🧬 Concept : Sex Determination in Humans and Other Organisms

Introduction to Sex Determination 

Sex determination is the biological process by which the sex of an organism (male or female) is established during development. In most organisms, sex is determined by genetic factors, though in some species, environmental factors also play a role.
The study of sex determination is an important part of genetics, as it explains:

  • Why males and females differ
    • 

  • How sex chromosomes behave during inheritance
    • 

  • How Mendelian principles apply to sex-linked traits

Sex determination is closely related to the behavior of sex chromosomes during meiosis and fertilization.

Chromosomes and Sex Determination

Chromosomes are thread-like structures present in the nucleus of a cell that carry genes. Types of Chromosomes
  1. Autosomes – Chromosomes that do not determine sex
  2. Sex chromosomes (Allosomes) – Chromosomes that determine sex
In humans:
  • Total chromosomes = 46 (23 pairs)
  • Autosomes = 22 pairs
  • Sex chromosomes = 1 pair

Sex Chromosomes in Humans

SexChromosomes
MaleXY
FemaleXX
  • X chromosome → Large, carries many genes
  • Y chromosome → Smaller, carries genes for male development

Sex Determination in Humans (XX–XY Type)

Humans follow the XX–XY type of sex determination, discovered by Henking.
Key Point
  • Females are homogametic (XX) → produce only one type of gamete (X)
  • Males are heterogametic (XY) → produce two types of gametes (X and Y)

Mechanism of Sex Determination in Humans

  1. Female produces ova with only X chromosome
  2. Male produces two types of sperms:
    • X-bearing sperm
    • Y-bearing sperm
  3. Fertilization outcomes:
    FertilizationZygoteSex
    X ovum + X spermXX Female
    X ovum + Y spermXY Male
📌 Important Fact: 👉 Sex of the child is determined by the father, not the mother.

Role of Y Chromosome and SRY Gene

The Y chromosome carries a crucial gene called SRY (Sex-determining Region of Y).
Functions of SRY Gene
  • Initiates development of testes
  • Leads to male sexual differentiation
  • Absence of SRY → female development

Sex Determination in Other Organisms

Different organisms show different mechanisms of sex determination.

1. XX–XO Type of Sex Determination

Seen in:
  • Grasshoppers
  • Cockroaches
  • Some insects
Chromosomal Pattern
SexChromosomes
FemaleXX
MaleXO (only one X, no Y)
Male
  • Male produces two types of sperms: X and O
  • Presence or absence of X determines sex

2. ZZ–ZW Type of Sex Determination

Seen in:
  • Birds
  • Some reptiles
  • Butterflies
Chromosomal Pattern
SexChromosomes
MaleZZ
FemaleZW
📌 Important Difference from Humans: 👉 Female is heterogametic in this system.

3. Haplodiploid Sex Determination

Seen in:
  • Honey bees
  • Ants
  • Wasps
Mechanism
  • Fertilized eggs (diploid) → Females
  • Unfertilized eggs (haploid) → Males
This system is also called arrhenotoky.

4. Environmental Sex Determination

In some organisms, sex is influenced by environmental factors such as temperature. Seen in:
  • Crocodiles
  • Turtles
  • Some lizards
Example
  • Higher temperature → female
  • Lower temperature → male (species-dependent)
📌 Note: This type is not genetically controlled.

Comparison of Different Sex Determination Systems

System MaleFemaleExample
XX–XYXY XX Humans
XX–XO XO XX Grasshopper
ZZ–ZWZZ ZW Birds
HaplodiploidHaploid Diploid Honey bee

Sex Determination vs Sex Differentiation

Sex Determination Sex Differentiation
Decides sexDevelopment of sexual organs
Genetic Hormonal
Occurs at fertilization Occurs during development

Common Misconceptions (NCERT Clarification)

❌ Mother determines sex
✅ Father determines sex
❌ Y chromosome has no function
✅ Y chromosome carries SRY gene

Importance of Studying Sex Determination

  • Helps understand genetic disorders (e.g., Turner syndrome, Klinefelter syndrome
  • Important for NEET & CBSE exams
  • Explains inheritance of sex-linked traits
  • Forms foundation of medical genetics

NCERT Exam-Oriented Key Points

✔ Humans follow XX–XY system
✔ Male is heterogametic
✔ Female is homogametic
✔ Sex is determined at fertilization
✔ Y chromosome determines maleness

Conclusion

Sex determination is a fundamental concept in genetics that explains how organisms develop as male or female. The XX–XY system in humans, along with other mechanisms like ZZ–ZW, XX–XO, and haplodiploid systems, highlights the diversity of genetic control over sex.