Why does a tall plant give tall offspring, and why do children resemble their parents? The answer is genetics — the science of heredity. The NDA General Ability Test asks 2–3 sure-shot questions on DNA, genes and Mendel's laws every cycle. In this Cavalier guide you will learn how traits pass from parent to child, explained in plain, exam-ready language.
Why this topic matters for NDA
In the NDA General Ability Test (GAT), General Science carries a fixed block of marks, and genetics is one of Biology's most repeated themes. Questions like "Who is the father of genetics?" or "What is the full form of DNA?" appear almost every cycle, often word for word from earlier papers.
These are pure recall marks — no calculation, no diagrams to draw. Once you fix a few names, definitions and Mendel's laws in your head, you bank easy marks that careless candidates often lose by guessing. Because the same handful of facts repeats across years, genetics gives you one of the best returns on study time in the whole GAT syllabus.
Another reason this topic deserves attention is that it connects to everyday life — blood groups, why siblings look different, and why some diseases run in families. The examiner can phrase a question from any of these angles, but the underlying fact is always one you can prepare in advance.
NDA genetics questions test names, definitions and full forms, not deep theory. Memorise scientists, key terms and the three Mendelian laws — that covers about 90% of what is asked.
Heredity and variation
Heredity is the passing of characters (traits) from parents to their offspring. Because of heredity, a dog gives birth to a puppy and a mango seed grows into a mango tree — not anything else. The traits that are passed on include things you can see, like height and eye colour, and things you cannot, like blood group.
The branch of biology that studies heredity and how traits are inherited is called genetics. The term was coined by William Bateson. The actual carriers of these inherited characters are the genes, which a child receives half from the mother and half from the father.
Along with similarities, offspring also show small differences from their parents and from each other — these differences are called variations. Variations arise mainly during sexual reproduction, when genes from two parents mix in new combinations. They are vital because they help a species survive in changing surroundings and provide the raw material for evolution through natural selection.
- Heredity — transmission of traits from parent to offspring.
- Variation — differences among individuals of the same species.
- Gregor Johann Mendel — the "Father of Genetics".
Mendel and his pea plant experiments
Gregor Johann Mendel, an Austrian monk, is called the Father of Genetics. Between 1856 and 1863 he carried out breeding experiments on the garden pea plant (Pisum sativum) in his monastery garden.
He chose the pea plant because it has clear contrasting traits (tall/short, round/wrinkled seeds), it self-pollinates, it grows fast, and it produces many offspring — perfect for counting results.
Mendel studied seven pairs of contrasting characters, such as plant height (tall vs dwarf), seed shape (round vs wrinkled), seed colour (yellow vs green) and flower colour (violet vs white).
- Mendel's work was published in 1866 but ignored for decades.
- It was rediscovered in 1900 by de Vries, Correns and Tschermak.
- He is honoured as the founder of modern genetics.
Mendel's three laws of inheritance
From his careful counting, Mendel gave three famous laws. These are the heart of the chapter for NDA.
1. Law of Dominance
When two contrasting traits are crossed, only one appears in the first generation (F1). The trait that shows up is dominant; the hidden one is recessive. Example: tall (T) is dominant over dwarf (t), so a Tt plant looks tall.
2. Law of Segregation
During the formation of gametes (sex cells), the two factors (alleles) of a trait separate so that each gamete carries only one factor. This is the only law with no exceptions, so it is also called the Law of Purity of Gametes.
3. Law of Independent Assortment
When two pairs of traits are studied together, each pair is inherited independently of the other. Seed shape is passed on without being affected by seed colour, and so on. This is why a child can have the mother's eye colour but the father's hair type — the two traits travel separately into the gametes.
A simple way to remember the order: Dominance decides what shows, Segregation decides how factors separate, and Independent assortment decides how different traits mix. Many NDA questions simply ask which scientist gave these laws — the answer is always Mendel.
For a monohybrid cross (one trait), the F2 ratio is 3:1. For a dihybrid cross (two traits), the F2 ratio is 9:3:3:1. These two numbers are very frequently asked.
Genes, alleles and chromosomes
A gene is the basic unit of heredity — a small segment of DNA that carries the instructions for one trait, such as eye colour. The term "gene" was coined by Wilhelm Johannsen.
Different forms of the same gene are called alleles. For height, T (tall) and t (dwarf) are two alleles of the same gene.
Genes are located on thread-like structures inside the nucleus called chromosomes, which are made of DNA and proteins. The term "chromosome" was given by W. Waldeyer.
- Humans have 23 pairs = 46 chromosomes in each body cell.
- 22 pairs are autosomes; 1 pair are sex chromosomes.
- Females are XX; males are XY.
- The father's sperm decides the sex of the child (X or Y).
Students often say the mother decides the baby's sex. Wrong! The mother always gives an X. The father contributes either X (girl) or Y (boy), so he determines the sex of the child.
What is DNA and what does it look like?
DNA stands for Deoxyribonucleic Acid. It is the chemical that stores all the genetic information of a living organism — the master "instruction manual" of life.
In 1953, James Watson and Francis Crick described the shape of DNA as a double helix — like a twisted ladder. They received the Nobel Prize in 1962 for this discovery.
Each strand of DNA is made of repeating units called nucleotides. A nucleotide has three parts: a sugar (deoxyribose), a phosphate group, and a nitrogen base.
