The sex of a newborn baby is determined by the chromosome inherited from

1. Fundamentals of Heredity and Traits (basic)

To understand genetics, we must first understand heredity: the mechanism by which characteristics are reliably passed from one generation to the next. In sexual reproduction, both parents contribute an equal amount of genetic material to the offspring, ensuring that the child has two copies of every gene Science, Class X (NCERT 2025 ed.), Chapter 8, p. 133. While this process ensures a 'similar design' to the parents, it also introduces variations—small differences that make each individual unique and can sometimes even help a species survive better in its environment Science, Class X (NCERT 2025 ed.), Chapter 8, p. 129.

One of the most fundamental traits determined at birth is biological sex. In humans, this is not decided by environmental factors but by sex chromosomes. While 22 pairs of our chromosomes are identical in both males and females (called autosomes), the 23rd pair is different. Females possess a perfect pair of X chromosomes (XX). Males, however, have a mismatched pair: one normal-sized X and one much shorter Y chromosome (XY).

During reproduction, the mother always contributes an X chromosome through her egg, as that is the only type she has. The father, however, produces two types of sperm: 50% carry an X chromosome and 50% carry a Y chromosome. The sex of the newborn is determined entirely by which type of sperm fertilizes the egg. If a sperm carrying an X chromosome joins the egg, the child will be a girl (XX); if a sperm carrying a Y chromosome joins the egg, the child will be a boy (XY) Science, Class X (NCERT 2025 ed.), Chapter 8, p. 132.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.129; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.133

2. Chromosomes: The Physical Basis of Inheritance (basic)

To understand inheritance, we must first look inside the nucleus of a cell. Here, the physical basis of life is organized into thread-like structures called chromosomes. These chromosomes act as the vehicles for DNA (Deoxyribo Nucleic Acid), which is the master "blueprint" for our body design Science, Class X (NCERT 2025 ed.), Chapter 7, p.113. Because DNA provides the information source for making proteins, any change in this information can lead to different proteins and, ultimately, altered body traits.

Humans typically have 23 pairs of chromosomes. Most of these (22 pairs) are identical in shape and size for both men and women. However, the 23rd pair—known as the sex chromosomes—is unique. While women possess a perfect pair of XX chromosomes, men have a mismatched pair consisting of one normal-sized X and one smaller Y chromosome Science, Class X (NCERT 2025 ed.), Chapter 8, p.132. During the formation of reproductive cells (sperm and eggs), the number of chromosomes is halved to ensure that when they combine, the child doesn't end up with double the DNA Science, Class X (NCERT 2025 ed.), Chapter 7, p.120.

The determination of a child's sex is a beautiful exercise in probability. Since a mother is XX, all her eggs will carry an X chromosome. However, a father is XY, meaning 50% of his sperm will carry an X and 50% will carry a Y. As shown in the table below, the outcome depends entirely on which sperm reaches the egg first:

Sperm Type	Egg Type	Resulting Zygote	Sex of Child
X Chromosome	X Chromosome	XX	Girl
Y Chromosome	X Chromosome	XY	Boy

Therefore, scientifically speaking, the sex of the newborn is determined by whether the paternal chromosome is X or Y Science, Class X (NCERT 2025 ed.), Chapter 8, p.133.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 7: How do Organisms Reproduce?, p.113, 120; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132, 133

3. Mendelian Laws of Inheritance (intermediate)

To understand genetics, we must start with Gregor Mendel, who discovered that inheritance is not a messy 'blending' of traits, but follows precise mathematical rules. Mendel proposed that traits are controlled by discrete 'factors' (which we now call genes). Every sexuallying reproducing organism possesses two copies of these genes for each trait — one inherited from each parent Science, Chapter 8, p.129. This equal contribution is the foundation of heredity.

Through his experiments with pea plants, Mendel established three pillars of inheritance. First is the Law of Dominance: when two different versions of a gene (alleles) are present, one often masks the other. For instance, in his first-generation (F1) plants, the 'tall' trait completely hid the 'short' trait Science, Chapter 8, p.130. Second is the Law of Segregation, which states that these two alleles separate during the formation of gametes (sperm/egg), so each gamete carries only one copy. This explains why the 'short' trait, though hidden in the F1 generation, reappeared in 25% of the F2 generation plants.

Finally, the Law of Independent Assortment reveals that different traits (like seed color and seed shape) are inherited independently of one another Science, Chapter 8, p.131. A modern application of these rules is human sex determination. Humans have 23 pairs of chromosomes, one of which determines sex. Females have an XX pair, while males have an XY pair. Because a mother can only contribute an X chromosome, the sex of the child is determined entirely by whether the father contributes an X or a Y chromosome Science, Chapter 8, p.132.

