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By Swati Singh
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Updated on 24 Apr 2025, 12:09 IST
Have you ever wondered how the sex of a baby is decided? Whether a baby will be a boy or a girl is not just a matter of chance—it is based on scientific principles of genetics and chromosomes. This process is called the determination of sex. Understanding this concept is very important for students preparing for NEET, Class 12 board exams, and other competitive tests.
In this article, we will explain the chromosomal basis of sex determination, the role of X and Y chromosomes, and how this knowledge helps us understand certain genetic disorders too.
Sex determination is the biological system that decides whether an individual will be male or female. This process happens at the time of fertilization, when a sperm meets an egg. The chromosomes inside the egg and sperm carry the information that determines the baby’s sex.
Humans have a total of 46 chromosomes in each body cell, arranged in 23 pairs:
Sex chromosomes come in two types:
| Chromosome Type | Function | Examples |
| Autosomes | Carry genes for body traits like eye color, height, etc. | Pairs 1–22 |
| Sex Chromosomes | Determine biological sex and carry genes related to reproduction | Pair 23: XX or XY |
This system is known as the chromosomal sex determination system.
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🧠 Fun Fact: It is the father’s sperm that determines the baby’s sex.
The Y chromosome contains a special gene called the SRY gene (Sex-determining Region Y). This gene starts the process of forming testes and male characteristics.
Gonads are organs that produce sex cells (sperm or egg).
This process is called gonadal differentiation, which is a part of the broader genetic basis of sex determination.
Sometimes, chromosomal disorders can affect sex development. Here are two common examples:
These are examples of genetic disorders affecting sex determination.
Here are some frequently asked questions in NEET Biology related to this topic:
Q1. What determines the sex of a baby?
→ The type of sperm (X or Y) that fertilizes the egg.
Q2. What is the role of the SRY gene?
→ It starts male development by forming testes.
Q3. Can environmental factors influence sex determination?
→ In some animals like reptiles, temperature can influence sex, but in humans, it is purely genetic.
Students preparing for Class 12 Biology Chapter 4 (Reproductive Health) should revise:
These are often part of MCQs in NEET and CBSE board papers.
| Concept | Description |
|---|---|
| Chromosomal Basis | Sex is decided by XX (female) or XY (male) |
| Responsible Gene | SRY gene on the Y chromosome |
| Gonadal Formation | Testes in males, ovaries in females |
| Disorders | Turner (XO), Klinefelter (XXY) |
| NEET Tip | Focus on diagrams, MCQs, and NCERT line-by-line |
Understanding the determination of sex is not only important for exams like NEET but also helps us understand the basics of human genetics. It involves the X and Y chromosomes, especially the SRY gene, and leads to gonadal differentiation. Any issues in this process can cause genetic disorders like Turner or Klinefelter syndrome.
So next time someone asks, “How is the sex of a baby determined?”, you’ll know it’s all in the genes!
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Sex determination is the biological process that decides whether a baby will be male or female. In humans, it is based on the combination of sex chromosomes: XX for female and XY for male.
Sex chromosomes (X and Y) determine the biological sex of an individual. Autosomes are the other 22 pairs of chromosomes that control general body traits. Humans have 1 pair of sex chromosomes and 22 pairs of autosomes.
The SRY gene (Sex-determining Region Y), found on the Y chromosome, starts the development of testes in an embryo. It plays a key role in forming male reproductive organs.
The father’s sperm decides the baby’s sex. The egg always has an X chromosome, but the sperm can carry either X or Y.
X + X = Girl
X + Y = Boy
Gonadal differentiation is the process where the embryo develops either testes (male gonads) or ovaries (female gonads) based on the presence or absence of the SRY gene.
No. In humans, sex is genetically determined and not influenced by the environment. However, in some animals like reptiles, temperature can affect sex during embryo development.
Turner Syndrome (XO): A female has only one X chromosome. It causes short height and infertility.
Klinefelter Syndrome (XXY): A male has an extra X chromosome, which may affect physical and learning abilities.
According to NEET Biology and NCERT Class 12, the chromosomal basis of sex determination is that females are XX and males are XY. The presence of Y and the SRY gene triggers male development.
Yes. Important sex determination diagrams include:
Human karyotypes (XX and XY)
Gonadal development flowchart
Punnett square for sex prediction
Yes. Sex-linked inheritance refers to traits passed through X or Y chromosomes, while sex determination is about deciding the baby’s sex. Both involve sex chromosomes.