Genetics & Heredity

Why This Chapter Matters

Genetics questions show up in almost every SSC CGL, CHSL, MTS and RRB NTPC paper, usually one to two marks, and they are some of the easiest marks in the entire Science section once you know the core facts cold. Ask a hundred toppers which biology topic they never get wrong, and most will say genetics, because the questions repeat: who discovered the laws of inheritance, how many chromosomes does a human cell carry, which disease is sex-linked. You are not being tested on understanding evolution here. You are being tested on a short, fixed list of facts, and this chapter gives you exactly that list.

The single biggest mistake aspirants make with this topic is mixing up dominant and recessive. Students memorise that tallness is dominant in Mendel's pea plants, then panic when a question flips the scenario and asks about a recessive trait showing up in a child of two normal parents. Recessive does not mean rare or weak, it means hidden unless both copies of the gene carry it. Get that one idea straight early, and half the confusion in this chapter disappears.

1. Heredity, Variation and Why Mendel Chose Peas

Heredity is the passing of traits from parents to offspring. Variation is the difference in traits among individuals of the same species, and it is variation that gives evolution something to work with. The scientist who cracked the mathematical rules behind heredity was Gregor Mendel, an Austrian monk who ran breeding experiments on pea plants (Pisum sativum) in a monastery garden between 1856 and 1863. He is called the Father of Genetics, and the word "genetics" itself was coined later, in 1905, by William Bateson.

Mendel picked pea plants for reasons that still make sense to any exam-setter testing experimental design: peas have clearly contrasting traits with no in-between shades (a flower is either purple or white, a seed is either round or wrinkled), they self-pollinate naturally so Mendel could control crosses precisely, they grow fast, and a single plant produces a large number of offspring, giving him big enough numbers to spot ratios.

He studied seven contrasting pairs of traits: seed shape (round/wrinkled), seed colour (yellow/green), flower colour (violet/white), pod shape (inflated/constricted), pod colour (green/yellow), flower position (axial/terminal) and stem height (tall/dwarf). Think of Mendel's pea garden as a school attendance register where every child is marked either present or absent, no half-days allowed. That binary, no-middle-ground nature of pea traits is exactly what let Mendel see clean 3:1 ratios instead of a blur.

2. Mendel's Three Laws of Inheritance

Exam trap: Questions often ask "how many laws did Mendel give," and the expected answer is three, even though the Law of Dominance is sometimes clubbed with the Law of Segregation in older textbooks. Go with three for SSC/RRB purposes: Dominance, Segregation, and Independent Assortment.

Law of Dominance

When Mendel crossed a pure tall pea plant with a pure dwarf one, every single offspring in the first generation (called the F1 generation) was tall. The dwarf trait did not blend in or vanish, it simply stayed hidden. Mendel called the trait that showed up the dominant trait (tall) and the one that stayed hidden the recessive trait (dwarf). This is the Law of Dominance: in a hybrid, only one of the two contrasting alleles expresses itself, masking the other.

Law of Segregation

Mendel then let the F1 tall plants self-pollinate. In the next generation, the F2 generation, dwarf plants reappeared, and the ratio of tall to dwarf came out close to 3:1. The hidden dwarf trait had not disappeared, it had just been carried silently by the F1 plants. This told Mendel that each parent carries two "factors" (what we now call alleles) for every trait, and during gamete formation these two factors separate, with only one going into each gamete. This is the Law of Segregation, and it is also called the Law of Purity of Gametes because each gamete carries a pure, single copy of the trait, never a mixed one.

Picture two ticket-counter queues at a railway station that never merge, one queue for the trait from the father, one for the mother. Each parent hands over only one ticket (one allele) per trait into the gamete. The queues stay separate right up to the point of fertilisation, when the child's cell finally holds two tickets again, one from each parent.

Law of Independent Assortment

When Mendel tracked two traits at once, say seed shape and seed colour, he found that the inheritance of one trait did not affect the inheritance of the other. Round/wrinkled and yellow/green sorted into new combinations independently, giving a 9:3:3:1 ratio in the F2 generation of a dihybrid cross. This is the Law of Independent Assortment: genes for different traits are inherited independently of each other, provided the genes sit on different chromosomes (a detail Mendel got lucky on, since the seven pea traits he chose happen to sit on different chromosome pairs).

Memory hook: Remember D-S-I in the order Mendel discovered them, "Dominant traits Separate, then sort Independently." Say it as one phrase, dominant-separate-independent, and the sequence of the three laws stays fixed in your head.

