Non Mendelian Inheritance in Genetics

The non-Mendelian Inheritance is a form of genetic Inheritance which is not in accordance with Mendel’s law. In non-Mendelian genetics, the traits of an individual are linked to a single gene or chromosome from the nuclear DNA. Scientists stumbled across the phenomenon when they began exploring more and more case studies; they soon realised that there are various types of non-Mendelian Inheritances. In humans, some findings suggest that along with this type of genetic inheritance, there are other environmental factors like lack of vitamin D, adolescent obesity etc. contributing to certain types of genetic disorders. 

Types of Non-Mendelian Inheritance

1. Incomplete Dominance

In this type, the principle of dominance, as discovered by Mendel is not applicable; however, the principle of uniformity, is seen. In the incomplete dominance, the genetic traits mix which produces an intermediate phenotype in terms of physical traits. The pink rose is a great example in which the white and red varieties of rose are hybridised and the resulting offspring is pink rose.

2. Codominance 

In an offspring of an organism, if we see traits from two alleles, then it is a result of codominance. The blood group in humans is a good example of non-Mendelian genetics codominance. Someone with the blood group AB expresses the allele of both blood groups A and B. Another example of Co-dominance Inheritance is in varieties of domestic fowl or chicken; in them, the varieties of black and white feathers are co-dominant and when the fowls of both these separate traits are bred, then the offspring show both white and black feathers. 

There are even subtypes of co-dominance as follows.

• Multiple Alleles: Some population shows the presence of multiple alleles of one gene. For example, in rabbits, there is a C gene that defines the colour of coat in the breed. There are four common alleles of this gene, viz., CC giving black or brown fur; CchCch giving grey fur also known as chinchilla colouration; ChCh giving white body fur and dark ears, face, feet and tail; and cc giving a pure white fur and reddish eyes as a result of albinism. 

• Pleiotropy: In Pleiotropy, one gene affects multiple characteristics of the individual. The disease ‘Phenylketonuria’ is an example. It originates from the defect of a single gene on chromosome 12; however, it has an impact on multiple systems like the integumentary system of the skin and nervous system. Albinism is another example of one gene affecting the skin, eyes and hair colours.

• Lethality Due to Alleles: At times, the combination of the multiple alleles can make the survival of the individual difficult to impossible; e.g., a hybrid between two heterozygous yellow mice makes them give birth to yellow and brown mice in a ratio of 2:1. Mice that have homozygous alleles die during the gestation period, especially during the embryonic development. These kinds of lethal alleles can be found in dominant or recessive forms, and they can express the individual’s traits in homozygous or heterozygous cases.

3. Polygenic Inheritance

There are some traits that are controlled by more than one gene. Height in human beings, e.g., is controlled by more than 400 different genes. Similarly, the pigmentation of the skin is controlled by at least four genes. In fruit flies, the reddish-brown pigment in the eyes is a result of at least three genes. 

4. Extranuclear Inheritance

Extranuclear Inheritance is also known as cytoplasmic Inheritance and some times is also known as Mitochondrial Inheritance. In this type of Inheritance, some DNA of the mitochondria is passed from the mother to the offspring. Although, mostly the Extranuclear Inheritance from the mitochondria; nevertheless, it may occur from the chloroplast too. In cloning, e.g., there is a risk of transfer of genes from the mitochondria of the donor cell. There are even some genetic disorders that pass from the mother to the offspring that have their origin in the mitochondrial DNA. In the spermatozoa, e.g., there is no cytoplasm; therefore, the phenotype of traits related to extranuclear DNA is derived from the mother.

5. Gender - Linked Inheritance

In this type of Non-Mendelian Inheritance, we see particular traits in an individual that are related to gender. Disorders like colour-blindness and haemophilia are genetic and gender - related.

Environmental Effects

While studying genetic inheritance it is imperative to note that most real-world traits and attributes are not simply determined by the inherited genotype, environmental factors have a crucial role to play and can greatly influence how a particular genotype is translated into the organism's phenotype. A quick example of this is the colour phenotype of hydrangea flowers. Hydrangea of the same genetic variety may vary in colour from blue to pink depending on the pH of the soil they are in, regardless of the same genotype being inherited by the many progenies.

Common Misconceptions Related to Non-mendelian Inheritance

Pleiotropy versus polygenic inheritance. Some people confuse the two.  However, they are not the same phenomena. The major difference lies in the number of genes that affect/ control the phenotypic expression. Between the two, pleiotropy shows how one gene affects multiple characteristics (e.g. Marfan syndrome) while polygenic inheritance is when more than one gene controls a single trait in an organism, such as skin pigmentation in humans.

Secondly, codominance and incomplete dominance are also not the same types of inheritance. In codominance, neither of the alleles shows dominance over the other and so both are equally expressed in the heterozygote. However, in the case of incomplete dominance, there is true blending such that a third intermediate heterozygote is formed (such as a pink flower when the parents' phenotypes are red and white).

Epistasis

In the pattern of inheritance if the expression of one or more genes is expressed due to hindrance from another genetic factor it is known as epistasis. Epistasis makes it impossible even for dominant alleles to exert their influence on the resultant phenotype of the organism. An example of epistasis is coat genetics in dogs.  Here the homozygous nature for coat colour genes along with allele "e e" on the Extension-locus makes it impossible to produce any other pigment than pheomelanin. Even as the allele "e" is a recessive type on the extension-locus itself, the presence of two copies forces the dominance of other coat colour genes. Domestic cats also possess a gene that has a similar effect on the X-chromosome.

Non-random Segregation

In this type of chromosomal segregation, there is an obvious deviation from the usual distribution (which is supposed to be "random") of chromosomes either during meiosis or during the mitotic division of cells.