Statement I: Human beings have 23 pair of chromosomes, one of which is the sex chromosome and the remaining 22 autosomes. The X-linked diseases are related to mutations on a sex chromosome. Statement II: Colour blindness results from a mutation in X-chromosome.

1. Foundations: DNA, Genes, and Chromosomes (basic)

Welcome to your first step in mastering genetics! To understand evolution, we must first look at the "blueprints" of life. Every living organism is built according to a set of instructions. In humans, these instructions are stored in the nucleus of our cells in the form of DNA (Deoxyribonucleic Acid). Think of DNA as the master library of information required to build and operate your body. When this information changes, the proteins produced by the cell change, which can ultimately lead to different body designs Science, Class X (NCERT 2025 ed.), Chapter 7: How do Organisms Reproduce?, p.113.

While DNA is the material, it is organized into functional units called genes and structural packages called chromosomes. A gene is a specific segment of DNA that codes for a particular trait. To keep this massive amount of information organized, the DNA is tightly coiled into thread-like structures called chromosomes. In a typical human cell, we have 46 chromosomes, which always come in 23 pairs. We inherit one set of 23 from our mother and another set of 23 from our father.

Out of these 23 pairs, 22 pairs are known as autosomes, where the maternal and paternal copies are generally similar in size and gene content. However, the 23rd pair is unique—these are the sex chromosomes. They determine an individual's biological sex. In females, this is a "perfect pair" consisting of two X chromosomes (XX). In males, it is a mismatched pair consisting of one normal-sized X chromosome and one shorter Y chromosome (XY) Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132.

Feature	Autosomes	Sex Chromosomes
Quantity	22 Pairs (44 total)	1 Pair (2 total)
Function	Govern general body traits	Determine biological sex (XX or XY)
Structure	Matched pairs	Can be mismatched (XY in males)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 7: How do Organisms Reproduce?, p.113; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132

2. The Human Karyotype: Autosomes and Allosomes (basic)

In the blueprint of human life, our genetic information is organized into thread-like structures called chromosomes. When we look at the complete set of these chromosomes under a microscope, we call it a karyotype. In a healthy human cell, there are exactly 46 chromosomes, which are organized into 23 pairs. These pairs are not all the same; they are divided into two distinct categories: Autosomes and Allosomes (also known as sex chromosomes).

Autosomes make up the first 22 pairs. These chromosomes are homologous, meaning the maternal and paternal copies carry the same sets of genes in the same order. They govern the vast majority of our physical traits, such as eye color, height, and metabolic functions, and they are essentially identical in both males and females Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132. The 23rd pair, however, is unique. These are the Allosomes, which determine an individual's biological sex. In females, this pair is a matching set of two X chromosomes (XX). In males, the pair is "mismatched," consisting of one X chromosome and a much smaller Y chromosome (XY) Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132.

Understanding this distinction is crucial because of how traits are inherited. Since the X chromosome is much larger than the Y, it carries many genes that the Y does not. This leads to X-linked inheritance. For example, if a gene on the X chromosome undergoes a mutation, a male (XY) will express that trait even if it is recessive, because he lacks a second X chromosome to provide a healthy backup copy. This is why conditions like red-green color blindness are more common in males.

Feature	Autosomes	Allosomes (Sex Chromosomes)
Number of Pairs	22 Pairs (1-22)	1 Pair (23rd)
Function	General somatic (body) traits	Sex determination and sex-linked traits
Male Composition	Identical pairs	XY (Mismatched)
Female Composition	Identical pairs	XX (Perfect pair)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.133

3. Mendelian Inheritance and Genetic Disorders (intermediate)

To understand how genetic disorders are passed down, we must first look at the 'architecture' of our cells. In humans, DNA is organized into 23 pairs of chromosomes. Each pair consists of one chromosome from the mother and one from the father. Of these, 22 pairs are autosomes (general body traits), while the 23rd pair consists of sex chromosomes — XX in females and XY in males Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132. Because genes are located on these chromosomes, their inheritance follows specific patterns first observed by Gregor Mendel.

Genetic disorders often arise from mutations in these genes. A trait is dominant if only one copy of the mutated gene is needed to cause the disorder, whereas it is recessive if both copies must be mutated Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.130. A fascinating category is X-linked recessive disorders. Since males have only one X chromosome, a single mutated gene on that X will cause the disorder. In contrast, females (XX) are often 'carriers' because their second, healthy X chromosome can compensate for the mutated one.

Red-green colour blindness is a classic example of such a disorder. It is caused by a mutation on the X chromosome. While this specific disease illustrates the principle of sex-linked inheritance, it is important to remember that the existence of colour blindness is a consequence of our genetic structure, not the reason for it. Our biological framework (having 23 pairs of chromosomes) is the broader rule that allows such specific conditions to manifest.