3. Genotype, Phenotype and the Monohybrid Cross

A genotype is the actual genetic makeup an organism carries, written as a pair of letters like TT, Tt or tt. A phenotype is what you actually see, the physical expression, like "tall" or "dwarf." An organism with two identical alleles (TT or tt) is called homozygous, and one with two different alleles (Tt) is called heterozygous.

In a monohybrid cross between a pure tall (TT) and pure dwarf (tt) plant, the F1 generation is all Tt, and phenotypically all tall, because T is dominant over t. Self-pollinating Tt with Tt gives F2 in the genotypic ratio 1 TT : 2 Tt : 1 tt, and phenotypic ratio 3 tall : 1 dwarf. This 3:1 phenotypic ratio is one of the most frequently tested numbers in this entire chapter, so lock it in.

Exam trap: Do not confuse the 3:1 ratio (monohybrid, one trait) with the 9:3:3:1 ratio (dihybrid, two traits). A question naming "seed shape and seed colour together" is asking about a dihybrid cross, and the answer ratio is 9:3:3:1, not 3:1.

4. DNA and RNA — Structure Basics

Genes are physically made of DNA (deoxyribonucleic acid), the molecule that stores hereditary information in nearly every living cell. DNA's double helix structure was worked out in 1953 by James Watson and Francis Crick, building on X-ray diffraction data from Rosalind Franklin. DNA looks like a twisted ladder, the "double helix", with two strands winding around each other.

Each strand is a chain of units called nucleotides, and each nucleotide has three parts: a sugar (deoxyribose in DNA), a phosphate group, and a nitrogenous base. DNA carries four bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These bases pair up in a fixed, lock-and-key way across the two strands: A always pairs with T, and G always pairs with C. This is called complementary base pairing, and it is the rule that lets DNA copy itself accurately every time a cell divides.

RNA (ribonucleic acid) is DNA's working cousin. It differs from DNA in three exam-friendly ways: RNA uses the sugar ribose instead of deoxyribose, RNA is usually single-stranded instead of double-stranded, and RNA replaces the base Thymine with Uracil (U). RNA's main job is to carry the genetic instructions from DNA (which stays safely inside the nucleus) out to the ribosomes, where proteins actually get built.

Think of DNA as the original master file locked in the head office (the nucleus) and RNA as the photocopy that gets couriered out to the factory floor (the ribosome) to actually do the work. The master file never leaves the office; only working copies do.

Feature	DNA	RNA
Full form	Deoxyribonucleic acid	Ribonucleic acid
Sugar	Deoxyribose	Ribose
Strands	Double-stranded (helix)	Usually single-stranded
Bases	A, T, G, C	A, U, G, C
Location	Mainly nucleus	Nucleus and cytoplasm
Main role	Stores genetic information	Carries information, builds proteins
Discoverers of structure	Watson and Crick (1953)	—

5. Chromosomes and Human Chromosome Numbers

A chromosome is a thread-like structure made of tightly coiled DNA wrapped around proteins called histones, found inside the nucleus of a cell. Genes are arranged along chromosomes at fixed positions called loci. A human body cell (somatic cell) carries 46 chromosomes, arranged as 23 pairs. One chromosome of each pair comes from the mother, one from the father.

Of these 23 pairs, 22 pairs are autosomes (chromosomes not involved in directly deciding sex), and one pair is the sex chromosomes, which does decide sex. A human female carries two X chromosomes, written XX, while a human male carries one X and one Y, written XY. Since the mother can only contribute an X, it is the father's sperm, carrying either an X or a Y, that determines the sex of the child. This single fact corrects a very common, and very wrong, folk belief that the mother "decides" the baby's sex.

Exam trap: Do not confuse the chromosome number of a body cell (46, diploid, written 2n) with that of a gamete (sperm or egg), which carries only 23 chromosomes (haploid, written n), exactly half. Fertilisation restores the full 46 when a 23-chromosome sperm meets a 23-chromosome egg.

A few other chromosome numbers examiners like to slip in: chimpanzee 48, dog 78, fruit fly (Drosophila, the classic genetics lab organism) just 8, and rice 24. You do not need to memorise the full list, but knowing that "more advanced" does not mean "more chromosomes" is itself useful; humans do not have the highest chromosome count in the animal kingdom, a fact examiners enjoy testing precisely because it surprises people.

6. Alleles, Dominant and Recessive Traits in Humans

An allele is one version of a gene. For any given gene, an individual carries two alleles, one on each chromosome of a pair. When the two alleles differ, the dominant one is expressed in the phenotype while the recessive one stays hidden, exactly as with Mendel's peas. A recessive trait only shows up in the phenotype when an individual is homozygous recessive, meaning both copies carry the recessive version.

