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.
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.
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.
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.
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.
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.
Question 1: What is the Difference Between Mendelian and Non-Mendelian Inheritances?
Answer: There are two major types of inheritances, namely, Mendelian and non-Mendelian inheritances. In the Mendelian type of Inheritance, the traits of the parents are passed down to their offspring by alleles of one gene that is either dominant or recessive. In the non-Mendelian type of Inheritance, there are different genes acting to show one trait, or various traits resulting from one gene. Some times, the traits are also seen in the phenotypes resulting from both the alleles that are different. The interplay of various genes or one gene affecting the physiology of the individual can give rise to diseases that are sometimes fatal.
Question 2: Do the Non-Mendelian Traits Depend on Dominant or Recessive Genes?
Answer: The non-Mendelian traits do not depend on the concept of dominant or recessive genes. In Incomplete Dominance, for example, the traits result from a mixed pattern. Whereas, in Codominance, there are mixed traits of both the parents seen in the offspring. Multiple Alleles is a phenomenon in co-dominance Inheritance which is present in some populations; in which, there are various traits seen in different individuals resulting from one gene but from different alleles. Moreover, in Polygenic Inheritance, there are numerous genes contributing to one trait; height in humans, e.g., is a result of the interplay of approximately 400 genes. Therefore, there is no relevance of dominant or recessive genes in non-Mendelian traits.