Fertilisation & Sex Determination

Fertilisation — Sperm Meets Egg

Fertilisation = the fusion of a male gamete (sperm) with a female gamete (secondary oocyte/ovum) → produces a diploid zygote.

Where and when?

• Location: the ampullary-isthmic junction of the fallopian tube (NOT in the uterus). Fertilisation occurs only if ovum and sperms reach the junction simultaneously (NCERT).
• Timing: Within ~24 hours of ovulation (the egg's viability window).
• Sperm window: Sperms can survive ~3-5 days in the female tract.

Definition

Fertilisation = fusion of male and female gametes producing a diploid zygote.

Number of sperms vs egg

• ~200-300 million sperms in ejaculate
• Only a few hundred reach the ampulla
• ONLY ONE fuses with the egg
• The rest die or are cleared by the female immune system

Three big questions this section answers

1. How does the sperm penetrate the egg?
2. How does the egg block other sperms after fusion?
3. How is the baby's sex determined?

[NEET Important] Fertilisation site = ampulla of fallopian tube. Memorise — common NEET MCQ.

The Steps of Fertilisation

Fertilisation has 4 distinct molecular events:

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Step 1: Sperm reaches the egg

Sperm swims up from the vagina/uterus into the fallopian tube ampulla. After capacitation (5-6 hours in female tract), it's ready to fertilise.

It encounters the egg surrounded by:

• Cumulus oophorus — outermost layer of follicular cells.
• Corona radiata — middle layer of cells.
• Zona pellucida — innermost glycoprotein coat (just outside the egg's plasma membrane).

Step 2: Acrosomal reaction

When the sperm head contacts the zona pellucida:

• Acrosomal membrane fuses with sperm membrane → releases enzymes.
• Hydrolytic enzymes (acrosin, hyaluronidase) locally digest the zona pellucida.
• Creates a path for the sperm head to reach the egg's plasma membrane.

Step 3: Sperm-egg fusion (syngamy)

• Sperm membrane fuses with egg plasma membrane.
• Sperm nucleus enters the egg cytoplasm.
• This triggers calcium signalling in the egg.

Step 4: Cortical reaction (polyspermy block)

The egg immediately responds:

• Cortical granules (small vesicles just under the egg's plasma membrane) release their contents into the perivitelline space.
• These contents harden the zona pellucida → makes it impenetrable to other sperms.
• This prevents polyspermy (multiple sperms fertilising the egg).

Why polyspermy must be prevented

If 2 sperms fertilise an egg → triploid embryo (extra chromosomes) → not viable. The cortical reaction is critical for normal development.

Step 5: Meiosis II completion

Sperm entry triggers the secondary oocyte (arrested in metaphase-II) to complete Meiosis II → ovum + second polar body.

Step 6: Pronuclear fusion → zygote

• Sperm nucleus (♂ pronucleus) and egg nucleus (♀ pronucleus) fuse.
• This is syngamy in the strict sense.
• Result: diploid zygote (2n = 46 chromosomes in humans).

[NEET Important] All these events happen rapidly — within minutes of sperm entry. Memorise the sequence.

Sex Determination — XY System

In humans (and all mammals), sex is determined at fertilisation by the XY chromosome system.

The basics

Sex	Chromosomes
Female	XX (homozygous for X)
Male	XY (heterozygous)

Egg and sperm contributions

• Eggs: All eggs carry an X chromosome (since female is XX).
• Sperms: Half carry X, half carry Y (since male is XY).

Fertilisation outcomes

Sperm contribution	Egg contribution	Zygote	Sex
X	X	XX	Female
Y	X	XY	Male

Who determines the baby's sex?

The father. Specifically, his sperm — because the sperm provides either X (→ daughter) or Y (→ son), while the egg always provides X. The father is the sex-deciding parent.

Cultural impact

This biology is critical for combating gender bias and false claims that women are responsible for the baby's sex. Mothers cannot "produce" sons or daughters — the chromosome from the sperm decides.

[NEET Important + Social impact] "Who determines the baby's sex?" Answer: The father (via his sperm's X or Y chromosome).

Other sex determination systems (Class XI recall)

• XO system (some insects): female XX, male XO (single X).
• ZW system (birds): female ZW, male ZZ. Notice the reversal!
• Haplodiploidy (bees): female diploid, male haploid (from unfertilised eggs).
• Environmental (some reptiles): temperature-dependent.

Small Memory Capsule

Section 10 in 60 seconds.