This is why two visibly normal parents can have a child with a recessive genetic disorder: both parents can be carriers, heterozygous, phenotypically normal, but each silently carrying one recessive allele. When two carriers have children, there is a real statistical chance (commonly one in four for a simple recessive condition) that a child inherits the recessive allele from both parents and shows the disorder.

Some human traits commonly cited in Indian textbooks as dominant versus recessive: free earlobes are dominant over attached earlobes, the ability to roll the tongue is dominant over the inability to, and dark eye colour is dominant over light eye colour. Treat these as illustrative, exam-style simplifications; real human trait inheritance is often more complex than a single gene, but for SSC/RRB purposes, this dominant/recessive framing is what gets tested.

7. Sex-Linked Genetic Disorders

Sex-linked disorders are caused by genes carried on the sex chromosomes, almost always the X chromosome, since the Y chromosome carries very few genes. Because males have only one X chromosome (XY), a single recessive allele on their one X is enough to cause the disorder, since there is no second X to mask it. Females have two X chromosomes (XX), so a female needs the recessive allele on both X chromosomes to show the disorder; a female with just one copy is a carrier, usually unaffected. This is exactly why sex-linked recessive disorders show up far more often in men than in women, and it is one of the most reliably asked concepts in this chapter.

Colour blindness is the classic example. Most commonly it is red-green colour blindness, where a person struggles to tell red and green apart, caused by a recessive allele on the X chromosome. It is far more common in men than women for the reason above.

Haemophilia is a disorder where blood fails to clot properly, so even a minor cut can bleed for a dangerously long time, because a needed clotting-factor protein is missing or faulty. It is also X-linked recessive, so it follows the same male-skewed pattern as colour blindness. Haemophilia carries a well-known place in world history: it ran through European royal families in the nineteenth and twentieth centuries, with Queen Victoria identified as a carrier who passed the allele to several royal houses across Europe, which is why some textbooks nickname it the "royal disease."

Think of the X chromosome as carrying a spare key. A woman has two keys (two X chromosomes), so if one key is faulty, the spare still works and the lock (her health) is fine, though she still carries the faulty key and can pass it on. A man has only one key, no spare, so if his single key is faulty, the lock stays broken.

Memory hook: For the two X-linked recessive disorders in this chapter, remember "Colour blindness and Haemophilia, Carried Hidden, mostly hit males." The C-H pairing keeps both names together, and "carried hidden" reminds you both are recessive and more common in men.

8. Down Syndrome and Chromosomal Disorders

Not every genetic disorder comes from a faulty allele; some come from an error in chromosome number itself. Down syndrome is the most exam-relevant example: instead of the usual two copies, a person with Down syndrome has three copies of chromosome 21, a condition called trisomy 21. It happens due to a fertilisation-stage error where chromosome 21 fails to separate properly (called non-disjunction). Down syndrome is linked to intellectual disability of varying degree and a set of characteristic physical features, and the chance of it rises with increasing maternal age, a fact sometimes tested directly.

Turner syndrome (a female with only one X chromosome, written XO, instead of XX) and Klinefelter syndrome (a male with an extra X, written XXY, instead of XY) are two other sex-chromosome disorders occasionally seen in exam question banks, worth a passing recognition even if you do not go deep into their symptoms.

9. From Mendel to Modern Genetics — Key Names

A short table of scientists tied to genetics is one of the highest-yield things you can carry into the exam hall, since "who discovered/proposed X" questions are common and quick.

Scientist	Contribution
Gregor Mendel	Laws of Inheritance (Dominance, Segregation, Independent Assortment), 1856-1863
William Bateson	Coined the term "genetics" (1905)
Walter Sutton & Theodor Boveri	Chromosomal theory of inheritance, linking Mendel's factors to chromosomes
James Watson & Francis Crick	Double helix structure of DNA (1953)
Rosalind Franklin	X-ray diffraction images (Photo 51) crucial to discovering DNA's structure
Hugo de Vries	Theory of mutation
Thomas Hunt Morgan	Gene-chromosome linkage using fruit flies (Drosophila)

Exam trap: Watson and Crick are jointly credited with the double helix model, but questions sometimes test whether you know Rosalind Franklin's data was essential to that discovery even though she is not always listed alongside them in the Nobel Prize citation (the Nobel was awarded in 1962, after her death in 1958, and Nobel Prizes are not given posthumously).

