Genetics, Heredity & Biotechnology – NDA Biology Notes

Every living thing inherits traits from its parents. Your height, eye colour, and blood group- all of these are partly determined by instructions passed from parent to child. Genetics is the science of how these instructions are stored, transmitted, and expressed.

The instructions are stored in DNA. Specific segments of DNA that control specific traits are called genes. Genes are passed from parent to offspring through gametes (sperm and egg). Understanding how this passing works, and what happens when something goes wrong, is what this chapter is about.

For the NDA exam, genetics is a high-frequency chapter with questions clustering around hereditary disease classification, Mendelian ratios (especially the 3:1 vs 1:1 confusion), sex determination, X-linked diseases, allele definitions, and biotechnology. Every section below marks exactly where exam questions originate.

1. What is Genetics?

Genetics is the science of heredity and variation, the study of how traits are inherited from parents and how variation arises between organisms. The physical basis of inheritance is DNA. Specific segments of DNA that govern specific traits are called genes.

Genes are located on chromosomes inside the cell nucleus. They are passed from parent to offspring through gametes, sperm from the father, eggs from the mother. During sexual reproduction, these gametes fuse, combining genetic information from both parents in the offspring.

2. Hereditary vs Non-Hereditary Diseases

The most exam-dense cluster in this chapter is distinguishing genetic diseases from non-genetic ones. This distinction has been tested directly across three NDA papers.

Category	Diseases	Cause	Exam
Hereditary (Genetic)	Haemophilia	X-linked gene mutation: blood fails to clot	[NDA 2006-II]
Hereditary (Genetic)	Colour Blindness	X-linked recessive: cannot distinguish colours (usually red/green)	[NDA 2007-II | NDA 2011-I]
Hereditary (Genetic)	Down’s Syndrome	Chromosomal: extra copy of chromosome 21 (Trisomy 21)
Hereditary (Genetic)	Sickle-Cell Anaemia	Gene mutation: abnormal haemoglobin (HbS) in RBCs	[NDA 2012-II]
NOT Hereditary	Cataract	Age-related and UV-related clouding of the eye lens	[NDA 2006-III
NOT Hereditary	Pellagra	Vitamin B₃ (Niacin) deficiency	—
NOT Hereditary	Osteoporosis	Hormonal and environmental: bone density loss	[NDA 2007-II]
NOT Hereditary	Xerophthalmia	Vitamin A deficiency: NOT genetic	[NDA 2007-II]

★ IMPORTANT When a question asks, “Which is NOT a genetic disease?” Xerophthalmia is the standard correct answer.  [NDA 2007-II] Xerophthalmia is caused by Vitamin A deficiency, a nutritional disease, not a genetic one. Learn the complete hereditary list (Haemophilia, Colour Blindness, Down’s Syndrome, Sickle-Cell Anaemia) and the non-hereditary list together.

3. Chromosomes: The Carriers of Genes

Genes are located on chromosomes, which are long threads of DNA wound around histone proteins. During cell division, chromosomes become visible under a microscope.

Chromosomes are composed of DNA and proteins (histones). Not lipids. Not vitamins. Not carbohydrates.

Fact	Detail	PYQ
Total chromosomes in humans	46 (23 pairs) [NDA 2011-I]	[NDA 2011-I]
Autosomes	22 pairs: carry genes for most body traits	—
Sex chromosomes	1 pair: determine biological sex of the individual	[NDA 2011-I]
Female	XX: two X chromosomes	—
Male	XY: one X and one Y chromosome	—
Chromosome composition	DNA + histone proteins (NOT lipids, NOT vitamins, NOT carbohydrates)

4. The X Chromosome: Sex Determination and X-Linked Diseases

Who Determines the Sex of the Baby?

The father determines the sex of the baby, not the mother.  [NDA 2013-I]

Every egg from the mother carries an X chromosome. This never varies. The father’s sperm can carry either an X or a Y chromosome.

• Father contributes X → baby is XX → female
• Father contributes Y → baby is XY → male

The mother’s contribution is always X. So, the father’s contribution, X or Y, is what decides the baby’s sex. The mother has no role in determining whether the baby is male or female.

