Genetics
Introduction
· Genetics is the branch of Biology which studies heredity (passing of traits from parents to children) and variation (differences between individuals).
· Traits like height, skin colour, blood group, ear shape, diseases etc. are controlled by genes and explained by genetics.
· The word genetics was first used by William Bateson in 1906.
· The modern study of genetics started with Gregor Mendel, who is called the Father of genetics.
Table of Contents
Basic terms in genetics
These terms are asked again and again in SSC, Railway, UPSC prelims and other exams.
|
Term |
Simple meaning (exam point) |
|
Trait |
Any observable character like height, colour, blood group. |
|
Heredity |
Passing of traits from parents to children. |
|
Variation |
Differences in traits among individuals of same species. |
|
Gene |
Unit of heredity; a segment of DNA which controls one character. |
|
Allele |
Different forms of the same gene (for example, tall vs dwarf). |
|
Dominant |
Allele which expresses itself in presence of another allele (written as capital letter, e.g. T). |
|
Recessive |
Allele which is hidden in presence of dominant allele (written as small letter, e.g. t) |
|
Genotype |
Genetic constitution (TT, Tt, tt). Not visible directly. |
|
Phenotype |
Physical appearance (tall plant, dwarf plant). |
|
Homozygous |
Both alleles same (TT or tt). |
|
Heterozygous |
Two different alleles (Tt). |
Example (simple):
· For height in pea plant:
· Gene for height has two alleles: T (tall, dominant), t (dwarf, recessive).
· TT and Tt will give tall plants, tt will
give dwarf
plant.
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DNA, gene, chromosome – simple difference
· DNA (Deoxyribonucleic acid) is the chemical that stores genetic information in almost all living organisms.
· Genes are small segments (parts) of DNA which control specific characters like eye colour or blood group.
· Chromosomes are tightly packed structures made of DNA and proteins, present in the nucleus of cells.
Table: DNA vs Gene vs Chromosome (for exams)
|
Feature |
DNA |
Gene |
Chromosome |
|
Definition |
Chemical molecule which stores genetic information. |
Small segment of DNA, unit of heredity for a character. |
Condensed structure made of DNA and proteins. |
|
Visibility |
Not visible under light microscope alone. |
Not visible under microscope. |
Visible under light microscope during cell division. |
|
Function |
Carries all genetic code of organism. |
Controls one trait or protein. |
Helps in organised distribution of DNA during cell division. |
|
Number |
DNA present in all chromosomes of cell. |
Many genes present on a chromosome. |
Fixed number for each species; humans have 46 (23 pairs). |
Example:
· Think of DNA as a big book of instructions.
· A gene is like one sentence or paragraph in that book.
· A chromosome is like one chapter which contains many sentences (genes).
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Mendel – Father of genetics
· Gregor Johann Mendel was an Austrian monk who did experiments on pea plants in the garden of his monastery.
· He studied how traits like height, seed colour, flower colour etc. are passed from one generation to the next.
Important exam points:
· Mendel is called Father of genetics.
· He worked on garden pea plant (Pisum sativum).
· He selected pea because it has:
· Many visible contrasting traits (tall/dwarf, round/wrinkled etc.).
· Short life cycle and easy to grow.
· Can do both self-pollination and cross-pollination.
Mendel’s experiments on pea plant
Mendel did two main types of crosses:
- Monohybrid cross – Cross between parents differing in one trait (e.g. plant height).
2. Dihybrid cross – Cross between parents differing in two traits (e.g. seed colour and seed shape).
Example: Monohybrid cross (height in pea)
· Parent (P generation):
· Pure tall plant (TT) × pure dwarf plant (tt).
· F₁ generation (first filial):
· All plants tall (Tt). Only dominant tall trait appears.
· F₂ generation (second filial):
- Selfing of F₁: Tt × Tt.
- Ratio in F₂:
- Phenotype (tall : dwarf) = 3 : 1.
· Genotype (TT : Tt : tt) = 1 : 2 : 1.
· This
3:1 ratio is a very important exam point.
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Mendel’s laws of inheritance
Mendel gave three main laws based on his experiments.
