The Bio Brief

 What is Sex Determination?

Sex determination is the process by which a baby becomes male (boy) or female (girl).

Different organisms use different methods to decide sex.

 Sex Determination in Different Organisms

 Some reptiles

 Temperature of eggs decides sex.

Warm temperature → One sex 

Cool temperature → Other sex

 Some animals (like snails)

 Can change sex during life.

So here:

 Sex is NOT fixed by genes.

In Humans – Sex is Genetically Determined

In humans:

Sex is decided by genes (chromosomes) inherited from parents.

 Human Chromosomes (Easy)

Humans have 23 pairs of chromosomes (46 total).

Out of these:

• 22 pairs → Body chromosomes (same in males and females)
• 1 pair → Sex chromosomes

Sex Chromosomes

Person	Sex Chromosomes
Female	XX
Male	XY

Important:

• Females have XX (perfect pair)
• Males have XY (one X + one Y)

 Who Decides the Sex of the Baby?

Mother always gives:

X chromosome

Father can give:

X OR Y chromosome

So:

 Father determines the sex of the child.

 Simple Flow Chart

Mother (X) + Father (X or Y)

              ↓

         Baby’s sex

 Possible Combinations

Case 1:

Mother X + Father X = XX (Girl)

Case 2:

Mother X + Father Y = XY (Boy)

 Simple Diagram (Punnett Square)

          Father

           X     Y

Mother X  XX    XY

Results:

• XX → Girl
• XY → Boy

So:

50% chance Girl

50% chance Boy

Key Points to Remember

• All children get X from mother.
• Father gives either X or Y.
• X from father → Girl
• Y from father → Boy
• Sex is genetically determined in humans.
• Probability is always 1:1 (50% boys, 50% girls).

 Exam One-Line Answer

In humans, sex is determined by sex chromosomes. Females are XX, males are XY, and the father decides the sex of the child.