Mendelian Deviations: With his work on the garden pea (Pisum sativum) in his church garden, Gregor Johann Mendel, a monk, naturalist, and a curious, innovative guy, achieved a paradigm shift. He was interested in inheritance patterns and spent his entire life studying them. Because of his pioneering contributions to genetics, he is known as the “Father of Genetics.” He proposed the Mendelian deviations thereafter.

His inheritance hypothesis is still important today. However, it was not deemed a significant discovery at the time of his discovery and was forgotten. Three European scientists rediscovered his work in 1900 and gave him credit. As research into inheritance progressed, experts began to see discrepancies in his assumptions. Read this article to know more about deviations from Mendelian laws, genetics, deviations from Mendelian Inheritance, etc.

The Central Idea of Deviations From Mendelian Inheritance

1. Heritable traits are carried by discrete units called factors (later termed genes by Johannsen) that are found in pairs. 
2. Law of Dominance: Alternative forms of the gene are called allele or allelomorphs. There are two variables, one dominant, which always expresses itself, and the other recessive allele, which expresses only in homozygous conditions.
3. Law of Segregation: The pair of alleles segregate randomly in gametes, i.e., each gamete receives only one allele of a gene.  Since gametes are haploid and contain only one allele for a character, they are considered pure; hence, this law is also called the law of purity of gametes.
4. Law of Independent Assortment: This law states that the alleles of two (or more)  genes get sorted into gametes independently of one another. In other words, the inheritance of one gene is independent of another gene.

Study Properties Of Genetic Material Here

These laws explain the transfer of traits from one generation to another and decide the genotype and phenotype of an individual. Genotype is the genetic makeup of an organism, whereas phenotype constitutes an organism’s external or physical appearance.

Two types of the cross are studied mainly to understand the inheritance pattern, and it affects genotype and phenotype. Monohybrid cross involves the study of the expression of one gene at a time and produces a genotypic ratio of 1:2:1 and a Phenotypic ratio of 3:1 in the F₂ generation. Dihybrid cross involves studying two genes at a time and produces a phenotypic ratio of 9:3:3:1 and a genotypic ratio of 1:2:1:2:4:2:1:2:1.

However, when studies were conducted on different traits of organisms, what surprised most geneticists was inconsistency with Mendel’s monohybrid and dihybrid cross results. After research, it was concluded that these were deviations from Mendel’s work due to different reasons and phenomena. Let’s discuss those in detail.   

Deviations From Mendelian Law of Dominance: Deviation Involving One Gene

Incomplete dominance

Incomplete dominance is defined as the phenomenon of partial dominance in which a gene is unable to express fully and shows only a partial phenotypic effect. It is a deviation from the law of independence. The result obtained due to incomplete dominance shows an intermediate phenotype between dominant and recessive traits. For example, in the snapdragon plant, the gene responsible for the red flower is dominant over the gene responsible for the white coloured flower. Still, the red colour could not fully express itself in heterozygous conditions due to incomplete dominance, and the pink flower is obtained. The genotypic and phenotypic ratio is 1:2:1 in the F₂  generation. Theoretically, in the case of incomplete dominance, the deviation is seen due to a non-functional enzyme or the absence of the enzyme altogether.

Fig: Cross Depicting Incomplete Dominance

Multiple Alleles

Multiple alleles are defined as genes that have more than two alleles and can be identified in a population study only. It is a deviation from Mendel’s law of dominance which states that factors occur in pairs. For example, human populations have four kinds of blood groups- A, B, AB, and O blood groups. The blood group deciding antigens (a sugar polymer) are coded by the I gene. These antigens remain on the plasma membrane of RBCs and are responsible for the blood group. The gene (I) has three alleles I^A, I^B, and i. I^A  produces antigen A making the blood group A, while I^B produces antigen B, making the blood group B.  ‘i, ‘the third allele is recessive to both  I^A and  I^B and does not produce any antigen; hence it is called O. The pattern of inheritance of the blood group is given below in the figure. Can you guess your blood group based using the following chart on information about the blood group of your parents?

Fig: Pattern of Inheritance in case of Multiple Alleles

Codominance

Codominance is an occurrence where both the alleles are simultaneously expressed. It is a deviation from the law of dominance which suggests that only one allele is dominant while the other is recessive. For example, flowers like Rhododendron have petals of two different colours, i.e., red and white colour. It is because alleles responsible for both the colours express themselves simultaneously. Another example of codominance can be seen in the human ABO blood group. People with the blood group  AB have an expression of both the alleles I^A and I^B.

Fig: Codominance in ABO Blood Group

Pleiotropy

While Mendel established that one gene is responsible for one trait, pleiotropy deviates from that and is presented with one gene expressing multiple characteristics. For example, pleiotropy in humans is phenylketonuria (PKU). This disorder is caused by the enzyme phenylalanine hydroxylase deficiency, which is necessary to convert the essential amino acid phenylalanine to tyrosine. A defect in the single gene that codes for this enzyme, therefore, results in the multiple phenotypes associated with PKU, including mental retardation, eczema, and pigment defects that make affected individuals lighter-skinned 

Fig: Pleiotropy

Lethal Alleles

 Alleles that cause the death of an organism are called lethal alleles. This is rather an unfortunate phenomenon found in genetics that may occur due to mutation. Lethal alleles are generally recessive in nature. Example: Cystic fibrosis, Tay-Sachs disease, sickle-cell anaemia in females. 

