Sex Determination in Humans

Introduction

Sex determination is the biological process by which the sex (male or female) of an individual is decided. According to NCERT, in humans, sex is determined by a pair of chromosomes called sex chromosomes. Humans have a total of 46 chromosomes (23 pairs), out of which 22 pairs are autosomes and 1 pair is sex chromosomes.

Female: XX
Male: XY

The sex of a child is determined at the time of fertilization.

What are Chromosomes?

Chromosomes are thread-like structures present in the nucleus of cells.

Functions of Chromosomes

Carry hereditary information.
Contain genes responsible for traits.
Transfer genetic information from parents to offspring.
Help in sex determination.

Type of Chromosome	Number in Humans	Function
Autosomes	44 (22 pairs)	Control body characteristics
Sex Chromosomes	2 (1 pair)	Determine sex

Sex Chromosomes in Humans

Female (XX)

Females possess two X chromosomes.
All ova (egg cells) contain only X chromosomes.

Male (XY)

Males possess one X chromosome and one Y chromosome.
Sperms are of two types:
- X-bearing sperm
- Y-bearing sperm

Therefore, males are responsible for determining the sex of the child.

How Does Sex Determination Occur?

Step 1: Formation of Gametes

Female:

Produces eggs (ova).
Every egg contains one X chromosome.

Male:

Produces sperms.
Half of the sperms contain X chromosome.
Half of the sperms contain Y chromosome.

Step 2: Fertilization

When sperm and egg fuse:

Egg	Sperm	Result	Child
X	X	XX	Girl
X	Y	XY	Boy

Thus, there is a 50% chance of having a girl and 50% chance of having a boy.

Sex Determination Chart (Punnett Square)

	X (Father)	Y (Father)
X (Mother)	XX (Girl)	XY (Boy)
X (Mother)	XX (Girl)	XY (Boy)

Interpretation

2 out of 4 combinations = Girl (XX)
2 out of 4 combinations = Boy (XY)

Probability:

Girl = 50%
Boy = 50%

Why Is the Father Responsible for Sex Determination?

Reason

Mother always contributes an X chromosome.
Father can contribute either:
- X chromosome
- Y chromosome

Therefore:

X sperm + X egg = Girl (XX)
Y sperm + X egg = Boy (XY)

Hence, the father’s sperm determines the sex of the child.

Important NCERT Point: The mother is not responsible for the birth of a girl or boy.

Importance of Sex Determination

Maintains male and female population balance.
Helps in understanding heredity.
Important for genetics studies.
Explains inheritance of sex-linked traits.
Helps understand chromosome behavior during reproduction.

Difference Between Male and Female Chromosomes

Feature	Male	Female
Sex Chromosomes	XY	XX
Types of Gametes Produced	X and Y sperms	Only X eggs
Determines Child’s Sex	Yes	No
Number of Sex Chromosomes	2	2

Key NCERT Facts

✔ Humans have 46 chromosomes.

✔ Females have XX chromosomes.

✔ Males have XY chromosomes.

✔ Eggs always carry X chromosome.

✔ Sperms carry either X or Y chromosome.

✔ Father determines the sex of the child.

✔ Probability of a boy or girl is equal (50%).

✔ The mother is not responsible for the child’s sex.

Practice Questions

Very Short Answer Questions

1. What are sex chromosomes?

Answer: Chromosomes that determine the sex of an individual are called sex chromosomes.

2. Which sex chromosomes are present in females?

Answer: XX chromosomes.

3. Which sex chromosomes are present in males?

Answer: XY chromosomes.

4. Which chromosome is present in all eggs?

Answer: X chromosome.

5. Which chromosome determines the sex of a child?

Answer: The chromosome contributed by the father (X or Y).

Short Answer Questions

1. Explain sex determination in humans.

Answer: In humans, females have XX chromosomes and males have XY chromosomes. Eggs carry only X chromosome, while sperms carry either X or Y chromosome. Fertilization of X egg by X sperm forms XX (girl), whereas fertilization by Y sperm forms XY (boy).

2. Why is the father responsible for determining the sex of a child?

Answer: Because the father produces two types of sperms (X and Y), while the mother produces only X-bearing eggs.

3. What is the probability of having a boy or a girl?

Answer: The probability of having either a boy or a girl is 50%.

Fill in the Blanks

Humans have ______ chromosomes.
Answer: 46
Females possess ______ chromosomes.
Answer: XX
Males possess ______ chromosomes.
Answer: XY
Eggs always carry ______ chromosome.
Answer: X
Sperms carry ______ or ______ chromosome.
Answer: X, Y
Sex determination occurs during ______.
Answer: fertilization
The father contributes either ______ or ______ chromosome.
Answer: X, Y
XX combination produces a ______ child.
Answer: female
XY combination produces a ______ child.
Answer: male
The mother contributes only ______ chromosome.
Answer: X

MCQs (Multiple Choice Questions)

1. Which chromosomes are present in females?

A) XY
B) XX
C) YY
D) XO

Answer: B) XX

2. Which chromosomes are present in males?

A) XX
B) YY
C) XY
D) XO

Answer: C) XY

3. Human beings have how many chromosomes?

A) 44
B) 48
C) 46
D) 23

Answer: C) 46

4. Eggs contain:

A) Y chromosome
B) X chromosome
C) XY chromosome
D) XX chromosome

Answer: B) X chromosome

5. Sperms may contain:

A) Only X chromosome
B) Only Y chromosome
C) X or Y chromosome
D) Neither X nor Y

Answer: C) X or Y chromosome

6. XX combination results in:

A) Boy
B) Girl
C) Both
D) None

Answer: B) Girl

7. XY combination results in:

A) Girl
B) Boy
C) Both
D) None

Answer: B) Boy

8. Who determines the sex of the child?

A) Mother
B) Father
C) Both equally through X/Y contribution from father
D) Environment

Answer: C) Both equally through X/Y contribution from father

9. How many pairs of sex chromosomes are present in humans?

A) 2
B) 22
C) 23
D) 1

Answer: D) 1

10. Sex determination occurs during:

A) Digestion
B) Respiration
C) Fertilization
D) Excretion

Answer: C) Fertilization

Quick Revision

Human chromosomes = 46
Autosomes = 44
Sex chromosomes = 2
Female = XX
Male = XY
Eggs = X only
Sperms = X or Y
XX = Girl
XY = Boy
Father determines the sex of the child
Probability of boy and girl = 50% each