo11 protein. One or more exonucleases digest the 5’ ends generated by the double-stranded breaks to produce 3’ single-stranded DNA tails; the meiosis-specific recombinase Dmc1 and the general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments. The recombinases catalyze invasion of the opposite chromatid by the single-stranded DNA from one end of the break. Next, the 3’ end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break.
The structure that results is a cross-strand exchange known as a Holliday junction. The contact between two chromatids that will soon undergo crossing-over is known as a chiasma; the Holliday junction is a
Genetic genealogy is the use of DNA testing in combination with traditional genealogical methods to infer relationships between individuals and find ancestors. Genetic genealogy involves the use of genealogical DNA testing to determine the level and type of the genetic relationship between individuals; this application of genetics became popular with family historians in the 21st century, as tests became affordable. The tests have been promoted by amateur groups, such as surname study groups, or regional genealogical groups, as well as research projects such as the genographic project; as of 2018, 18.5 million people had been tested. As this field has developed, the aims of practitioners broadened, with many seeking knowledge of their ancestry beyond the recent centuries for which traditional pedigrees can be constructed; the investigation of surnames in genetics can be said to go back to George Darwin, a son of Charles Darwin. In 1875, George Darwin used surnames to estimate the frequency of first-cousin marriages and calculated the expected incidence of marriage between people of the same surname.
He arrived at a figure between 2.25% and 4.5% for cousin-marriage in the population of Great Britain, higher among the upper classes and lower among the general rural population. One famous study examined the lineage of descendants of Thomas Jefferson’s paternal line and male lineage descendants of the freed slave, Sally Hemmings. Bryan Sykes, a molecular biologist at Oxford University tested the new methodology in general surname research, his study of the Sykes surname obtained results by looking at four STR markers on the male chromosome. It pointed the way to genetics becoming a valuable assistant in the service of genealogy and history; the first company to provide direct-to-consumer genetic DNA testing was the now defunct GeneTree. However, it did not offer multi-generational genealogy tests. In fall 2001, GeneTree sold its assets to Salt Lake City-based Sorenson Molecular Genealogy Foundation which originated in 1999. While in operation, SMGF provided free mitochondrial DNA tests to thousands.
GeneTree returned to genetic testing for genealogy in conjunction with the Sorenson parent company and was part of the assets acquired in the Ancestry.com buyout of SMGF. In 2000, Family Tree DNA, founded by Bennett Greenspan and Max Blankfeld, was the first company dedicated to direct-to-consumer testing for genealogy research, they offered eleven marker Y-Chromosome STR tests and HVR1 mitochondrial DNA tests. They tested in partnership with the University of Arizona. In 2007, 23andMe was the first company to offer a saliva-based direct-to-consumer genetic testing, it was the first to implement using autosomal DNA for ancestry testing, which all other major companies now use. By 2018, large DNA genealogy companies had over 18.5 million profiles. GEDmatch said; the publication of The Seven Daughters of Eve by Sykes in 2001, which described the seven major haplogroups of European ancestors, helped push personal ancestry testing through DNA tests into wide public notice. With the growing availability and affordability of genealogical DNA testing, genetic genealogy as a field grew rapidly.
By 2003, the field of DNA testing of surnames was declared to have “arrived” in an article by Jobling and Tyler-Smith in Nature Reviews Genetics. The number of firms offering tests, the number of consumers ordering them, rose dramatically. In 2018 a paper in Science Magazine estimated that a DNA genealogy search on anybody of European descent would result in a third cousin or closer match 60% of the time; the original Genographic Project was a five-year research study launched in 2005 by the National Geographic Society and IBM, in partnership with the University of Arizona and Family Tree DNA. Its goals were anthropological; the project announced that by April 2010 it had sold more than 350,000 of its public participation testing kits, which test the general public for either twelve STR markers on the Y-Chromosome or mutations on the HVR1 region of the mtDNA. In 2007, annual sales of genetic genealogical tests for all companies, including the laboratories that support them, were estimated to be in the area of $60 million.
The current phase of the project is Geno 2.0 Next Generation. As of 2018 one-million participants in over 140 countries have joined the project. Genetic genealogy has enabled groups of people to trace their ancestry though they are not able to use conventional genealogical techniques; this may be because they do not know one or both of their birth parents or because conventional genealogical records have been lost, destroyed or never existed. These groups include adoptees, Holocaust survivors, GI babies, child migrants, descendants of children from orphan trains and people with slave ancestry; the earliest test takers were customers most those who started with a Y-Chromosome test to determine their father's paternal ancestry. These men took part in surname projects; the first phase of the Genographic project brought new participants into genetic genealogy. Those who tested were as to be interested in direct maternal heritage as their paternal; the number of those taking mtDNA tests increased. The introduction of autosomal SNP tests based on microarray chip technology changed the demographics.
Women were as as men to test themselves. Members of the growing genetic genealogy community have been credited with making useful contributions to knowledge in the field. One of the earliest interest groups to emerge was the International Society of Genetic Genealogy, their stated goal is to promote DNA testing for genealogy. Members advocate the use of genetics in genealogical research and the group faci
Zygosity is the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous; the DNA sequence of a gene varies from one individual to another. Those variations are called alleles. While some genes have only one allele because there is low variation, others have only one allele because deviation from that allele can be harmful or fatal, 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, another allele may be rare. Sometimes, one allele is a disease-causing variation. 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 one from the female parent. Zygosity is a description of whether those two alleles have different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome; the words homozygous 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, nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing. A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes.
The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits. An individual, homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait; this allele called the "dominant allele", is represented by a capital letter. 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, homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait; this allele called the "recessive allele", is represented by the lowercase form of the letter used for the corresponding dominant trait. The genotype of an organism, homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp". A diploid organism is heterozygous at a gene locus when its cells contain two different alleles of a gene; the cell or organism is called a heterozygote for the allele in question, therefore, heterozygosity refers to a specific genotype.
Heterozygous genotypes are represented by a capital letter and a lowercase letter, such as "Rr" or "Ss". Alternatively, a heterozygote for gene "R" is assumed to be "Rr"; the capital letter is written first. If the trait in question is determined by simple dominance, a heterozygote will express only the trait coded by the dominant allele, the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex. A heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype - this is called a heterozygote advantage. A chromosome in a diploid organism is hemizygous; 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. Hemizygosity must not be confused with haploinsufficiency, which describes a mechanism for producing a phenotype. For organisms in which the male is heterogametic, such as humans 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.
Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are considered to be hemizygous, because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic individual can be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotype of each individual. 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. 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 n