Extrachromosomal and Non-Mendelian Inheritance

The laws of inheritance were established in the nineteenth century by an Austrian monk Gregor Mendel, since that we talk about Mendelian inheritance. Over the next years and decades some exceptions, which do not correspond with original ideas, were discovered.

Epigenetic Inheritance
Epigenetic influences modify genetic information (gene expression) without changing the primary structure of the DNA (nucleotide sequence). This modification could be changed (reprogrammed) during an individual's life, but it could be also transmitted to the next generation(s).

Methylation
Methylation is a type of chemical modification when methyl group binds to DNA basis. This change decide how a gene will be expressed or not. It goes against the third law of inheritance - law of dominance. Althought the inheritance of some disease seems to be autosomal, it can be transmitted in different way depends on from which parent you get a genetic information.

Methylation can sometimes protect its carrier from the outbreak of a disease, but his children have higher risk, if they do not have this methyl groups. There are two main diseases, which are influenced by this modification: fragile X syndrome and Huntington disease (E.g. Huntington disease inherited from father has earlier onset than the maternal one.)

Imprinting
Imprinting is important for two human diseases - Angelman and Prader-Willi syndromes. More information here.

Extranuclear Inheritance
The original Mendelian rules were based just on nuclear DNA, but as it was discovered later, some of the organelles also have their own DNA:
 * 1) Mitochondrial DNA - also called mtDNA.
 * 2) Chloroplasts in plants.

Extranuclear DNA is also double stranded as the nuclear is and it is located in nucleoid. Human mtDNA consists of 17 000 bp, which code 13 genes important for oxidative phosphorylation. The maternal type of inheritance is characterized by strict transmition of mtDNA from mother to offspring. It is very important in the origin of mitochondrial diseases - e.g. Leber´s hereditary optic neuropathy, Kearns-Sayre disease. Mitochondrial diseases are chronical and degenerative and affect mainly muscles, brain, kidneys or glands.

Genetic Linkage
Some genes do not assort independently, because they are located on the same chromosome and there is a linkage between them, so they have to be transfered together. More information.

Alelic Interaction
Codominant inheritance means that there are two alleles and both of them have their patterns in offsprings. Normally the dominant one ovrpower the recessive one, but not in this case. E.g. blood ABO system, where allele A and B have same importance.

Penetrance
The frequency of expression of an allele, which can be find in the genotype. E.g. if you have 10 children and 6 of them have 21 fingers, the penetrance of polydactyly is 6/10 - it is 60%.

Not all phenotypes that are expressed are manifested to the same degree. For polydactyly, an extra digit may occur on one or more appendages, and the digit can be full size or just a stub. Therefore, when the P allele is present it expresses variable expressivity.

Gonosomal Inheritance
X-linked disorders affected only male (they have one X chromosome), because female are heterozygous carriers. If female have one faulty X chromosome and the second one in order, they do not suffer from all of symptoms. This can be later change by an inactivation of X chromosome.

X-linked diseases are: Duchenne and Becker muscular dystrophy, Hemophilia A nad B, diabetes insipidus, fragile X syndrome

Related articles

 * Genetic Linkage
 * Genomic Imprinting
 * Prader-Willi Syndrome
 * Angelman Syndrome
 * Laws of Inheritance