Term	Description	Example in Mendel's Peas
Dominant Trait	The trait that is expressed even if only one copy is present.	Tallness (T)
Recessive Trait	The trait that is masked unless two copies are present.	Shortness (t)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.129; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.130; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.131; Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132

4. Chromosomal and Genetic Disorders (exam-level)

To understand genetic and chromosomal disorders, we must first look at the architecture of inheritance. In humans, our genetic blueprint is not a single long strand but is organized into 23 pairs of chromosomes. Each cell carries two copies of every chromosome—one inherited from the mother and one from the father Science, Class X (NCERT 2025), Heredity, p.132. This stability is maintained through a specialized process where germ cells (sperm and eggs) undergo a division that halves their DNA content. Without this reduction, the DNA amount would double every generation, creating a "cellular mess" that would prevent life from functioning Science, Class X (NCERT 2025), How do Organisms Reproduce?, p.120.

Disorders generally fall into two categories: Mendelian disorders (caused by a mutation in a single gene) and Chromosomal disorders (caused by the absence, excess, or abnormal arrangement of entire chromosomes). A classic example of chromosomal logic is sex determination. In humans, females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). Since females only produce eggs with an X chromosome, the sex of the offspring is determined entirely by the father's sperm; if the sperm contributes a Y chromosome, the child is male Science, Class X (NCERT 2025), Heredity, p.132. When chromosomes fail to segregate properly during cell division—a phenomenon called non-disjunction—it leads to conditions like Down Syndrome (an extra copy of chromosome 21) or Turner’s Syndrome (missing an X chromosome).

Feature	Mendelian Disorders	Chromosomal Disorders
Cause	Alteration or mutation in a single gene.	Absence or excess of one or more chromosomes.
Detection	Pedigree analysis or DNA sequencing.	Karyotyping (visualizing the chromosome set).
Examples	Sickle-cell anemia, Haemophilia.	Down Syndrome, Klinefelter’s Syndrome.

Understanding these disorders is vital for the UPSC syllabus because they intersect with public health, biotechnology, and ethical issues in genetic screening. For instance, while chromosomal analysis can help detect life-altering disorders, it has also been misused for illegal sex-selective practices, leading to strict regulations like the PNDT Act in India.

Sources: Science, Class X (NCERT 2025), Heredity, p.132; Science, Class X (NCERT 2025), How do Organisms Reproduce?, p.120

5. Biotechnology: DNA and Gene Editing (exam-level)

At the heart of biotechnology lies DNA (Deoxyribonucleic Acid), the molecular 'instruction manual' for life. Every human is unique because they receive a shuffled combination of genetic material from both parents through specialized reproductive cells called gametes (sperm and egg) Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.222. To ensure that the offspring does not end up with double the amount of genetic material every generation, these gametes carry only half the genetic set of the parent. When they fuse during fertilization, they restore the complete set, creating a unique individual with traits inherited from both sides Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.221. In humans, a specific part of this genetic code determines biological sex through the X and Y chromosomes. Biological females possess two X chromosomes (XX), meaning all eggs they produce carry an X. Biological males possess one X and one Y chromosome (XY), meaning half their sperm carry an X and the other half carry a Y. Because the mother always contributes an X, the father’s sperm is the deciding factor: if it contributes an X, the child is female (XX); if it contributes a Y, the child is male (XY) Science, Class X NCERT, Heredity, p.132-133. Modern biotechnology takes our understanding of this 'code' a step further through Gene Editing. Tools like CRISPR-Cas9 act as molecular scissors, allowing scientists to target a specific sequence of DNA to either 'snip out' a faulty gene or 'paste in' a functional one. This technology is being leveraged globally and in India through initiatives like the Biotech-KISAN programme, which applies scientific solutions to agricultural challenges, and various R&D funds managed by the Department of Science & Technology (DST) to fight diseases like COVID-19 Indian Economy, Nitin Singhania, Agriculture, p.332 Indian Economy, Nitin Singhania, Sustainable Development and Climate Change, p.617.

Sources: Science, Class VIII NCERT, Our Home: Earth, a Unique Life Sustaining Planet, p.221-222; Science, Class X NCERT, Heredity, p.132-133; Indian Economy, Nitin Singhania, Agriculture, p.332; Indian Economy, Nitin Singhania, Sustainable Development and Climate Change, p.617

6. Classification: Autosomes and Allosomes (intermediate)

In our journey through genetics, it is essential to understand that DNA is not just a loose thread of information; it is organized into discrete structures called chromosomes. In humans, every cell contains 23 pairs of these chromosomes, with one member of each pair inherited from each parent to ensure the stability of the species' DNA Science, Class X (NCERT 2025 ed.), Chapter 8, p.132. We classify these 23 pairs into two vital categories: Autosomes and Allosomes.

