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Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with
Mendel's laws Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized ...
. These laws describe the inheritance of traits linked to single
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ...
s on
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
s in the nucleus. In
Mendelian inheritance Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
, each parent contributes one of two possible
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s for a trait. If the
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
s of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
s expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values. Non-Mendelian inheritance plays a role in several disease which affected the processes.


Types


Incomplete dominance

In cases of intermediate inheritance due to
incomplete dominance In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
, the principle of dominance discovered by Mendel does not apply. Nevertheless, the principle of uniformity works, as all offspring in the F1-generation have the same genotype and same phenotype. Mendel's principle of segregation of genes applies too, as in the F2-generation homozygous individuals with the phenotypes of the P-generation appear. Intermediate inheritance was first examined by
Carl Correns Carl Erich Correns (19 September 1864 – 14 February 1933) was a German botanist and geneticist notable primarily for his independent discovery of the principles of heredity, which he achieved simultaneously but independently of the botanis ...
in
Mirabilis jalapa ''Mirabilis jalapa'', the marvel of Peru or four o'clock flower, is the most commonly grown ornamental species of ''Mirabilis'' plant, and is available in a range of colors. ''Mirabilis'' in Latin means wonderful and Jalapa (or Xalapa) is the st ...
sed for further genetic experiments. ''
Antirrhinum majus ''Antirrhinum majus'', the common snapdragon (often - especially in horticulture - simply "snapdragon"), is a species of flowering plant belonging to the genus ''Antirrhinum''. The plant was placed in the family Plantaginaceae following a rev ...
'' also shows intermediate inheritance of the pigmentation of the blossoms.


Co-dominance

In cases of
co-dominance In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
, the genetic traits of both different alleles of the same gene-locus are clearly expressed in the phenotype. For example, in certain varieties of chicken, the allele for black feathers is co-dominant with the allele for white feathers. Heterozygous chickens have a colour described as "erminette", speckled with black and white feathers appearing separately. Many human genes, including one for a protein that controls cholesterol levels in the blood, show co-dominance too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels.


Genetic linkage

When genes are located on the same chromosome and no crossing over took place before the segregation of the chromosomes into the
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s, the
genetic trait Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wo ...
s will be inherited in connection, because of the
genetic linkage Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be sep ...
. These cases constitute an exception to the Mendelian rule of independent assortment.


Multiple alleles

In Mendelian inheritance, genes have only two alleles, such as ''a'' and ''A''. Mendel consciously chose pairs of genetic traits, represented by two alleles for his inheritance experiments. In nature, such genes often exist in several different forms and are therefore said to have
multiple alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. They display a pattern of a dominance-hierarchy that can produce four coat colors. In the genes for the dog coat colours there are four alleles on the Agouti-locus. The allele "aw" is dominant over the alleles "at" and "a" but recessive under "Ay". Many other genes have multiple alleles, including the human genes for
ABO blood type The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes. For human blood transfusions, it is the most important of the 43 different blood type (or group) classification system ...
.


Epistasis

If one or more genes cannot be expressed because of another genetic factor hindering their expression, this
epistasis Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is ...
can make it impossible even for dominant alleles on certain other gene-loci to have an effect on the phenotype. An example in
dog coat genetics Dogs have a wide range of coat colors, patterns, textures and lengths. Dog coat color is governed by how genes are passed from dogs to their puppies and how those genes are expressed in each dog. Dogs have about 19,000 genes in their genome but on ...
is the homozygosity with the allele "e e" on the Extension-locus making it impossible to produce any other pigment than pheomelanin. Although the allele "e" is a recessive allele on the extension-locus itself, the presence of two copies leverages the dominance of other coat colour genes. Domestic cats have a gene with a similar effect on the X-chromosome.


Sex-linked inheritance

Genetic traits located on
gonosome A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In some species there are her ...
s sometimes show specific non-Mendelian inheritance patterns. Individuals can develop a recessive trait in the phenotype dependent on their sex—for example,
colour blindness Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
and
haemophilia Haemophilia, or hemophilia (), is a mostly inherited genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. This results in people bleeding for a longer time after an injury, easy bruising ...
(see gonosomal inheritances). As many of the alleles are dominant or recessive, a true understanding of the principles of
Mendelian inheritance Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
is an important requirement to also understand the more complicated inheritance patterns of sex-linked inheritances.