X-Linked Diseases: Why Males Are More Affected

Some diseases are caused by mutations on the X chromosome. These are called X-linked diseases.  [NDA 2011-I]

Sex	X Chromosomes	If One X Has Faulty Gene	Consequence
Male (XY)	One X only	No second X to compensate	Disease expressed immediately: far more frequently affected
Female (XX)	Two X chromosomes	Other X often compensates	Carrier: has the faulty gene but may not show the disease

This is why X-linked diseases are far more common in males than females. Males have only one X chromosome. One faulty copy is enough to cause disease.

5. Colour Blindness and Haemophilia: X-Linked Conditions

Colour Blindness

Colour blindness is an X-linked recessive disorder. A mutation on the X chromosome causes the inability to distinguish certain colours, most commonly red and green.  [NDA 2007-II | NDA 2011-I] Because it is X-linked, males are far more frequently affected than females.

Haemophilia

Haemophilia is an X-linked hereditary disease. Blood fails to clot normally due to deficiency of specific clotting factors. It is passed from carrier mothers to sons. It is almost exclusively expressed in males.  [NDA 2006-II]

6. Down’s Syndrome: Trisomy 21

Down’s Syndrome is a chromosomal disorder, not caused by a gene mutation, but by an extra copy of chromosome 21. Instead of the normal two copies, there are three copies (trisomy 21).

Feature	Correct?
It is a genetic (chromosomal) disorder	Correct
Affected individuals show early ageing	Correct
There is intellectual disability (mental retardation)	Correct
A furrowed tongue with an open mouth is a diagnostic feature	NOT a characteristic feature: this appeared as a wrong option

7. Sickle-Cell Anaemia: RBCs Only

Sickle-cell anaemia is a genetic disease. A mutation in the haemoglobin gene produces an abnormal haemoglobin variant called HbS. Under low-oxygen conditions, HbS causes red blood cells to take a rigid, crescent (sickle) shape.

The disease affects red blood cells only, not white blood cells, not thrombocytes (platelets), not plasma composition.  [NDA 2012-II] Sickle-shaped RBCs block small blood vessels and are destroyed faster than normal RBCs, causing anaemia.

8. Alleles: Different Versions of the Same Gene

Each gene can exist in different versions. These different versions are called alleles. For example, the gene for plant height exists in two alleles, tall (T) and short (t). T and t are alleles of the same gene.  [NDA 2021-II]

Term	What It Is	Example	Is It an Allele?
Allele	A different variant (version) of the same gene [NDA 2021-II]	T (tall) and t (short) are alleles of the height gene	Yes: this is the definition
Genotype	The full genetic constitution of an organism	TT, Tt, or tt are genotypes	No: genotype uses allele symbols but is not itself an allele
Isomer	Chemistry term: molecules with the same formula, different structure	C₆H₁₂O₆ can be glucose or fructose (isomers)	No: completely different concept
Sib pair	A pair of biological siblings	Two brothers or sisters	No: this refers to family relationships

9. Mendelian Genetics: The Classic Cross

Gregor Mendel discovered the basic rules of inheritance by crossing pea plants and recording outcomes across generations. His experiments established the laws of segregation and independent assortment that underpin all of modern genetics.

Setting Up the Cross

P generation (parental): Pure tall plant (TT) × Short plant (tt)

F₁ generation (first offspring): All offspring are Tt, hybrid tall. They look tall because T is dominant over t. But they carry the recessive t allele inside.

10. F₂ Generation: Three Questions, Three Different Answers

When F₁ plants (Tt) are selfed (Tt × Tt), the F₂ generation is produced. The same cross produces three different answers depending on what the question asks. All three must be known precisely.

★ IMPORTANT Q1: What is the phenotypic ratio of tall to short in F₂? Answer: 3 tall: 1 short (TT + Tt + Tt = 3 tall-appearing plants; tt = 1 short-appearing plant)   Q2: What is the genotypic ratio in F₂? Answer: 1 TT: 2 Tt: 1 tt (this is the full breakdown of all four possible offspring)   Q3: What is the ratio of pure tall to short in F₂? Answer: 1 TT: 1 tt (TT = ¼ of F₂; tt = ¼ of F₂; the two Tt plants are excluded because they are hybrid tall, not pure tall)   The most common confusion: 3:1 is the phenotypic ratio (includes hybrid Tt in “tall”). 1:1 is pure-breeding only (TT and tt each = ¼ of F₂). These are different questions with different answers.