Fig: Death Due To Presence Of Recessive Allele In Homozygous Condition

Sex Linkage

Autosomal genes are always found in pairs as per Mendel’s law, but in the case of sex chromosomes, It does not follow the Mendelian principle. The female has paired X chromosomes, but the male has one X and one Y chromosome. Genes present in sex chromosomes are not paired in males as males contain one X and one Y chromosome. Differences in chromosomal combinations produce different morphology and sexual character of males and females.

Pseudodominance

Pseudodominance refers to a condition where a recessive character starts to express itself due to the absence of a dominant allele. A dominant allele may be deleted due to mutation, or it may be absent altogether, as in the case of human males—for example, the occurrence of colour blindness in the human male. The gene responsible for colour blindness is recessive, yet due to the presence of single X chromosomes, human males are affected even in the presence of one recessive allele.

Deviation Involving Multiple Gene

Epistasis

Epistasis is defined as the genetic phenomenon in which interaction between two different gene variants at two or more locus on the chromosome produces a unique phenotype that could not be expected by considering the combined effects of the genes. Epistasis Gene Interaction can be of various types: 

Recessive Epistasis: When recessive alleles at one locus mask the expression of both dominant as well as recessive alleles at another locus, it is called recessive epistasis: example- grain colour in maize.

Dominant Epistasis: When a dominant allele at one locus can mask the expression of both dominant as well as recessive alleles at another locus. Example- fruit colours in this cucumber.

Duplicate Recessive Epistasis (complementary gene): When recessive alleles at either of the two loci can mask the expression of any of the two dominant alleles at the two loci, it is called duplicate recessive epistasis: example- flower colour in sweet pea.

Duplicate Dominant Epistasis: When a dominant allele at either of two loci can mask the expression of recessive alleles at both loci, it is known as duplicate dominant epistasis. Example- Development of awn in rice.

Polymeric Gene Interaction – When two dominant alleles have similar effects when they are separate but produce enhanced effects when they come together. Example- fruit shape in summer squash.

Deviations of Mendelian Genetics: Polygenic Inheritance

Polygenic inheritance is defined as the kind of inheritance in which a trait is regulated by the cumulative effect of multiple genes: For example- Human skin colour. 

Other Factors 

Linkage

Linkage is defined as the tendency of two genes to travel together from one generation to another. Linkage shows the failure of two genes to assort independently due to their physical association. Morgan coined the term and explained it after his studies on Drosophila melanogaster (fruit fly). The chances of two genes getting linked depend on their proximity to each other.

Fig: Chances for Two Genes to be Linked

Environmental Effects

Genes are mostly responsible for the basic characters of an organism, but they can be greatly affected by environmental factors. These are studied under an entirely new discipline called epigenetics, which studies heritable change due to environmental factors that do not alter genetic makeup. For example, if a dwarf plant is treated with the growth hormone gibberellin, it will cause an increase in height without changing the genetic makeup.

Cytoplasmic Inheritance

Cell organelles like mitochondria and chloroplast contain their genome. This DNA also has an impact on the phenotype of an individual. Mitochondrial genes are associated with maternal inheritance in humans as these genes are transferred only from the mother. Genes related to disorders like dementia, hypertension, lymphoma, and retinopathy are present in mitochondria.

Fig: Mitochondrial Genome

Deviations From Mendelian Laws: Summary

Mendel made a remarkable discovery with his experiments on garden peas (Pisum sativum). His genetic principles are still studied to understand inheritance biology. However, several deviations were found in later research. Incomplete dominance and codominance both are deviations from the law of dominance. Similarly, other deviations include multiple alleles, epistasis, polygenic inheritance. Linkage is a deviation from Mendel’s law of independent assortment. Environmental factors also impact phenotype and produce altered effects from that of genotype.  But this story doesn’t end here. So much new research is coming forward every day.

Frequently Asked Questions (FAQs) From Deviation From Mendelian Genetics

Below we have mentioned the frequently asked questions on deviations from Mendelian Genetics:

Q.1: How does Codominance show deviation from Mendelian inheritance?
Ans: In Mendel’s inheritance, one allele is dominant over the other allele, while in codominance, both alleles are dominant. 

Q.2: What are the three principles of Mendelian genetics?
Ans: 3 principles of Mendelian genetics are:

• Law of dominance
• Law of segregation
• Law of independent assortment

Q.3: Can mutation cause deviation from Mendelian genetics?
Ans: Yes! Mutation can provide the alleles with some novel character that might behave the genes otherwise and not as per the Mendelian inheritance.

Q.4: What is the difference between incomplete dominance and codominance?
Ans: In the case of incomplete dominance, neither of the alleles is completely dominant; however, in the case of codominance, neither of the genes is recessive. 
In the case of incomplete dominance, the phenotype of neither of the parents is expressed in the F₁ generation, whereas in the case of the codominance phenotype of both the parents is expressed in the F₁  generation.

Q.5: Give an example of a disease caused due to Mendelian inheritance.
Ans: Mendelian diseases are those which follow the Mendelian pattern of inheritance. For example- Sickle cell anaemia,  cystic fibrosis, haemophilia, etc.

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