CDS Biology · Genetics and Evolution

Heredity and DNA

What chromosomes and genes are made of, how Mendel's crosses give their ratios, how the sex of a child is decided, and the enzymes and products of genetic engineering.

All Genetics and Evolution notes

Why this matters

Nine CDS questions from 2017 (II) to 2026 (II), and both HARD questions of the chapter are here: how many insulin genes a person has, and what Photo 51 shows. Five are about chromosomes and DNA, one is a Mendel ratio, one is about a girl's chromosomes, and two are about genetic engineering.

Concept 1 of 4: Chromosomes, DNA and genes

Every cell keeps its instructions in long threads of DNA inside the nucleus. To fit, the DNA is wound around little spools of protein called histones, the way thread is wound on a reel, and the wound-up thread is a chromosome. A gene is one stretch of that DNA that holds the recipe for one protein. A change in the DNA is a mutation.

Definition

Chromosomes and DNA (NCERT Class 10 and 12):

A chromosome is made of DNA and proteins. The main proteins are the histones: basic proteins, found in the nucleus, around which DNA is wound to form nucleosomes.
DNA is a double helix (Watson and Crick, 1953). Its bases pair as adenine with thymine and guanine with cytosine.
The double helix was worked out with the help of Photo 51, an X-ray picture of DNA taken by Rosalind Franklin (with Raymond Gosling).
A gene is a stretch of DNA that codes for a protein. Humans have one functional insulin gene; the hormone's two chains are cut from one long chain made from it.
A mutation is a change in the genetic material (the DNA, or the RNA of an RNA virus).
A human body cell has 46 chromosomes (23 pairs).

Term	Fact
Histones	Basic proteins in the nucleus, bound to DNAQ CDS 2019 (I): histones are proteins present in the nucleus in association with DNA.
Photo 51	X-ray picture of DNA by Rosalind FranklinQ CDS 2024 (I), HARD: Photo 51 is an image of DNA molecules.
Double helix	Watson and Crick, 1953
Base pairs	A with T, G with C
Insulin gene	One functional gene in adult humansQ CDS 2017 (II), HARD: adult humans have one functional insulin gene.
Mutation	A change in the genetic materialQ CDS 2022 (I): a mutation in a virus is a change in its genetic material.
Human chromosome number	46, in 23 pairs

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (II) 2023 — General Knowledge · Q12Easy

Example 1 · Genetics and Evolution · Heredity and DNA

Chromosomes are composed of DNA and :

Histones live in the nucleus

Histones are proteins wound up with DNA in the nucleus. They are not membrane proteins of the mitochondria, and they are not joined to lipids or carbohydrates in the cytosol.

Photo 51 is DNA, not a virus

Photo 51 is Rosalind Franklin's X-ray picture of DNA. It has nothing to do with the COVID-19 virus, the common cold or the Moon.

Drill 4 more on chromosomes, dna and genes

Concept 2 of 4: Mendel's monohybrid cross

Each pea plant carries two copies of the gene for height, one from each parent, and passes on only one of them to each seed. Tall (T) wins over short (t), so a plant with even one T is tall. Cross a pure tall plant with a short one and every child is Tt, and tall. Cross two of those children, and the four ways the copies can meet give one TT, two Tt and one tt.

Definition

Mendel's rules for one character (NCERT Class 10 and 12):

Dominant trait (T, tall) shows even when only one copy is present; recessive (t, short) shows only in tt.
Pure (true-breeding) plants have two identical copies: TT or tt. A hybrid has Tt.
F1 of TT × tt: all Tt, all tall.
F2 (F1 × F1): genotypes 1 TT : 2 Tt : 1 tt, phenotypes 3 tall : 1 short.
A test cross (unknown tall × tt) shows the genotype: all tall means TT; half tall and half short means Tt.
For two characters (a dihybrid cross) the F2 phenotypes come out 9 : 3 : 3 : 1.

Monohybrid cross

TT \times tt \;\to\; F_1:\ Tt \qquad Tt \times Tt \;\to\; F_2:\ 1\,TT : 2\,Tt : 1\,tt

Tdominant copy (tall)
trecessive copy (short)
F1, F2first and second generations

Worked example

Two hybrid tall pea plants (Tt) are crossed. Find the genotype and phenotype ratios of the offspring.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2021 · CDS (II) 2021 — General Knowledge · Q14Moderate

Example 2 · Genetics and Evolution · Heredity and DNA

In pea, a pure tall plant (TT) is crossed with a short plant (tt). What will be the ratio of pure tall plants to short plants in the

\mathrm{F_2}

generation ?

Pure tall means TT only

A pure tall plant is TT. The Tt plants are tall too, but they are hybrids. So in the F2 of TT × tt, 'tall' is 3 in 4, while 'pure tall' is only 1 in 4.

3 : 1 is what you see, 1 : 2 : 1 is what they carry

3 : 1 is the F2 phenotype ratio (tall : short). 1 : 2 : 1 is the genotype ratio (TT : Tt : tt). Read which one the question asks for.

Concept 3 of 4: How sex is decided

A woman has two X chromosomes, so every egg she makes carries an X. A man has one X and one Y, so half his sperm carry X and half carry Y. The egg is always the same; it is the sperm that decides. An X sperm makes a girl, and a Y sperm makes a boy.

Definition

Sex determination in humans (NCERT Class 10):

Of the 23 pairs, 22 pairs are autosomes and 1 pair is the sex chromosomes.
Female: XX. Every egg carries X.
Male: XY. Half the sperm carry X, half carry Y.
Egg (X) + sperm (X) gives XX, a girl: one X from the mother and one X from the father.
Egg (X) + sperm (Y) gives XY, a boy.
So the father's sperm decides the sex of the child, and each birth has an equal chance of a boy or a girl.

