The UPSC mains GS-3 paper syllabus has various subjects like Economics, Science and Technology. Science and technology in UPSC is mostly concerned with current Affairs. but it is necessary to understand the basics of science. Basics of Biology starts with Cells, This article will give aspirant an idea of the basics of Human body cells.


  • What is genetics
  • Law of inheritance
  • Functions of Genes
  • DNA


The exploration of the working and major codes of variation and heredity is termed as Genetics. The groundwork on which heredity stands is known as inheritance. It is defined as the procedure by which characteristics are handed down from one generation to the other. Gregor Johann Mendel is known as the “Father of Modern Genetics” for his discoveries on the basic principles of heredity.

Variation, as the name suggests is the amount of dissimilarity that exists in between children and their parentages. It can be determined to keep in view the behaviouristic, cytological, physiological, and morphological characters of individuals fitting into similar species.

 Some of the major reasons that variation are

  • Genetic/Chromosomal rearrangement.
  • Mutated genes due to the influence of the ecosystem.
  • Crossing over.

Let us have a detailed look at genetics notes to learn about genes and the principle of inheritance.


Garden Pea (Pisum Sativum) was the plant that Mendel experimented on for 7 years to get to the point to propose the laws of inheritance in live creatures. Mendel carefully chose seven distinct characteristics of Pisum Sativum for the investigation concerning hybridization. Mendel used true-breeding lines i.e. those that go through constant self-pollination and display steady characteristic heritance.

 Principles of Inheritance

  1. When Mendel observed the monohybrid cross he proposed two laws of inheritance-
  2. Law of Dominance – Distinct elements termed as factors control the characteristics. These factors at all times exist as a couple. One of the constituent genes of the couple dominates over the former.
  3. Law of Segregation – Alleles don’t blend and the two characteristics are recuperated all through the gamete formation (in the F2 generation). The characters apart from each other and pass on to diverse gametes. Comparable types of gametes are produced by Homozygous and heterozygous produces diverse sorts of a gamete with varied characteristics.

 Incomplete Dominance

It is the discovery that was done after Mendel’s work. Incomplete dominance is the situation in which both the alleles do not display a dominant trait resulting in a fine combination or a midway amid the characteristics of the alleles.


When two alleles lack the dominant-recessive association and thus the duo affects the creature together.

Law of Independent Assortment

Separation of one set of the characteristic is autonomous of the other set of the characters when they are pooled in a hybrid.

The Chromosomal Theory of Inheritance

Both genes and chromosomes exist in sets of two. The homologous chromosome contains the two alleles of a gene pair in the homologous sites. The coupling and split of a set of chromosomes will cause a split in the set of genes (factor) they carry. This united knowledge is termed as the Chromosomal Theory of Inheritance.

 Sex Determination

A particular nuclear arrangement was perceived by Henking. He perceived that this particular nuclear arrangement was found in only fifty per cent of sperms. He termed this body as x. Later it became clear that only Ovum’s that obtain only the x chromosome is born female and those that don’t have such a case are born male. Thus, the X- chromosome was termed as sex chromosome and the remaining ones were termed as autosomes.

The occurrence due to which a modification in DNA happens and causes a variation in the phenotype and genotype of a creature is termed as a Mutation.

  • Genetic Disorders
  • Disorders of a Mendelian nature include:
  • Sickle Cell Anaemia.
  • Disorders of a chromosomal nature include:
  • Down’s syndrome.
  • Klinefelter’s Syndrome.
  • Turners Syndrome.

Characteristics determined by Genes

The human cell contains 23 pairs of chromosomes. The trait is one of the characteristics determined by one or more genes. Abnormal genes and genes that are formed due to new mutations also result in certain traits. Genes vary in size depending on the code or the protein they produce. All cells in the human body contain the same DNA. The difference between the cells occurs due to the different type of genes that are turned on and therefore produce a variety of proteins.

Reasons for hereditary

Genes come in pairs in the same way as the chromosomes. Each parent of a human being carries two copies of their genes and each parent passes one copy of genes to their child. This is the reason why the child has many characteristics of both the parents like hair colour, same eyes etc.


Genes control the functions of DNA and RNA.

  1. Proteins are the most important materials in the human body which not only help by being the building blocks for muscles, connecting tissue and skin but also takes care of the production of the enzyme.
  2. These enzymes play an important role in conducting various chemical processes and reactions within the body. Therefore, protein synthesis is responsible for all activities carried on by the body and are mainly controlled by the genes.
  3. Genes consist of a particular set of instructions or specific functions. For example, the globin gene was instructed to produce haemoglobin. Haemoglobin is a protein that helps to carry oxygen in the blood.


  1. DNA is the material that carries all the information about how a living thing will look and function. For instance, DNA in humans determines such things as what color the eyes are and how the lungs work. Each piece of information is carried on a different section of the DNA. These sections are called genes.
  2. DNA is short for deoxyribonucleic acid. It is in every cell of every living thing. DNA is found in structures of the cell called chromosomes. Both DNA and chromosomes are tiny. Scientists need to use very powerful microscopes to see them.


  1. When DNA works correctly, it helps keep the body functioning properly. DNA helps cells to make the substances called proteins, which the cells need to live. DNA also allows living things to reproduce. The genes in DNA pass along physical traits from parents to children.
  2. Sometimes there are mistakes in DNA. These mistakes are called mutations. They can cause diseases and other problems.


  1. DNA has a complex structure. It is made of chemical substances that are linked together like a chain. Each piece of DNA has two long strands, or chains. The two strands are joined together. They form a shape like a ladder that has been twisted into a spiral.
  2. Chemicals called phosphates and sugars make up the sides of the ladder. DNA also has chemicals called bases. Each base on one strand is joined to a base on the other strand. The linked bases form the rungs of the ladder.

Genetic Code

  1. There are four different bases in DNA: adenine, thymine, guanine, and cytosine. These four chemicals are repeated in different orders over and over again in each strand of DNA. Human DNA contains about 3 billion pairs of these bases.
  2. The order in which the bases are arranged is very important. It forms a code that tells cells to make certain kinds of proteins. The differences in these proteins is what makes different living things—such as a cactus, a gerbil, and two different people—different.



Every part of your body is made of tiny building blocks called cells, which fit together like bricks in a wall . These are called tissues



Cells provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions.


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