Extranuclear inheritance

Extranuclear inheritance (also known as cytoplasmic inheritance) is a form of non-Mendelian inheritance also first discovered by Carl Correns in 1908. While working with ''
Mirabilis jalapa ''Mirabilis jalapa'', the marvel of Peru or four o'clock flower, is the most commonly grown ornamental species of ''Mirabilis'' plant, and is available in a range of colors. ''Mirabilis'' in Latin means wonderful and Jalapa (or Xalapa) is the st ...
,'' Correns observed that leaf colour was dependent only on the genotype of the maternal parent. Based on these data, he determined that the trait was transmitted through a character present in the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. T ...
of the
ovule In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the '' nucellus'' (or remnant of the megasporangium), and the ...
. Later research by Ruth Sager and others identified DNA present in
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s as being responsible for the unusual inheritance pattern observed. Work on the poky strain of the mould ''
Neurospora crassa ''Neurospora crassa'' is a type of red bread mold of the phylum Ascomycota. The genus name, meaning "nerve spore" in Greek, refers to the characteristic striations on the spores. The first published account of this fungus was from an infestation ...
'' begun by Mary and Hershel Mitchell ultimately led to the discovery of genetic material in the mitochondria, the
mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DN ...
. According to the
endosymbiont An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...
theory, mitochondria and chloroplasts were once free-living organisms that were each taken up by a eukaryotic cell. Over time, mitochondria and chloroplasts formed a
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasi ...
relationship with their eukaryotic hosts. Although the transfer of a number of genes from these organelles to the nucleus prevents them from living independently, each still possesses genetic material in the form of double stranded DNA. It is the transmission of this organellar DNA that is responsible for the phenomenon of extranuclear inheritance. Both chloroplasts and mitochondria are present in the cytoplasm of maternal gametes only. Paternal gametes (
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum ...
for example) do not have cytoplasmic mitochondria. Thus, the
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
of traits linked to genes found in either chloroplasts or mitochondria are determined exclusively by the maternal parent. In humans,
mitochondrial disease Mitochondrial disease is a group of disorders caused by mitochondrial dysfunction. Mitochondria are the organelles that generate energy for the cell and are found in every cell of the human body except red blood cells. They convert the energy of ...
s are a class of diseases, many of which affect the muscles and the eye.


Polygenic traits

Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. ''Polygenic'' means "many genes" are necessary for the organism to develop the trait. For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The broad variety of skin colour in humans comes about partly because at least four different genes probably control this trait.


Non-random segregation

Non-random segregation of chromosomes is a deviation from the usual distribution of chromosomes during meiosis and in some cases of mitosis.


Gene conversion

Gene conversion Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces ...
can be one of the major forms of non-Mendelian inheritance. Gene conversion arises during DNA repair via DNA recombination, by which a piece of DNA sequence information is transferred from one DNA helix (which remains unchanged) to another DNA helix, whose sequence is altered. This may occur as a
mismatch repair DNA mismatch repair (MMR) is a system for recognizing and repairing erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination, as well as repairing some forms of DNA damage. Mismatc ...
between the strands of DNA which are derived from different parents. Thus the mismatch repair can convert one
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
into the other. This phenomenon can be detected through the offspring non-Mendelian ratios, and is frequently observed, e.g., in fungal crosses.


Infectious heredity

Another form of non-Mendelian inheritance is known as infectious heredity. Infectious particles such as
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky' ...
es may infect host cells and continue to reside in the cytoplasm of these cells. If the presence of these particles results in an altered phenotype, then this phenotype may be subsequently transmitted to progeny. Because this phenotype is dependent only on the presence of the invader in the host cell's cytoplasm, inheritance will be determined only by the infected status of the maternal parent. This will result in a uniparental transmission of the trait, just as in extranuclear inheritance. One of the most well-studied examples of infectious heredity is the killer phenomenon exhibited in
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
. Two double-stranded
RNA virus An RNA virus is a virusother than a retrovirusthat has ribonucleic acid ( RNA) as its genetic material. The nucleic acid is usually single-stranded RNA (ssRNA) but it may be double-stranded (dsRNA). Notable human diseases caused by RNA viruse ...
es, designated L and M, are responsible for this phenotype. The L virus codes for the
capsid A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or m ...
proteins of both viruses, as well as an
RNA polymerase In molecular biology, RNA polymerase (abbreviated RNAP or RNApol), or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that synthesizes RNA from a DNA template. Using the enzyme helicase, RNAP locally opens th ...
. Thus the M virus can only infect cells already harbouring L virus particles. The M viral RNA encodes a
toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849� ...
that is secreted from the host cell. It kills susceptible cells growing in close proximity to the host. The M viral RNA also renders the host cell immune to the lethal effects of the toxin. For a cell to be susceptible it must therefore be either uninfected or harbour only the L virus. The L and M viruses are not capable of exiting their host cell through conventional means. They can only transfer from cell to cell when their host undergoes mating. All progeny of a mating involving a doubly infected yeast cell will also be infected with the L and M viruses. Therefore, the killer phenotype will be passed down to all progeny. Heritable traits that result from infection with foreign particles have also been identified in ''
Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many specie ...
''. Wild-type flies normally fully recover after being anesthetized with carbon dioxide. Certain lines of flies have been identified that die off after exposure to the compound. This carbon dioxide sensitivity is passed down from mothers to their progeny. This sensitivity is due to infection with σ (Sigma) virus, a rhabdovirus only capable of infecting ''Drosophila''. Although this process is usually associated with viruses, recent research has shown that the ''
Wolbachia ''Wolbachia'' is a genus of intracellular bacteria that infects mainly arthropod species, including a high proportion of insects, and also some nematodes. It is one of the most common parasitic microbes, and is possibly the most common repro ...
'' bacterium is also capable of inserting its genome into that of its host.