10. Why This Matters Beyond the Exam Hall

Genetic counselling clinics in India today routinely test couples for carrier status of conditions like thalassemia and haemophilia before marriage or childbirth, precisely because of the recessive-carrier logic you just learned in this chapter. Blood group inheritance, discussed in the circulatory system chapter, also follows the same dominant-recessive logic you have built here. Once you understand that hidden recessive alleles can sit quietly in a family for generations before two carriers happen to marry, a lot of everyday health news, from genetic screening camps to royal-family history trivia, suddenly makes a lot more sense.

Quick Revision — One-Line Facts

• Gregor Mendel is called the Father of Genetics, based on his pea plant experiments (1856-1863).
• The word "genetics" was coined by William Bateson in 1905.
• Mendel gave three laws: Dominance, Segregation, and Independent Assortment.
• The Law of Dominance says one allele masks the other in a heterozygote.
• The Law of Segregation says the two alleles for a trait separate during gamete formation.
• The Law of Independent Assortment says genes for different traits sort independently.
• A monohybrid cross (Tt x Tt) gives an F2 phenotypic ratio of 3:1.
• A dihybrid cross gives an F2 phenotypic ratio of 9:3:3:1.
• Genotype is the genetic makeup; phenotype is the observable trait.
• Homozygous means two identical alleles; heterozygous means two different alleles.
• DNA stands for deoxyribonucleic acid; RNA stands for ribonucleic acid.
• DNA's double helix structure was discovered by Watson and Crick in 1953, using Rosalind Franklin's X-ray data.
• DNA bases: Adenine, Thymine, Guanine, Cytosine; A pairs with T, G pairs with C.
• RNA uses Uracil instead of Thymine and the sugar ribose instead of deoxyribose.
• RNA is usually single-stranded; DNA is double-stranded.
• A human body (somatic) cell has 46 chromosomes in 23 pairs.
• Of the 23 pairs, 22 are autosomes and 1 pair is sex chromosomes.
• Human females are XX; human males are XY.
• The father's sperm determines the sex of the child, not the mother.
• A human gamete (sperm or egg) carries 23 chromosomes, half the body-cell number.
• An allele is one version of a gene; a recessive trait shows only when homozygous recessive.
• Carriers are heterozygous individuals who show no symptoms but can pass a recessive allele on.
• Colour blindness and haemophilia are classic X-linked recessive disorders.
• X-linked recessive disorders are more common in males because males have only one X chromosome.
• Queen Victoria was a well-documented carrier of the haemophilia allele in European royal history.
• Down syndrome is caused by trisomy 21, three copies of chromosome 21 instead of two.
• Down syndrome risk rises with increasing maternal age.
• Turner syndrome is XO (female, one X); Klinefelter syndrome is XXY (male, extra X).
• Thomas Hunt Morgan linked genes to chromosomes using fruit fly (Drosophila) experiments.
• Fruit flies have only 8 chromosomes, far fewer than humans, showing chromosome count does not track complexity.
• Chromosomes are made of DNA coiled around proteins called histones.

Memory Tables

Table 1 — Mendel's Three Laws at a Glance

Law	What it states	Key ratio/evidence
Law of Dominance	One allele masks the other in a heterozygote	F1 all show dominant trait
Law of Segregation	The two alleles of a trait separate into different gametes	F2 monohybrid ratio 3:1
Law of Independent Assortment	Genes for different traits are inherited independently	F2 dihybrid ratio 9:3:3:1

Table 2 — DNA vs RNA vs Chromosome (Quick Comparison)

Feature	DNA	RNA	Chromosome
What it is	Genetic material (double strand)	Working copy (usually single strand)	Coiled, organised DNA + protein package
Sugar/base note	Deoxyribose; A,T,G,C	Ribose; A,U,G,C	— (structural unit, not a molecule type)
Human count	One genome per cell	Varies (many copies made as needed)	46 per body cell, 23 per gamete
Location	Nucleus	Nucleus and cytoplasm	Nucleus

Table 3 — Sex-Linked Disorders and Chromosomal Disorders

Disorder	Cause	Pattern	Key fact
Colour blindness	Recessive allele on X chromosome	X-linked recessive	Far more common in males
Haemophilia	Recessive allele on X chromosome, faulty clotting factor	X-linked recessive	Famously carried in European royal families via Queen Victoria
Down syndrome	Extra copy of chromosome 21	Trisomy (chromosome number error)	Risk increases with maternal age
Turner syndrome	Only one X chromosome	XO (female)	Chromosomal disorder, not inherited from a single faulty allele
Klinefelter syndrome	Extra X chromosome in a male	XXY (male)	Chromosomal disorder, not inherited from a single faulty allele