- Adenine (A) always pairs with Thymine (T).
- Guanine (G) always pairs with Cytosine (C).
- The two strands are held together by these base pairs (A–T, G–C).
This fixed pairing rule (called complementary base pairing) is why DNA can copy itself accurately when cells divide. Each old strand acts as a template to build a new partner strand, so two identical DNA molecules are produced — one for each daughter cell. This copying is called replication, and it is the reason your genetic instructions stay the same in every cell of your body.
In eukaryotic cells, most DNA is stored safely inside the nucleus, tightly coiled around proteins to form chromosomes. A small amount of DNA is also found in the mitochondria, which is interesting because it is inherited only from the mother.
DNA vs RNA
Cells contain a second nucleic acid called RNA (Ribonucleic Acid), which mainly helps build proteins from the DNA's instructions. Telling the two apart is a common NDA question.
- DNA has the sugar deoxyribose; RNA has ribose.
- DNA is usually double-stranded; RNA is usually single-stranded.
- DNA uses the base Thymine (T); RNA replaces it with Uracil (U).
- DNA stores information; RNA mostly carries it out (protein synthesis).
So in RNA the pairing becomes A–U and G–C. A handy memory hook: DNA is the Double-stranded Director; RNA is the single-stranded worker.
How genes make proteins
A gene does not directly build your body parts — it works by making proteins. Proteins act as enzymes, hormones and building blocks, and they ultimately decide your traits. This flow of information is called the central dogma of molecular biology.
- DNA → RNA → Protein.
- Making RNA from DNA is called transcription.
- Making protein from RNA is called translation.
The order of the four bases (A, T, G, C) along a gene is the genetic code. A group of three bases, called a codon, codes for one amino acid — and chains of amino acids fold into proteins. So a tiny change in the base sequence can change the protein, and therefore the trait.
This is why DNA is called the "blueprint of life": it does not act by itself but issues precise instructions that the cell follows step by step to stay alive and grow.
Mutations and genetic disorders
A mutation is a sudden, permanent change in the DNA or genes of an organism. Mutations can be caused by radiation, chemicals or errors during copying. They create new variations and are raw material for evolution — but harmful mutations can cause disease.
Some disorders are hereditary (passed from parents through genes). NDA likes to test a few examples:
- Haemophilia — blood does not clot properly; sex-linked.
- Colour blindness — inability to tell certain colours apart; sex-linked.
- Sickle-cell anaemia — abnormal, sickle-shaped red blood cells.
- Down's syndrome — caused by an extra copy of chromosome 21.
Haemophilia and colour blindness are sex-linked disorders carried on the X chromosome, so they appear far more often in males. Do not confuse them with disorders caused by extra chromosomes like Down's syndrome.
Worked example
A pure tall pea plant (TT) is crossed with a pure dwarf pea plant (tt). Find the appearance of the F1 generation and the ratio in the F2 generation.
Notice that the dwarf trait disappears in F1 but reappears in F2. This proves Mendel's Law of Dominance and the Law of Segregation in one neat cross.
Previous-year style question
Q. In DNA, the base Adenine always pairs with which of the following bases?
Answer: Thymine. In DNA the pairing rule is A–T and G–C. (In RNA, Adenine instead pairs with Uracil, because RNA has no Thymine.)
Lock in these one-liners: DNA = Deoxyribonucleic Acid, double helix by Watson and Crick (1953), pairing A–T, G–C, Father of Genetics = Mendel. These exact facts are repeated year after year.
Quick revision
- Heredity = passing of traits; Mendel = Father of Genetics; worked on pea plants.
- Mendel's laws: Dominance, Segregation, Independent Assortment. Monohybrid F2 = 3:1; dihybrid = 9:3:3:1.
- Gene = unit of heredity on a chromosome; humans have 46 (23 pairs); XX female, XY male; father decides sex.
- DNA = double helix (Watson & Crick, 1953); bases pair A–T, G–C; RNA uses U instead of T.
- Disorders: haemophilia & colour blindness (sex-linked), sickle-cell anaemia, Down's syndrome (extra chromosome 21).
Revise this box the night before your exam and these genetics marks are safely in your pocket. Best wishes from The Cavalier!
Frequently asked questions
Who is known as the Father of Genetics?
Gregor Johann Mendel, an Austrian monk, is called the Father of Genetics for his pioneering breeding experiments on garden pea plants between 1856 and 1863.
What is the full form of DNA and who discovered its structure?
DNA stands for Deoxyribonucleic Acid. Its double-helix structure was described by James Watson and Francis Crick in 1953, for which they won the Nobel Prize in 1962.
What are the base-pairing rules in DNA?
In DNA, Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). In RNA, Thymine is replaced by Uracil, so Adenine pairs with Uracil.
How many chromosomes do humans have and who decides the sex of a child?
Humans have 46 chromosomes (23 pairs) in each body cell. The father determines the sex of the child, since he contributes either an X chromosome (girl) or a Y chromosome (boy).
What is the difference between a gene and an allele?
A gene is a segment of DNA that carries instructions for a particular trait, while alleles are the different forms of the same gene, such as T (tall) and t (dwarf) for plant height.
What is a mutation?
A mutation is a sudden, permanent change in the DNA or genes of an organism. It can be caused by radiation or chemicals and is a source of variation that drives evolution.
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