Law of Dominance
· When two different alleles (T and t) come together, only the dominant allele expresses itself in F₁ generation.
· Recessive allele remains hidden in F₁ but appears again in F₂.
Example:
- TT (tall) × tt (dwarf) → F₁ = all Tt (tall).
- Tall (T) is dominant, dwarf (t) is recessive.
Law of Segregation
· Every individual has two alleles for each gene, one from each parent.
· During gamete formation, the two alleles separate (segregate), so each gamete receives only one allele.
· This explains why recessive trait can reappear in F₂ generation.
Example:
· Tt → gametes will be of two types: T and t, not Tt.
Law of Independent Assortment
· When we study more than one trait, the alleles of different genes assort (separate) independently during gamete formation.
· This law is explained by Mendel’s dihybrid cross (two traits at a time).
Example:
· Seed colour: Yellow (Y, dominant), Green (y, recessive).
· Seed shape: Round (R, dominant), Wrinkled (r, recessive).
· Cross between
YYRR and yyrr gives F₂
phenotypic ratio 9:3:3:1 (yellow round : yellow wrinkled : green round : green
wrinkled).
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Important genetic terms for exams
These small terms often appear in assertion–reason or one-liner type questions.
· Haploid (n): Cell with single set of chromosomes (e.g. gametes – sperm, ovum).
· Diploid (2n): Cell with two sets of chromosomes (normal body cells).
· Autosomes: Non-sex chromosomes (in humans 22 pairs).
· Sex chromosomes: Chromosomes which determine sex (in humans 1 pair: XX or XY).
· Karyotype: Photograph showing complete set of chromosomes of an individual.
·Genome: Total genetic material (all genes) present in an organism.
Example for humans:
· Total chromosomes = 46 (23 pairs).
· 22 pairs of autosomes + 1 pair of sex chromosomes (XX or XY).
Blood groups and genetics (ABO and Rh)
Blood group is a very important topic for competitive exams because it appears in Biology and also in medical and defence related questions.
ABO blood group system
· ABO blood group is controlled by one gene with three alleles: Iᴬ, Iᴮ and i.
· Iᴬ and Iᴮ are codominant (both express together), while i is recessive.
|
Genotype |
Phenotype (Blood group) |
|
IᴬIᴬ or Iᴬi |
Group A |
|
IᴮIᴮ or Iᴮi |
Group B |
|
IᴬIᴮ |
Group AB (both A and B, example of codominance) |
|
Ii |
Group O |
Example:
· Father = group A (Iᴬi), Mother = group B (Iᴮi).
· Child can be A, B, AB or O depending on allele combination.
Rh factor (positive and negative)
· Rh factor is another protein present on red blood cells.
· If present → Rh positive (Rh⁺), if absent → Rh negative (Rh⁻).
· Most Indians are Rh positive.
Important exam point:
A dangerous condition called erythroblastosis fetalis
can occur when Rh negative mother carries Rh positive fetus in later
pregnancies.
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Sex determination (XX–XY system)
· In humans, sex is determined by sex chromosomes.
· Female: 46, XX; Male: 46, XY.
Key idea:
· Mother always gives X chromosome in egg (ovum).
· Father gives either X or Y in sperm.
· If sperm has X → combination XX → female baby.
· If sperm has Y → combination XY → male baby.
Thus sex of child is decided by father’s chromosome, not by mother (frequently asked concept question).
Mutation and genetic disorders
Mutation
· Mutation is a sudden, permanent change in DNA or chromosome.
Mutation can create new variations, but some mutations cause diseases or defects.
· Types (basic for exams):
· Gene mutation: Small change in DNA sequence (point mutation, insertion, deletion).
Chromosomal mutation: Large change (deletion, duplication, rearrangement of chromosome parts).
· Example:
· Change in one base of DNA causing abnormal haemoglobin can lead to sickle cell anemia.