Feature	Autosomal Inheritance	X-Linked Recessive Inheritance
Chromosomes Involved	Pairs 1 to 22	The 23rd pair (Sex chromosomes)
Gender Bias	Affects males and females equally	Affects males much more frequently
Example	Cystic Fibrosis, Sickle Cell Anemia	Red-green colour blindness, Haemophilia

Sources: Science, Class X (NCERT 2025 ed.), 8: Heredity, p.130-132

4. Connected Topic: Biotechnology and Recombinant DNA (intermediate)

Concept: Connected Topic: Biotechnology and Recombinant DNA

5. Connected Topic: Modern Gene Editing (CRISPR-Cas9) (exam-level)

In our journey through genetics, we’ve seen how traits are passed down through 23 pairs of chromosomes and how mutations on specific chromosomes, like the X chromosome, can lead to disorders such as colour blindness Science, Class X NCERT, p.132. But what if we could go into the cell and "rewrite" those errors? This is where CRISPR-Cas9 comes in. Often called "molecular scissors," it is a revolutionary tool that allows scientists to edit parts of the genome by removing, adding, or altering sections of the DNA sequence.

To understand how it works, think of DNA as a massive library of instruction manuals. If there is a typo in one book, CRISPR-Cas9 acts as a search-and-replace function. It consists of two main components:

• Guide RNA (gRNA): This is a small piece of pre-designed RNA. Its job is to act like a GPS, finding and binding to a specific sequence of DNA that needs to be changed.
• Cas9 Enzyme: This is the "scissors" part. Once the Guide RNA finds the target, the Cas9 protein cuts the DNA at that precise location.

Once the DNA is cut, the cell recognizes the damage and tries to repair it. Scientists can "trick" this natural repair process by providing a healthy DNA template, which the cell then uses to fix the break, effectively rewriting the genetic code Indian Economy, Nitin Singhania, p.301. This technology is far more precise, cheaper, and faster than older methods of genetic modification. It holds the potential to cure hereditary diseases at their root, rather than just treating symptoms.

Sources: Science, Class X NCERT, Heredity, p.132; Indian Economy, Nitin Singhania, Agriculture, p.301

6. Connected Topic: Mitochondrial DNA and Maternal Inheritance (intermediate)

While we often focus on the DNA found in the cell nucleus, there is a second, smaller genome hiding in our cells: Mitochondrial DNA (mtDNA). Unlike the 23 pairs of chromosomes in the nucleus that are inherited equally from both parents Science, Class X (NCERT 2025 ed.), Heredity, p.129, mtDNA follows a unique path called maternal inheritance. This means that your mitochondrial genetic code comes exclusively from your mother, her mother, and so on, stretching back through an unbroken maternal line.

The reason for this lies in the mechanics of fertilization. An egg cell is massive and packed with cytoplasm containing thousands of mitochondria to power the initial stages of life. In contrast, a sperm cell is built for speed; its few mitochondria are located in the tail or midpiece to provide energy for swimming. Upon fertilization, only the sperm's nucleus usually enters the egg, or any paternal mitochondria that do enter are actively identified and destroyed by the egg cell. Consequently, the resulting embryo's energy factories are entirely of maternal origin.

Because mtDNA does not undergo recombination (the shuffling of genes that happens with nuclear DNA), it remains remarkably stable over generations. This stability makes it a 'molecular clock' for scientists. As noted in History, Class XI (Tamilnadu State Board 2024 ed.), Early India, p.1, studying mtDNA allows us to track prehistoric human migrations and understand how different populations dispersed across the planet. If a mutation occurs in the mtDNA of one woman, it becomes a marker that can be tracked in all her maternal descendants thousands of years later.

Feature	Nuclear DNA	Mitochondrial DNA
Inheritance	Biparental (50% Mom, 50% Dad)	Uniparental (100% Maternal)
Structure	Linear chromosomes	Small, circular molecules
Recombination	Yes (shuffling occurs)	No (passed down as a clone)

Sources: Science , class X (NCERT 2025 ed.), Heredity, p.129; History , class XI (Tamilnadu state board 2024 ed.), Early India: From the Beginnings to the Indus Civilisation, p.1

7. Sex-Linked Inheritance Mechanics (intermediate)

To understand why certain conditions like colour blindness or haemophilia appear more frequently in men, we must look at our biological blueprint. Human cells contain 23 pairs of chromosomes. Of these, 22 pairs are autosomes (which look the same in both males and females), while the 23rd pair consists of the sex chromosomes. Women possess two X chromosomes (XX), while men possess one X and one Y chromosome (XY). Science, Class X (NCERT 2025 ed.), Heredity, p. 132. Because the X chromosome is much larger than the Y, it carries many essential genes that the Y chromosome lacks. Any mutation in these genes leads to X-linked inheritance.