Egg from mother	Sperm from father	Child
X	X	XX: girlQ CDS 2020 (II): a normal girl gets one X from the mother and one X from the father.
X	Y	XY: boy

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2020 · CDS (II) 2020 — General Knowledge · Q109Moderate

Example 3 · Genetics and Evolution · Heredity and DNA

In human beings, the chromosomes that determine birth of a normal female child are

The mother can only give X

A woman is XX, so every egg carries one X. Options that give a child two X chromosomes from the mother, or an X and a Y from the father, cannot happen.

Concept 4 of 4: Genetic engineering

Genetic engineering moves a useful gene from one living thing into another. Special enzymes act as scissors and glue: one kind cuts DNA at exact spots, another sticks the pieces together. The gene is carried into the new host on a small ring of DNA called a plasmid. Bacteria given the human insulin gene make human insulin, and cotton given a gene from a bacterium kills the insects that eat it.

Definition

Tools and products (NCERT Class 12, Biotechnology):

Nucleases cut DNA. Exonucleases remove nucleotides from the ends of DNA; endonucleases cut at positions within it.
Restriction endonucleases cut DNA at specific sequences. They are used in genetic engineering to form recombinant molecules of DNA, made of DNA from different genomes.
DNA ligase joins the cut pieces together. A plasmid (a small ring of bacterial DNA) is the usual vector that carries the gene into the host.
An organism given a gene from another species is transgenic (a genetically modified organism, GMO).
It is used in bacteria, plants and animals: human insulin made by E. coli; Bt cotton with an insect-killing gene from Bacillus thuringiensis; Golden Rice rich in vitamin A; transgenic animals, made by putting the gene in at a very early stage.
One worry: genes for insect resistance may spread from crops to wild plants.

Tool or product	What it is or does
Restriction endonuclease	Cuts DNA at a specific sequence; used to form recombinant DNAQ CDS 2026 (II): exonucleases remove end nucleotides AND restriction endonucleases are used to form recombinant DNA; both statements are correct.
Exonuclease	Removes nucleotides from the ends of DNA
DNA ligase	Joins pieces of DNA
Plasmid	Small ring of bacterial DNA used as a vector
Human insulin	Made by genetically engineered bacteria
Bt cotton	Carries an insect-killing gene from Bacillus thuringiensisQ CDS 2022 (I): the false statement is that genetic modification is used only in plants.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (II) 2026 — General Knowledge · Q117Moderate

Example 4 · Genetics and Evolution · Heredity and DNA

Which of the following statements is/are associated with role of enzymes in genetic engineering ? 1. Exonucleases remove nucleotides from the ends of DNA 2. Restriction endonucleases are used to form recombinant molecule of DNA Select the answer using the code given below :

Genetic engineering is not only for plants

It is used in bacteria (human insulin), plants (Bt cotton) and animals (transgenic animals). A statement limiting it to plants is false.

Scissors and glue

Restriction enzymes cut DNA; ligase joins it. Exonucleases nibble from the ends, endonucleases cut inside.

Drill 1 more on genetic engineering

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Mendel's monohybrid cross
Monohybrid cross
$TT \times tt \;\to\; F_1:\ Tt \qquad Tt \times Tt \;\to\; F_2:\ 1\,TT : 2\,Tt : 1\,tt$

Reference tables (3)

Chromosomes, DNA and genes7 rows

Term	Fact
Histones	Basic proteins in the nucleus, bound to DNAQ CDS 2019 (I): histones are proteins present in the nucleus in association with DNA.
Photo 51	X-ray picture of DNA by Rosalind FranklinQ CDS 2024 (I), HARD: Photo 51 is an image of DNA molecules.
Double helix	Watson and Crick, 1953
Base pairs	A with T, G with C
Insulin gene	One functional gene in adult humansQ CDS 2017 (II), HARD: adult humans have one functional insulin gene.
Mutation	A change in the genetic materialQ CDS 2022 (I): a mutation in a virus is a change in its genetic material.
Human chromosome number	46, in 23 pairs

How sex is decided2 rows

Egg from mother	Sperm from father	Child
X	X	XX: girlQ CDS 2020 (II): a normal girl gets one X from the mother and one X from the father.
X	Y	XY: boy

Genetic engineering6 rows

Tool or product	What it is or does
Restriction endonuclease	Cuts DNA at a specific sequence; used to form recombinant DNAQ CDS 2026 (II): exonucleases remove end nucleotides AND restriction endonucleases are used to form recombinant DNA; both statements are correct.
Exonuclease	Removes nucleotides from the ends of DNA
DNA ligase	Joins pieces of DNA
Plasmid	Small ring of bacterial DNA used as a vector
Human insulin	Made by genetically engineered bacteria
Bt cotton	Carries an insect-killing gene from Bacillus thuringiensisQ CDS 2022 (I): the false statement is that genetic modification is used only in plants.

Watch out for (7)

Histones live in the nucleus
→ Chromosomes, DNA and genes
Photo 51 is DNA, not a virus
→ Chromosomes, DNA and genes
Pure tall means TT only
→ Mendel's monohybrid cross
3 : 1 is what you see, 1 : 2 : 1 is what they carry
→ Mendel's monohybrid cross
The mother can only give X
→ How sex is decided
Genetic engineering is not only for plants
→ Genetic engineering
Scissors and glue
→ Genetic engineering

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