Genetic disorders (examples)
Some common exam-oriented genetic disorders:
|
Disorder |
Type |
Cause (simple) |
|
Down syndrome |
Chromosomal disorder |
Extra copy of chromosome 21 (Trisomy 21). |
|
Turner syndrome |
Chromosomal disorder |
Female with only one X chromosome (XO). |
|
Klinefelter syndrome |
Chromosomal disorder |
Male with XXY sex chromosomes. |
|
Sickle cell anemia |
Gene disorder |
Mutation in gene for haemoglobin. |
|
Hemophilia |
Gene disorder |
X-linked disease, blood does not clot properly. |
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Important exam points and recap table
|
Topic |
Key exam points (short) |
|
Definition of genetics |
Study of heredity and variation. |
|
Father of genetics |
Gregor Johann Mendel; worked on pea plant (Pisum sativum). |
|
Genetic material |
DNA is main genetic material; genes are segments of DNA; chromosomes are packed DNA structures. |
|
Human chromosomes |
46 (23 pairs): 22 pairs autosomes + 1 pair sex chromosomes. |
|
Blood groups |
ABO system: A, B, AB, O; Rh factor: Rh⁺ / Rh⁻. |
|
Laws of Mendel |
Dominance, Segregation, Independent Assortment. |
|
F₂ ratios |
Monohybrid = 3:1; Dihybrid = 9:3:3:1. |
|
Sex determination |
XX (female), XY (male); father decides sex. |
|
Mutation |
Sudden change in gene or chromosome; can cause genetic disorders. |
MCQs
Q1. Who is called the Father of Genetics?
(A) Charles Darwin
(B) Gregor Johann Mendel
(C) Watson and Crick
(D) William Bateson
Ans: B
Gregor Mendel did experiments on pea plants and gave laws of
inheritance. He is called the Father of Genetics.
Q2. The term "Genetics" was first used by which scientist?
(A) Gregor Mendel
(B) Charles Darwin
(C) William Bateson
(D) Thomas Morgan
Ans: C
William Bateson used the word "Genetics" for the
first time in 1906.
Q3. On which plant did Mendel perform his famous experiments?
(A) Maize
(B) Sunflower
(C) Garden Pea (Pisum sativum)
(D) Rice
Ans: C
Mendel chose garden pea because it has many contrasting traits,
short life cycle and is easy to grow.
Q4. Which is the correct definition of "Heredity"?
(A) Differences
between parents and children
(B) Passing of traits from parents to offspring
(C) Sudden change in genetic material
(D) Study of chromosomes
Ans:B
Heredity means transfer of characters (traits) from parents to
their children through genes.
Q5. Which of the following is the correct meaning of "Variation"?
(A) Passing of
traits from parents to offspring
(B) The degree by which offspring differ from their parents
(C) Study of DNA structure
(D) Process of cell division
Ans: B
Variation means differences between parents and their offspring
or among individuals of the same species.
Q6. What is a "Gene"?
(A) A full
chromosome
(B) A type of protein
(C) A unit of heredity; a segment of DNA
(D) A type of RNA
Ans: C
A gene is the basic unit of heredity and is a small segment of DNA that
controls a specific trait.
Q7. Which of the following is the correct phenotypic ratio of a Monohybrid cross in F₂ generation?
(A) 1:2:1
(B) 9:3:3:1
(C) 3:1
(D) 1:1
Ans: C
In a monohybrid cross (Tt × Tt), the F₂ phenotypic ratio is 3 (dominant) : 1
(recessive).
Q8. What is the genotypic ratio of a Monohybrid cross (Tt × Tt) in F₂ generation?
(A) 3:1
(B) 1:2:1
(C) 9:3:3:1
(D) 1:1:1:1
Ans: B
Tt × Tt gives TT : Tt : tt = 1 : 2 : 1 in F₂ generation.
Q9. Mendel's Law of Dominance states that:
(A) Both alleles
express together
(B) Alleles assort independently
(C) Only the dominant allele expresses itself in F₁ when two different alleles are
present
(D) Gametes carry two alleles
Ans: C
In F₁,
only the dominant allele shows its effect; the recessive allele remains hidden.