Genomic imprinting

Genomic imprinting represents yet another example of non-Mendelian inheritance. Just as in conventional inheritance, genes for a given trait are passed down to progeny from both parents. However, these genes are epigenetically marked before transmission, altering their levels of expression. These imprints are created before gamete formation and are erased during the creation of germ line cells. Therefore, a new pattern of imprinting can be made with each generation. Genes are imprinted differently depending on the parental origin of the
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
that contains them. In mice, the
insulin-like growth factor 2 Insulin-like growth factor 2 (IGF-2) is one of three protein hormones that share structural similarity to insulin. The MeSH definition reads: "A well-characterized neutral peptide believed to be secreted by the liver and to circulate in the b ...
gene undergoes imprinting. The
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
encoded by this gene helps to regulate body size. Mice that possess two functional copies of this gene are larger than those with two mutant copies. The size of mice that are heterozygous at this locus depends on the parent from which the wild-type
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
came. If the functional allele originated from the mother, the offspring will exhibit
dwarfism Dwarfism is a condition wherein an organism is exceptionally small, and mostly occurs in the animal kingdom. In humans, it is sometimes defined as an adult height of less than , regardless of sex; the average adult height among people with dw ...
, whereas a paternal allele will generate a normal-sized mouse. This is because the maternal ''Igf2'' gene is imprinted. Imprinting results in the inactivation of the ''Igf2'' gene on the chromosome passed down by the mother. Imprints are formed due to the differential
methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These ...
of paternal and maternal alleles. This results in differing expression between alleles from the two parents. Sites with significant methylation are associated with low levels of
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. T ...
. Higher gene expression is found at unmethylated sites. In this mode of inheritance, phenotype is determined not only by the specific allele transmitted to the offspring, but also by the sex of the parent that transmitted it.


Mosaicism

Individuals who possess cells with genetic differences from the other cells in their body are termed mosaics. These differences can result from
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
s that occur in different tissues and at different periods of development. If a mutation happens in the non-gamete forming tissues, it is characterized as somatic.
Germline In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny ( offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. T ...
mutations occur in the egg or sperm cells and can be passed on to offspring. Mutations that occur early on in development will affect a greater number of cells and can result in an individual that can be identified as a mosaic strictly based on phenotype. Mosaicism also results from a phenomenon known as
X-inactivation X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged in ...
. All female mammals have two
X chromosome The X chromosome is one of the two sex-determining chromosomes ( allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO s ...
s. To prevent lethal gene dosage problems, one of these chromosomes is inactivated following
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proc ...
. This process occurs randomly for all of the cells in the organism's body. Because a given female's two X chromosomes will almost certainly differ in their specific pattern of alleles, this will result in differing cell phenotypes depending on which chromosome is silenced. Calico cats, which are almost all female, demonstrate one of the most commonly observed manifestations of this process.


Trinucleotide repeat disorders

Trinucleotide repeat disorders also follow a non-Mendelian pattern of inheritance. These diseases are all caused by the expansion of
microsatellite A microsatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from one to six or more base pairs) are repeated, typically 5–50 times. Microsatellites occur at thousands of locations within an organism's genome. ...
tandem repeat Tandem repeats occur in DNA when a pattern of one or more nucleotides is repeated and the repetitions are directly adjacent to each other. Several protein domains also form tandem repeats within their amino acid primary structure, such as armadil ...
s consisting of a stretch of three
nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
s. Typically in individuals, the number of repeated units is relatively low. With each successive generation, there is a chance that the number of repeats will expand. As this occurs, progeny can progress to premutation and ultimately affected status. Individuals with a number of repeats that falls in the premutation range have a good chance of having affected children. Those who progress to affected status will exhibit symptoms of their particular disease. Prominent trinucleotide repeat disorders include
Fragile X syndrome Fragile X syndrome (FXS) is a genetic disorder characterized by mild-to-moderate intellectual disability. The average IQ in males with FXS is under 55, while about two thirds of affected females are intellectually disabled. Physical features m ...
and
Huntington's disease Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...
. In the case of Fragile X syndrome it is thought that the symptoms result from the increased methylation and accompanying reduced expression of the fragile X intellectual disability gene in individuals with a sufficient number of repeats.


See also

*
Meiotic drive Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will effect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenoty ...
*
CoRR Hypothesis The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation (" redox") state of its gene products. CoRR is short for "co-location for redox regulatio ...
*
Epigenetic inheritance Transgenerational epigenetic inheritance is the transmission of epigenetic markers from one organism to the next (i.e., from parent to child) that affects the traits of offspring without altering the primary structure of DNA (i.e. the sequence of ...
*
Gene drive A gene drive is a natural process and technology of genetic engineering that propagates a particular suite of genes throughout a population by altering the probability that a specific allele will be transmitted to offspring (instead of the Mend ...
* Intragenomic conflict


References


External links


non-Mendelian inheritance
at
Duke University Duke University is a private research university in Durham, North Carolina. Founded by Methodists and Quakers in the present-day city of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco and electric power industrialist ...
{{DEFAULTSORT:Non-Mendelian Inheritance Extended evolutionary synthesis Classical genetics