Autosomes represent the first 22 pairs of chromosomes. These are essentially the same in both males and females and carry the genetic instructions for the vast majority of our physical characteristics—such as height, skin color, and metabolic functions. Allosomes, also known as sex chromosomes, make up the 23rd pair. These are unique because they differ between the sexes: females possess two X chromosomes (XX), while males possess one X and one Y chromosome (XY) Science, Class X (NCERT 2025 ed.), Chapter 8, p.132.

Feature	Autosomes	Allosomes (Sex Chromosomes)
Quantity	22 pairs (44 total)	1 pair (2 total)
Function	Govern somatic (body) traits	Determine biological sex
Pairing	Homologous (identical) in both sexes	Can be heterologous (XY in males)

The determination of a child's sex is a direct result of how these allosomes are inherited during fertilization. Since a mother is XX, every egg she produces will naturally carry one X chromosome. However, because a father is XY, he produces two types of motile germ cells (sperm): half carry an X chromosome and half carry a Y chromosome Science, Class X (NCERT 2025 ed.), Chapter 8, p.132. Therefore, the sex of the newborn is determined entirely by the paternal chromosome. If the sperm contributes an X, the result is a girl; if it contributes a Y, the result is a boy Science, Class X (NCERT 2025 ed.), Chapter 8, p.133.

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

7. The Mechanism of Sex Determination (intermediate)

In the grand design of biology, nature employs diverse strategies to determine the sex of an individual. While some species rely on environmental cues—such as certain reptiles where the incubation temperature of eggs determines sex, or snails that can actually change their sex—human beings follow a strictly genetically determined path Science, Class X (NCERT 2025 ed.), Chapter 8, p. 132. Understanding this mechanism is crucial not just for biology, but for debunking deep-seated social myths regarding the "responsibility" for a child’s sex.

Human cells contain 23 pairs of chromosomes. Of these, 22 pairs are identical in both males and females (called autosomes). However, the 23rd pair, known as the sex chromosomes, differs. Females possess a perfect pair of X chromosomes (XX), meaning every egg a woman produces will carry an X chromosome. In contrast, males have a mismatched pair consisting of one normal-sized X and one shorter Y chromosome (XY). Consequently, when a male produces sperm through meiosis, 50% of the sperm carry the X chromosome and 50% carry the Y chromosome Science, Class X (NCERT 2025 ed.), Chapter 8, p. 132.

Parent	Chromosomes	Gametes Produced
Female (Mother)	XX	100% X-carrying eggs
Male (Father)	XY	50% X-carrying sperm / 50% Y-carrying sperm

The sex of the newborn is determined at the moment of fertilization. Since the mother always contributes an X chromosome, the outcome depends entirely on the father’s contribution. If a sperm carrying an X chromosome fertilizes the egg, the result is a girl (XX). If a sperm carrying a Y chromosome fertilizes the egg, the result is a boy (XY). Scientifically, the paternal chromosome is the deciding factor. From a demographic perspective, this biological 50/50 probability is often skewed by socio-cultural factors like pre-birth sex determination, leading to unfavorable sex ratios in states like Haryana and Punjab Geography of India, Majid Husain (McGrawHill 9th ed.), Cultural Setting, p. 82.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 8: Heredity, p.132; Geography of India, Majid Husain (McGrawHill 9th ed.), Cultural Setting, p.82

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of chromosomal inheritance and the distinction between autosomes and sex chromosomes, this question asks you to apply that logic to human reproduction. In humans, sex is determined by the XY system. You have learned that females possess two identical sex chromosomes (XX), meaning every egg produced carries an X chromosome. In contrast, males possess one X and one Y chromosome (XY), producing two distinct types of sperm. This biological blueprint is the essential building block needed to solve this problem.

To arrive at the correct answer, follow the logic of gametic contribution: since the mother can only provide an X chromosome, she is a 'constant' in the equation. The variable factor is the father, who contributes either an X or a Y chromosome during fertilization. If the sperm carrying a Y chromosome fertilizes the egg, the resulting XY combination produces a male; if an X-carrying sperm succeeds, the XX combination produces a female. Therefore, the father (B) is the sole biological determinant of the newborn's sex, as outlined in Science, Class X (NCERT).

UPSC frequently uses options like 'the mother' or 'grandparents' to test your ability to distinguish between genetic mechanism and social myths or lineage. While grandparents pass down the genes that eventually form the parents' genotypes, they do not directly determine the sex of the newborn at the moment of conception. Option A is a common trap based on outdated societal misconceptions, but as a student of science, you must focus on the heterogametic parent (the one producing different types of gametes) to find the decider in sex determination.