Q10. Mendel's Law of Segregation is also called:
(A) Law of
Dominance
(B) Law of Purity of Gametes
(C) Law of Independent Assortment
(D) Law of Linkage
Ans: B
The Law of Segregation says each gamete receives only one
allele from each pair, so gametes are "pure."
Q11. The phenotypic ratio of a Dihybrid cross in F₂ generation is:
(A) 3:1
(B) 1:2:1
(C) 1:1:1:1
(D) 9:3:3:1
Ans: D
In dihybrid cross (two traits), the F₂ phenotypic ratio is 9:3:3:1 — yellow
round : yellow wrinkled : green round : green wrinkled.
Q12. Mendel's Law of Independent Assortment applies to genes located on:
(A) Same
chromosome
(B) Non-homologous chromosomes
(C) X chromosome only
(D) Mitochondrial DNA
Ans: B
Genes on different (non-homologous) chromosomes assort
independently during gamete formation.
Q13. When two alleles of a gene are both expressed in a heterozygote, this is called:
(A) Incomplete
dominance
(B) Epistasis
(C) Codominance
(D) Dominance
Ans: C
In codominance, both alleles express fully. Example: Blood
group AB (Iᴬ and Iᴮ
both express).
Q14. In incomplete dominance, the phenotypic ratio in F₂ is:
(A) 3:1
(B) 9:3:3:1
(C) 1:2:1
(D) 1:1
Ans: C
In incomplete dominance, F₂
ratio is 1 red : 2 pink : 1 white (1:2:1) because neither allele is fully
dominant.
Q15. How many pairs of chromosomes are present in a normal human body cell?
(A) 44
(B) 46
(C) 23 (pairs)
(D) 48
Ans: C
Humans have 46 chromosomes arranged in 23 pairs — 22 pairs of
autosomes and 1 pair of sex chromosomes.
Q16. The genetic material in most living organisms is:
(A) RNA
(B) Protein
(C) DNA
(D) Glucose
Ans:C
DNA (Deoxyribonucleic acid) is the main genetic material that stores hereditary
information in almost all organisms.
Q17. A karyotype shows:
(A) All genes in a
cell
(B) Complete set of chromosomes of an individual
(C) Structure of DNA double helix
(D) Process of protein synthesis
Ans: B
Karyotype is a photograph that shows the full set of
chromosomes of a person, arranged in pairs.
Q18. Which of the following is NOT a sex chromosome in humans?
(A) X chromosome
(B) Y chromosome
(C) Chromosome 1
(D) Both X and Y
Ans: C
Chromosome 1 is an autosome (non-sex chromosome). Sex
chromosomes in humans are X and Y only.
Q19. ABO blood group system in humans is an example of:
(A) Incomplete
dominance
(B) Codominance and multiple alleles
(C) Sex-linked inheritance
(D) Chromosomal mutation
Ans: B
ABO system has 3 alleles (Iᴬ, Iᴮ,
i). Iᴬ and Iᴮ are
codominant; i is recessive. This is an example of multiple alleles and
codominance.
Q20. A person with blood group O has genotype:
(A) IᴬIᴮ
(B) IᴬIᴬ
(C) ii
(D) Iᴬi
Ans: C
Blood group O is produced only by genotype ii (both alleles
recessive).
Q21. Blood group AB is called "Universal Recipient" because:
(A) It can donate
blood to all groups
(B) It has no antigens
(C) It has both A and B antigens and no antibodies in plasma
(D) It has the Rh factor
Ans: C
AB group has both A and B antigens but no anti-A or anti-B
antibodies, so it can receive blood from any group.
Q22. The sex of a human child is determined by:
(A) X chromosome
from mother
(B) Y chromosome from father
(C) Both parents equally
(D) Chromosome 22
Ans: B
Mother always gives X; father gives either X (girl) or Y (boy).
So the father's chromosome determines the sex of the child.
Q23. A female human being has sex chromosomes:
(A) XY
(B) XO
(C) XX
(D) YY
Ans: C
Normal human females have XX sex chromosomes. Males have XY.