Question	Answer	Who Is Counted as “Tall”
Phenotypic ratio (all tall : short)	3 : 1	TT + Tt + Tt = 3 (all look tall) : tt = 1 (short)
Genotypic ratio	1 TT: 2 Tt : 1 tt	All four genotypes listed separately
Pure tall to short ratio	1 TT : 1 tt	Only TT counts as pure tall; Tt (hybrid) is excluded

The most tested confusion: students read “3:1” as the answer to the pure tall: short question. That is wrong. 3:1 is the phenotypic ratio. It includes hybrid Tt plants in the “tall” group. When the question asks for pure tall (TT) to short (tt), the answer is 1:1.

11. Self-Pollination Test

A tall plant could be TT (pure tall) or Tt (hybrid tall). Both look the same externally. To determine which it is, use self-pollination.

Plant Selfed	Offspring Produced	Conclusion
TT (pure tall)	All tall offspring (100% tall)	Plant was TT: confirmed pure tall
Tt (hybrid tall)	Both tall and short offspring (3:1 ratio)	Plant was Tt: short offspring appearing confirms it

Cross-pollination does not reveal this. Another plant’s genes would also be involved. Self-pollination isolates the plant’s own genetic makeup. Short offspring appearing = the plant was Tt. No short offspring = the plant was TT.

12. Sources of Genetic Variation

Not all offspring are identical. Variation between individuals allows natural selection to work. Variation comes from three sources.

Source	Description	Produces Variation?
Sexual reproduction	Meiosis and fertilisation shuffle and combine genes from two parents; every offspring is genetically unique	Yes: major source
Mutations	Spontaneous or induced changes in DNA sequence; create new allele variants	Yes
Epigenetic changes	Heritable changes in gene expression without changing the DNA sequence itself	Yes
Asexual reproduction	Offspring are near-identical clones of the parent (binary fission, budding, vegetative propagation)	No: minimum variation

Asexual reproduction does NOT produce significant genetic variation. Offspring from asexual reproduction are near-identical clones of the parent.

Not all genetic variations are equally useful. Environmental conditions favour some variants over others. Variants with unfavourable traits face reduced survival and reproductive success. The statement “all variations in a species have equal chances of survival” is biologically false.

13. DNA and Gene Expression: Important Points

DNA Nucleotide Composition

Each unit of DNA is called a nucleotide. Every nucleotide has three components:

• A nitrogenous base (A, T, G, or C in DNA)
• A deoxyribose sugar
• A phosphate group

RNA uses ribose sugar: not deoxyribose. This is the key structural difference between DNA and RNA nucleotides.

Transcription: Copying DNA into RNA

Transcription is the process of copying genetic information from a DNA strand into a complementary RNA molecule.

Transcription is NOT translation (RNA → protein). NOT replication (DNA → DNA). NOT mutation (a change in sequence). Only transcription produces RNA from DNA.

Human Insulin: Two Genes

The human insulin molecule has two chains:

• α-chain: 21 amino acids
• β-chain: 30 amino acids

Adult humans have two functional insulin genes, one for each chain.

14. Applied Genetics: Biotechnology

DNA Fingerprinting

DNA fingerprinting identifies individuals through unique patterns in their DNA. The technique used is Southern Blotting.  

NOT ELISA (immunological assay, used for AIDS diagnosis). NOT RIA (radioimmunoassay, detects hormones). NOT Northern Blotting (which detects RNA, not DNA). Only Southern Blotting is used for DNA fingerprinting.

Genetic Screening

Genetic screening is the analysis of an individual’s DNA to determine the presence or absence of a specific gene or genetic variant. It identifies risk of inherited diseases before symptoms appear.  

It is not population-level gene analysis. Not pedigree tracing. Not infertility assessment. Individual DNA analysis is the precise definition.

Father of Genetic Engineering

Paul Berg is the father of genetic engineering. He pioneered recombinant DNA technology in the early 1970s.

Philip Drinker invented the iron lung. Thomas Addison described adrenal insufficiency. Alpheus Packard was a zoologist. None of these three is associated with genetic engineering.

GM Crops

Genetically modified (GM) crops have their genetic material altered through: introduction of new DNA sequences, removal of existing DNA sequences, and introduction of new traits.

The modification works at the DNA level, not RNA. Introduction of RNA is not the primary mechanism for GM crops.

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