Practice MCQs

Q1. Who is known as the Father of Genetics? (a) Charles Darwin (b) Gregor Mendel (c) James Watson (d) Louis Pasteur

Q2. Mendel conducted his famous breeding experiments on which plant? (a) Wheat (b) Maize (c) Pea plant (d) Rice

Q3. How many chromosomes are present in a normal human body cell? (a) 23 (b) 44 (c) 46 (d) 48

Q4. Which sex chromosome combination is found in a human male? (a) XX (b) XY (c) YY (d) XO

Q5. What is the F2 phenotypic ratio obtained in a typical Mendelian monohybrid cross? (a) 1:1 (b) 9:3:3:1 (c) 3:1 (d) 2:1

Q6. DNA is made up of a sugar-phosphate backbone and which of the following? (a) Amino acids (b) Nitrogenous bases (c) Fatty acids (d) Glucose units

Q7. Which scientists are credited with discovering the double helix structure of DNA? (a) Mendel and Bateson (b) Watson and Crick (c) Morgan and Sutton (d) Darwin and Wallace

Q8. In DNA, Adenine always pairs with which base? (a) Guanine (b) Cytosine (c) Thymine (d) Uracil

Q9. Which of the following is a sex-linked genetic disorder more common in males than females? (a) Diabetes (b) Colour blindness (c) Goitre (d) Rickets

Q10. An organism with two different alleles for a trait (for example Tt) is described as: (a) Homozygous (b) Heterozygous (c) Recessive (d) Mutant

Q11. Down syndrome in humans is caused by an extra copy of which chromosome? (a) Chromosome 18 (b) Chromosome 21 (c) Chromosome X (d) Chromosome 13

Q12. Which base is found in RNA in place of Thymine? (a) Guanine (b) Cytosine (c) Uracil (d) Adenine

Q13. A woman who carries one faulty recessive allele for haemophilia on one X chromosome but shows no symptoms is called a: (a) Mutant (b) Homozygote (c) Carrier (d) Dominant expresser

Q14. Which of the following correctly states Mendel's Law of Independent Assortment? (a) Alleles of the same gene always stay together in gametes (b) Genes for different traits are inherited independently of each other (c) Only dominant alleles pass to the next generation (d) All traits are always inherited together as a single unit

Q15. A human sperm or egg cell (gamete) normally carries how many chromosomes? (a) 46 (b) 44 (c) 23 (d) 21

Answer Key

Q	Answer	Reason
1	(b) Gregor Mendel	His 1856-1863 pea plant experiments gave the first mathematical laws of inheritance.
2	(c) Pea plant	Peas gave Mendel clear-cut, contrasting traits with no in-between forms, ideal for tracking ratios.
3	(c) 46	Human body cells are diploid (2n), carrying 23 pairs; only gametes carry the halved number, 23.
4	(b) XY	The father's sperm carries either X or Y, so the father's contribution decides the child's sex.
5	(c) 3:1	A monohybrid cross (Tt x Tt) gives genotypic ratio 1:2:1 but phenotypic ratio 3 dominant : 1 recessive.
6	(b) Nitrogenous bases	DNA nucleotides consist of a sugar, a phosphate group, and one of four nitrogenous bases (A, T, G, C).
7	(b) Watson and Crick	They proposed the double helix model in 1953, using Rosalind Franklin's X-ray diffraction data.
8	(c) Thymine	DNA base pairing is fixed: A pairs with T, and G pairs with C, never any other combination.
9	(b) Colour blindness	It is X-linked recessive; males have only one X, so a single faulty allele shows up unmasked.
10	(b) Heterozygous	Two different alleles for the same gene (Tt) makes an organism heterozygous, as opposed to homozygous (TT or tt).
11	(b) Chromosome 21	Down syndrome is trisomy 21, three copies of chromosome 21 instead of the normal two.
12	(c) Uracil	RNA replaces DNA's Thymine with Uracil; this is one of the three structural differences between DNA and RNA.
13	(c) Carrier	A heterozygous female carries one faulty allele silently since her other X chromosome compensates.
14	(b) Genes for different traits are inherited independently of each other	This is the precise statement of Mendel's third law, confirmed by the 9:3:3:1 dihybrid ratio.
15	(c) 23	Gametes are haploid (n), carrying exactly half the body-cell chromosome number so fertilisation restores 46.