Q24. Down syndrome is caused by:
(A) Deletion of
chromosome 21
(B) Extra copy of chromosome 21 (Trisomy 21)
(C) Missing X chromosome
(D) Gene mutation in chromosome 7
Ans: B
Down syndrome occurs when there are 3 copies of chromosome 21
instead of 2. Also called Trisomy 21.
Q25. Turner syndrome is characterised by which chromosomal constitution?
(A) XXY
(B) XYY
(C) XO
(D) XXX
Ans: C
Turner syndrome affects females who have only one X chromosome
(45, XO) instead of the normal XX.
Q26.
Klinefelter syndrome is represented by which sex chromosome pattern?
(A) XO
(B) XXY
(C) XYY
(D) XX
Ans: B
Klinefelter syndrome is a chromosomal disorder in males with
two X and one Y chromosome (47, XXY).
Q27. Haemophilia is an example of which type of inheritance?
(A) Autosomal
dominant
(B) Autosomal recessive
(C) X-linked recessive
(D) Y-linked inheritance
Ans: C
Haemophilia (blood does not clot properly) is controlled by a
gene on the X chromosome and is recessive. Males (XY) are more affected.
Q28. Sickle cell anemia is caused by:
(A) Extra
chromosome 21
(B) Missing X chromosome
(C) Point mutation in the gene for haemoglobin
(D) Deletion of chromosome 7
Ans: C
A single gene mutation changes one amino acid in haemoglobin, making
red blood cells sickle-shaped.
Q29. A sudden permanent change in the DNA of an organism is called:
(A) Variation
(B) Heredity
(C) Crossing over
(D) Mutation
Ans: D
Mutation is a sudden, permanent change in the genetic material
(DNA or chromosome) that can create new traits or cause diseases.
Q30. Cystic Fibrosis is an example of which type of genetic disorder?
(A) X-linked
dominant
(B) Autosomal recessive
(C) Chromosomal disorder
(D) Y-linked disorder
Ans: B
Cystic Fibrosis is caused by a recessive gene located on an
autosome (non-sex chromosome), so both parents must carry the allele for a
child to be affected.
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FAQs-
Q1. What is genetics in simple words?
Genetics is the study of how traits like height, colour or diseases pass
from parents to children and why people look similar but not exactly same.
Q2. Who is called the Father of genetics and why?
Gregor Johann Mendel is called the Father of genetics because he first explained
how traits are inherited using pea plant experiments and gave laws of
inheritance.
Q3. What is the difference between gene and chromosome?
A gene is a small part of DNA that controls one character, while a chromosome
is a large, packed structure made of DNA and proteins that contains many genes.
Q4. How many chromosomes are present in humans?
Human body cells have 46 chromosomes (23 pairs), including 22 pairs of
autosomes and 1 pair of sex chromosomes (XX or XY).
Q5. Which parent decides the sex of the child?
The father decides the sex of the child because his sperm can carry either X or
Y chromosome, while the mother’s egg always carries X.
Q6. What is mutation?
Mutation is a sudden, permanent change in DNA or chromosome which can create
new variations and sometimes cause diseases.
Q7. What is codominance? Give one example.
In codominance, both alleles express in the phenotype at the same time.
Example: Blood group AB, where both Iᴬ and Iᴮ are
expressed.
Q8. Which genetic disorders are important for exams?
Common exam-oriented names: Down syndrome, Turner syndrome, Klinefelter
syndrome, sickle cell anemia, hemophilia.
Conclusion
Genetics is a scoring and predictable topic in almost every competitive exam, from UPSC and SSC to Railway, Delhi Police, NEET and AIIMS. If you understand the simple ideas of genes, DNA, chromosomes, Mendel’s laws, blood groups, sex determination, mutations and common genetic disorders, you can easily answer most questions asked in the exam.
You do not need very high-level biology; you only need clear concepts, simple definitions, key ratios (3:1, 9:3:3:1), important names (Mendel, Down syndrome, sickle cell anemia etc.) and some practice of MCQs and PYQs. Revise these notes 2–3 times, make your own short mind map, and solve previous year questions so that genetics becomes a sure-shot chapter for gaining marks in the General Science / Biology section.
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