Fertilisation = sperm + egg → zygote

• Where: Ampullary-isthmic junction of the fallopian tube
• When: Within ~24 hours of ovulation
• Result: Diploid zygote (2n = 46 in humans)

Steps of fertilisation

1. Sperm reaches egg — passes through cumulus, corona radiata
2. Acrosomal reaction — enzymes digest zona pellucida
3. Sperm-egg fusion (syngamy)
4. Cortical reaction — blocks polyspermy
5. Meiosis II completes in oocyte
6. Pronuclear fusion → zygote

Key terms

• Acrosomal reaction: sperm releases enzymes to digest zona pellucida
• Cortical reaction: egg hardens zona pellucida to block other sperms
• Polyspermy block: prevents multiple sperms (would cause triploid, non-viable embryo)

Sex determination

Sperm chromosome	Egg chromosome	Zygote	Sex
X	X	XX	Female
Y	X	XY	Male

Father determines sex

• All eggs carry X
• Sperms carry X or Y
• Father's sperm chromosome decides → father is the sex-deciding parent

Sperm window vs egg window

• Sperm survives ~3-5 days in female tract
• Egg viable ~24 hours after ovulation
• Fertile window: ~5-6 days around ovulation

One-line takeaway

"Fertilisation occurs in the ampulla within 24 hr of ovulation; sperm undergoes acrosomal reaction → fuses with egg → triggers cortical reaction (blocks other sperms) → completes meiosis II → pronuclear fusion → diploid zygote. Father's sperm (X or Y) determines baby's sex."

Solved Examples

Example 1: Define fertilisation

Define fertilisation. Where and when does it occur?

Solution:

Fertilisation = the fusion of a male gamete (sperm) with a female gamete (secondary oocyte/ovum) to form a diploid zygote.

• Location: Ampullary-isthmic junction of the fallopian tube (NOT the uterus).
• Timing: Within ~24 hours of ovulation (egg viability window).

Answer: Sperm-egg fusion → zygote; ampulla of fallopian tube; within 24 hr of ovulation.

Example 2: Acrosomal reaction

What is the acrosomal reaction? Why is it important?

Solution:

Acrosomal reaction = the release of hydrolytic enzymes from the sperm's acrosome when it contacts the zona pellucida of the egg. The acrosomal membrane fuses with the sperm membrane, releasing:

• Acrosin (serine protease)
• Hyaluronidase

These enzymes locally digest the zona pellucida, creating a path for the sperm head to reach the egg's plasma membrane.

Why important: Without the acrosomal reaction, the sperm cannot penetrate the zona pellucida — fertilisation fails.

Answer: Acrosomal reaction = enzyme release from sperm acrosome that digests zona pellucida, allowing sperm to reach egg.

Example 3: Cortical reaction

What is the cortical reaction? What does it accomplish?

Solution:

Cortical reaction = the release of contents from cortical granules (small vesicles just under the egg's plasma membrane) into the perivitelline space immediately after sperm-egg fusion.

What it accomplishes:

• Hardens the zona pellucida → makes it impenetrable to other sperms.
• This PREVENTS POLYSPERMY (multiple sperms fertilising the egg).

Why crucial: If 2+ sperms entered the egg, the embryo would be triploid (3 sets of chromosomes) → not viable. Cortical reaction ensures only ONE sperm fertilises.

Answer: Cortical reaction = egg hardens zona pellucida after first sperm enters; blocks polyspermy.

Example 4: Sex determination

In humans, who determines the sex of the baby?

Solution:

The FATHER.

• Females are XX: all eggs carry an X chromosome.
• Males are XY: sperms carry either X or Y (50:50 ratio).

When fertilisation occurs:

Sperm	Egg	Zygote	Sex
X	X	XX	Female
Y	X	XY	Male

Since the egg always provides X, the sperm's chromosome (X or Y) decides the baby's sex.

Answer: Father determines sex (via his sperm's X or Y chromosome). NOT mother.

Example 5: Why polyspermy is prevented

Why must fertilisation be limited to one sperm? What mechanism prevents multiple sperms from entering?

Solution:

Why limited to one: If 2+ sperms fertilise an egg, the resulting embryo would be triploid (3 sets of chromosomes instead of 2). Triploidy is incompatible with normal development — almost all triploid human embryos die early.

Mechanism — Cortical Reaction:

1. As soon as one sperm fuses with the egg, calcium signalling triggers cortical granules.
2. Cortical granules release their contents into the perivitelline space.
3. The zona pellucida is chemically modified → becomes hard and impenetrable to additional sperms.
4. Any other sperms approaching are blocked.

This is the fast block to polyspermy (within seconds of fertilisation).

Answer: Polyspermy → triploidy → embryonic death. Cortical reaction hardens zona pellucida, blocking additional sperms.

Example 6: Where exactly is the egg?

When fertilisation occurs, what is the technical state of the female gamete (the "egg")?

Solution:

What's released at ovulation and waiting in the ampulla is TECHNICALLY a SECONDARY OOCYTE, NOT a fully mature ovum.

State at ovulation:

• Haploid ($n$)
• Arrested in metaphase-II of meiosis
• Surrounded by zona pellucida + corona radiata

When sperm enters:

• Calcium signalling → secondary oocyte completes Meiosis II
• Produces ovum + second polar body
• ONLY NOW it's a true ovum

So:

$$\text{Secondary oocyte (n, metaphase-II)} \xrightarrow{\text{sperm entry}} \text{Ovum (n) + Second polar body (n)}$$

Answer: What's released is a secondary oocyte; it becomes a true ovum only after sperm entry triggers Meiosis II completion.