The mechanics of this inheritance depend on whether a trait is dominant or recessive. In standard Mendelian genetics, a recessive trait only shows up if an individual has two copies of that gene (one from each parent). Science, Class X (NCERT 2025 ed.), Heredity, p. 130. However, because males have only one X chromosome, they are hemizygous—they don't have a second 'backup' X chromosome to mask a faulty gene. If a male inherits an X chromosome with a recessive mutation, he will definitely express the trait or disease. Females, having two X chromosomes, usually require mutations on both to show the disease. If they have only one mutated X, they are termed carriers; they appear healthy but can pass the mutation to their offspring.

Red-green colour blindness serves as a classic example of this mechanic. It is an X-linked recessive disorder. A son inherits his X chromosome solely from his mother and his Y from his father. Science, Class X (NCERT 2025 ed.), Heredity, p. 132. Therefore, if a mother is a carrier, there is a 50% chance her son will be colour blind. For a daughter to be colour blind, she would typically need to inherit a mutated X from both a carrier/affected mother and an affected father, making the condition statistically rarer in females.

Feature	Females (XX)	Males (XY)
Response to Recessive Mutation	Often a 'carrier' (no symptoms) unless both Xs are affected.	Always expresses the trait if the single X is affected.
Source of X Chromosome	One from Mother, one from Father.	Exclusively from Mother.

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.130; Science, class X (NCERT 2025 ed.), Heredity, p.132

8. Specific X-linked Disorders: Color Blindness and Hemophilia (exam-level)

To understand X-linked disorders, we must first look at our biological blueprint. Humans possess 23 pairs of chromosomes: 22 pairs of autosomes (which are identical in males and females) and one pair of sex chromosomes. In females, this pair is XX, while in males, it is XY Science, class X (NCERT 2025 ed.), Heredity, p.132. X-linked disorders occur when there is a mutation in a gene located specifically on the X chromosome. Because the Y chromosome is much smaller and lacks many of the genes found on the X, a male who inherits a defective X chromosome has no 'backup' copy to mask the mutation. This is why these disorders are significantly more common in men. Two classic examples of X-linked recessive disorders are Color Blindness and Hemophilia. Red-green color blindness is a condition where an individual cannot distinguish between certain shades of red and green. While we often study the light spectrum (VIBGYOR) in physics Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.167, the biological ability to perceive these wavelengths depends on proteins encoded by genes on the X chromosome. Hemophilia, often called the 'Royal Disease,' is a more severe condition where the blood's ability to clot is severely reduced, meaning even a minor injury can cause prolonged bleeding.

Feature	Males (XY)	Females (XX)
Expression	Express the trait with only one copy of the mutated gene.	Usually require two copies of the mutated gene to express the trait.
Carrier Status	Cannot be 'carriers'; they either have the condition or they don't.	Can be carriers (one normal gene, one mutated gene) without showing symptoms.
Inheritance	Pass the X chromosome only to their daughters.	Can pass the mutated X chromosome to both sons and daughters.

Sources: Science, class X (NCERT 2025 ed.), Heredity, p.132; Science, class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.167

9. Solving the Original PYQ (exam-level)

This question bridges the gap between basic chromosomal structure and the functional application of genetics. You have recently learned that the human karyotype consists of 23 pairs of chromosomes, split into 22 pairs of autosomes and 1 pair of sex chromosomes. Statement I is a foundational summary of this biological blueprint, as detailed in Science, class X (NCERT 2025 ed.). Statement II then tests your ability to identify a specific application of this rule: colour blindness. Because you know that the genes for red-green vision are located on the X-chromosome, you can confidently conclude that both statements are factually true.

The core challenge in UPSC Assertion-Reasoning questions is the logical link. To arrive at the correct answer, you must ask: "Does the existence of colour blindness explain why humans have 23 pairs of chromosomes?" The answer is clearly no. While Statement II is a perfect example of an X-linked disease mentioned in Statement I, it is not the cause or the reason for the general rule of human chromosomal counts. Therefore, the reasoning leads us to (B) Both the statements are individually true but statement II is not the correct explanation of statement I.

A common UPSC trap is Option (A), where students see two related, true facts and reflexively assume the second explains the first. Always remember: for Statement II to be a "correct explanation," it must answer the "Why?" or provide the mechanism behind Statement I. Options (C) and (D) are designed to catch students who are unsure of factual details—such as whether colour blindness is X-linked or Y-linked—but since you have mastered the NCERT fundamentals, you can easily bypass these distractors by verifying the accuracy of each individual claim first.