Non mendalianne.docx

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Non-Mendelian inheritance is a general term that refers to any pattern of inheritance in which

traits do not segregate in accordance with Mendels laws. These laws describe the inheritance of

traits linked to singlegenesonchromosomesin the nucleus. InMendelian inheritance, eachparent contributes one of two possibleallelesfor a trait. If thegenotypesof both parents in a

genetic cross are known, Mendels laws can be used to determine the distribution ofphenotypes

expected for the population of offspring. There are several situations in which the proportions ofphenotypes observed in the progeny do not match the predicted values.

Although inheritance of traits infungi,viruses, andbacteriaare all non-Mendelian, the phrase"non-Mendelian inheritance" is usually only used to describe the exceptions which occur in

eukaryoticreproduction.[1]

Non-Mendelian inheritance plays a role in several disease processes.[2]

Types

Extranuclear inheritance

Extranuclear inheritance(also known as cytoplasmic inheritance) is a form of non-Mendelian

inheritance first discovered byCarl Corrensin 1908.[3]

While working withMirabilis jalapaCorrens observed that leaf color 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

thecytoplasmof theovule. Later research byRuth Sagerand others identified DNA present inchloroplastsas being responsible for the unusual inheritance pattern observed. Work on the poky

strain of the moldNeurospora crassabegun byMaryandHershel Mitchell[4]

ultimately led to

the discovery of genetic material inmitochondriaas well.

According to theendosymbionttheory, mitochondria and chloroplasts were once free livingorganisms that were each taken up by a eukaryotic cell.

[5]Over time, mitochondria and

chloroplasts formed asymbioticrelationship 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 thisorganellarDNA that is responsible for the phenomenon of

extranuclear inheritance. Both chloroplasts and mitochondria are present in the cytoplasm of

maternal gametes only. Paternal gametes (spermfor example) do not have cytoplasmicmitochondria. Thus, thephenotypeof traits linked to genes found in either chloroplasts or

mitochondria are determined exclusively by the maternal parent.

In humans,mitochondrial diseasesare a class of diseases, many of which affect the muscles and

the eye.

Gene conversion

Gene conversioncan be one of major forms of non-Mendelian inheritance. Gene conversion is areparation process in DNArecombination, by which a piece of DNA sequence information is
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transferred from one DNA helix (which remains unchanged) to another DNA helix, whose

sequence is altered. This may occur as amismatch repairbetween the strands of DNA which are

derived from different parents. Thus the mismatch repair can convert onealleleinto the other.This phenomenon can be detected through the offspring non-Mendelian ratios, and is frequently

observed, e.g., in fungal crosses.[6]

Infectious heredity

Another form of non-Mendelian inheritance is known as infectious heredity. Infectious particles

such asvirusesmay infect host cells and continue to reside in the cytoplasm of these cells. If thepresence of these particles results in an altered phenotype, then this phenotype may be

subsequently transmitted to progeny.[7]

Because this phenotype is dependent only on the

presence of the invader in the host cells 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 exhibitedinyeast. Two double-strandedRNA viruses, designated L and M, are responsible for this

phenotype.[8]

The L virus codes for thecapsidproteins of both viruses, as well as anRNA

polymerase. Thus the M virus can only infect cells already harboring L virus particles. The Mviral RNA encodes atoxinwhich 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 harboronly the L virus.

The L and M viruses are not capable of exiting their host cell through conventional means. Theycan 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. Wild type flies normally full 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 Sigma virus, arhabdovirusonly capable of infectingDrosophila.[9]

Although this process is usually associated with viruses, recent research has shown that the

Wolbachiabacterium is also capable of inserting its genome into that of its host.[10][11]

Genomic imprinting

Main article:Genomic imprinting

Genomic imprinting represents yet another example of non-Mendelian inheritance. Just as inconventional inheritance, genes for a given trait are passed down to progeny from both parents.However, these genes areepigeneticallymarked before transmission, altering their levels of
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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 thechromosomethat

contains them. In mice, theinsulin-like growth factor 2gene undergoes imprinting. Theproteinencoded 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 typeallelecame. If the functional alleleoriginated from the mother, the offspring will exhibitdwarfism, 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.[12]

Imprints are formed due to the differentialmethylationof paternal and maternal alleles. This

results in differing expression between alleles from the two parents. Sites with significant

methylation are associated with low levels ofgene expression. Higher gene expression is found

at unmethylated sites.

[13]

In this mode of inheritance, phenotype is determined not only by thespecific 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 aretermed mosaics. These differences can result frommutationsthat occur in different tissues and at

different periods of development. If a mutation happens in the non-gamete forming tissues, it is

characterized assomatic.Germlinemutations occur in the egg or sperm cells and can be passed

on to offspring.[14]

Mutations that occur early on in development will affect a greater number ofcells and can result in an individual that can be identified as a mosaic strictly based on

phenotype.

Mosaicismalso results from a phenomenon known asX-inactivation. All female mammals have

twoX chromosomes. To prevent lethalgene dosageproblems, one of these chromosomes is

inactivated followingfertilization. This process occurs randomly for all of the cells in the

organisms body. Because a given females two X chromosomes will almost certainly differ intheir specific pattern of alleles, this will result in differing cell phenotypes depending on which

chromosome is silenced.Calico cats, which are almost all female,[15]

demonstrate one of the

most commonly observed manifestations of this process.[16]

Trinucleotide repeat disorders

Trinucleotide repeat disordersalso follow a non-Mendelian pattern of inheritance. These diseases

are all caused by the expansion ofmicrosatellitetandem repeatsconsisting of a stretch of three

nucleotides.[17]

In normal 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 permutation and ultimately affected status. Individuals with a number of

repeats that falls in the permutation range have a good chance of having affected children. Those

who progress to affected status will exhibit symptoms of their particular disease. Prominent
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trinucleotide repeat disorders includeFragile X syndromeandHuntington's disease. 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 mental retardation gene in individuals with asufficient number of repeats.

[18]
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