Important Notes of Biology for NEET- Principles of Inheritance and Variation

An Overview of Principles of Inheritance and Variation

Some people look almost the same as their parents and their siblings. The reason behind it is heredity. The principles of inheritance and variation class 12 notes will explain this topic in detail. Notably, this topic comprises several interlinked sub-topics which are also of prime importance. 

Moreover, if you are aiming for NEET-UG, the principles of inheritance and variation class 12 will take on extra emphasis. A considerable number of questions are generally asked from this topic. Often students find this portion tough simply because of lack of clear understanding of the concept. 

So, let us proceed with some critical aspects of this topic!

List of Some Fundamental Terms 

  • Genetics- Genetics is the study of genes and various inherited genetic properties of a species.

  • Inheritance- It is the mechanism through which offspring acquire a few specific genetic characteristics from their parent organisms within the same species. 

  • Variation- Genetic variation is basically the difference between cell organisms and DNA, among a group of organisms. There are several factors like genetic differences, environmental forces, etc. that leads to this variation.

Mendel’s Law of Inheritance

Gregor Johann Mendel is a scientist who is regarded as the father or founder of genetics. 

Mendel observed several garden pea plants mainly based on their opposite traits like tall or dwarf; green or yellow seeds. He cross-pollinated two contrasting pure breeds. 

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Below is the list of 7 pairs of pea plants with contrasting traits that Mendel selected for artificial breeding. 


Contrasting character (Dominant/Recessive)

Stem height


Flower colour


Flower position


Pod colour


Pod shape


Seed colour


Seed shape


The offspring resulting from it is called the first filial generation or F1. This F1 generation self-pollinates and gives birth to the second filial generation or F2. The genes that pass on from parents to offspring are called “factors”, and they exist in pairs which are called “alleles”. 

Based on this experiment, he propounded three laws.

  • Law of Dominance- In this law, he stated that various factors control the characters. Also, in a dissimilar pair, one unit is dominant, and the other one is recessive. Hence the characteristics of F1 will be dominant, and the different characters appearing in F2 generation will be recessive. 

  • Law of Segregation- He stated that, in a hybrid pair, the characteristics do not mix. Instead, they retain their distinctive traits. During the meiosis of chromosome, every gamete acquires one of the two alleles. Also, this law proves that the phenotypic ratio is 3:1 and dominant and recessive homozygous show dominant and recessive traits, respectively. 

  • Law of Independent Assortment- This law stated that during gamete and zygote formation, the genes passed on from parents to offspring independently, with separate characters and traits. 

Also, he conducted a test cross between a pea plant having dominant phenotype and recessive parent. The result is as follows.

  • Parent plant has to be homozygous for dominant traits if the offspring carry only dominant traits.

  • If the offspring show both phenotypes, the parent plants must be heterozygous for dominant traits.  

Incomplete Dominance 

In this dominance, a heterozygous organism combines the two traits. For example, a pink snapdragon receives both red and white allele. 


In simple words, co-dominance expressed by a heterozygous organism shows both traits without blending with each other. For example, a roan cow often has both white and red hairs. 

All these kinds of dominances comprise a significant part of the principles of inheritance and variation. 

Chromosomal Theory of Inheritance

  • Chromosomal theory of inheritance propounded by Boveri and Sutton, states that genes are located at the specific location of chromosomes.

  • Morgan studied fruit flies and confirmed this chromosomal theory. Through his experiment, he observed that the mutation affects the eye colour of fly. Also, this mutation is inherited differently by female and male flies. 

  • Based on this evidence, he concluded that the gene responsible for eye colour is located on the X chromosome. 

Linkage and Recombination

  • Linkage is the physical association of two genes located in the X chromosome.

  • The term recombination is the process by which two DNA get separated during meiosis.

  • The frequency of recombination and the presence of linkage allow chromosomes to map the positions of genes and DNA structure of an organism. Hence it is one of the most crucial parts of principles of inheritance and variation. 


This term refers to alteration or change in genetics of any organism that leads to genetic diversity of a species. It can be classified into two sections.

  • Somatic- This gene mutation happens during the lifetime of an individual. Environmental factors like UV rays and others are a fine example in this regard. As the name suggests, it takes place in general body cells and cannot be inherited from one generation to another. 

  • Hereditary- This mutation passes on from one generation to its future generation. It occurs in parent eggs or sperm cells. When an offspring is born from at least one muted parent cell, the mutation exists in all its body cells. 

Genetic Disorder

The principles of inheritance and variation also include genetic disorders. It can be divided into two segments.

  • Mendelian- This genetic disorder results in diseases like haemophilia, sickle cell anaemia, phenylketonuria etc. 

  • Chromosomal- This includes down syndrome, Turner’s syndrome, Klinefelter's syndrome, etc.   

Fill in the Blank: Trisomy of chromosome 18 results in ____________ syndrome. 

Answer: Edward’s. 

This topic discussed above is a significant part of any vital examination, be it class 12 board examination or NEET-UG. Hence from the beginning of the session, try to keep your concepts clear. In that matter, principles of inheritance and variation notes can prove to be a life saviour.

Once you get a firm grip on these topics, you can expect a drastic surge in your biology score. However, while preparing for topics like the principles of inheritance and variation and others, do not forget about your health. It is also imperative that you maintain your fitness with adequate rest and proper diet.

FAQ (Frequently Asked Questions)

1. What is Inheritance?

Ans. Inheritance is a mechanism through which some specific characteristics are passed on from parents to their offsprings. Understandably, principles of inheritance and variation turn out to be a vital part of every life form.

2. What is an Example of a Variation?

Ans. Some examples of variation are plant’s leaf variation, camouflage in animals, eye colour, etc. Notably, variations can be observed mostly in every life form, especially if they are multi-celled organisms. 

3. How is Variation Maintained in the Population?

Ans. Primarily variation in a population is maintained through genetic crossover or recombination that occurs during meiosis.