IB DP Biology Reproduction Study Notes - New Syllabus -2025

IB DP Biology Reproduction Study Notes

IB DP Biology Reproduction Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

How does asexual or sexual reproduction exemplify themes of change or continuity?
What changes within organisms are required for reproduction?

Standard level and higher level: 5 hours
Additional higher level: 3 hours

IBDP Biology 2025 -Study Notes -All Topics

D3.1.1 – Differences Between Sexual and Asexual Reproduction

🧠 What is Asexual Reproduction?

Involves one parent.
Offspring are genetically identical clones of the parent.
No fusion of gametes (sex cells).
Common in organisms like bacteria, some plants, and simple animals.

Advantages of Asexual Reproduction

Produces offspring quickly and efficiently.
Useful when the environment is stable and well-suited to the parent’s genetics.
No energy needed to find a mate.

🧠 What is Sexual Reproduction?

Involves two parents.
Offspring have new combinations of genes due to fusion of gametes.
Creates genetic variation in the population.
Common in most animals, plants, and many fungi.

Advantages of Sexual Reproduction

Produces genetic diversity.
Allows populations to adapt to changing environments.
Increases chances of survival against diseases and environmental shifts.

📊 Comparison Table: Sexual vs Asexual Reproduction

Feature	Asexual Reproduction	Sexual Reproduction
Number of parents	One	Two
Genetic variation	None (clones)	High (new gene combinations)
Speed of reproduction	Usually faster	Usually slower
Adaptation	Good for stable environments	Good for changing environments
Energy cost	Low	High (finding mate, gamete production)

📌 Summary Box
Asexual reproduction is fast and produces identical offspring, ideal for stable environments.
Sexual reproduction creates variation, helping species adapt to new or changing conditions.

D3.1.2 – Role of Meiosis and Fusion of Gametes in the Sexual Life Cycle

🧠 Meiosis: Creating Genetic Variation

Meiosis is a special type of cell division producing haploid gametes (sperm and egg).
It breaks up parental combinations of alleles, shuffling genes to create unique gametes.
This reduces the chromosome number by half (diploid → haploid).
Ensures offspring have the right number of chromosomes when gametes fuse.

🧠 Fusion of Gametes (Fertilization)

Fertilization is the joining of two haploid gametes.
It restores the diploid chromosome number in the zygote.
Combines alleles from both parents to create new gene combinations.
This increases genetic diversity in the offspring.

🔍 Key Concept

Meiosis generates diversity by shuffling alleles.
Fertilization mixes alleles from two parents.
Together, these processes ensure variation in sexually reproducing populations.

📌 Summary Box:

Process	Function	Result
Meiosis	Reduces chromosome number, shuffles genes	Haploid gametes with varied alleles
Fertilization	Fusion of gametes	Diploid zygote with new allele combinations

D3.1.3 – Differences Between Male and Female Sexes in Sexual Reproduction

🧠 Prime Difference: Gamete Size and Mobility

Male gametes (sperm) are small, mobile, and have little food reserve.
Female gametes (eggs) are larger, immobile, and contain food reserves to support early development.
The male gamete travels to the female gamete to fertilize it.

🌿 Differences in Numbers of Gametes

Aspect	Male Gametes (Sperm)	Female Gametes (Egg)
Size	Small	Large
Number produced	Millions produced continuously	Few produced, usually one per cycle
Mobility	Highly motile	Non-motile
Energy reserves	Minimal (little food stored)	Large (nutrient-rich cytoplasm)

🌿 Reproductive Strategies

Males produce many small gametes to increase chances of fertilization.
Females invest more in fewer, nutrient-rich gametes to support embryo development.
This leads to different reproductive roles and energy investments:
- Males maximize quantity.
- Females maximize quality and survival of offspring.

📌 Summary Box
Male gametes are small and mobile; female gametes are large and nutrient-rich.
Males produce many gametes; females produce fewer.
These differences shape distinct reproductive strategies and roles.

D3.1.4 – Anatomy of the Human Male and Female Reproductive Systems

🌿 Male Reproductive System Anatomy

Structure	Function
Testes	Produce sperm and testosterone
Epididymis	Store and mature sperm
Vas deferens	Transports sperm from epididymis to urethra
Seminal vesicles	Produce seminal fluid (nutrients for sperm)
Prostate gland	Adds fluids to semen to help sperm motility
Urethra	Conducts semen and urine out of the body
Penis	Organ for delivering sperm into female reproductive tract

🌿 Female Reproductive System Anatomy

Structure	Function
Ovaries	Produce eggs (ova) and hormones (estrogen, progesterone)
Fallopian tubes	Transport eggs from ovaries to uterus; site of fertilization
Uterus	Site of embryo implantation and fetal development
Cervix	Narrow opening to uterus; allows passage of sperm and childbirth
Vagina	Canal receiving sperm; birth canal
Vulva	External genitalia protecting internal organs

📌 Summary Box
Male system focuses on sperm production, maturation, and delivery.
Female system supports egg production, fertilization, and fetal development.
Knowing structure-function relationships is key for understanding reproduction.

D3.1.5 – Changes During the Ovarian and Uterine Cycles and Their Hormonal Regulation

Overview: The Menstrual Cycle

The menstrual cycle = combined ovarian + uterine cycles.
Lasts about 28 days in humans.
Controlled by hormones: FSH, LH, oestradiol, and progesterone.
Involves development of the egg and preparation of the uterus for pregnancy.

🌿 Ovarian Cycle Phases

Phase	Key Events	Hormonal Control
Follicular phase	Follicle in ovary matures; produces oestradiol	FSH stimulates follicle growth; oestradiol rises, triggering LH surge
Ovulation	Mature follicle releases the egg	LH surge causes follicle to rupture and release egg
Luteal phase	Corpus luteum forms, secretes progesterone	Progesterone prepares uterus; inhibits FSH and LH (negative feedback)

🌿 Uterine Cycle Phases

Phase	Description	Hormonal Influence
Menstrual phase	Shedding of uterine lining (menstruation)	Low levels of oestradiol and progesterone
Proliferative phase	Rebuilding of uterine lining	Rising oestradiol thickens endometrium
Secretory phase	Uterine lining maintained and enriched	Progesterone from corpus luteum stabilizes lining

🌿 Hormonal Regulation and Feedback

FSH (Follicle-Stimulating Hormone): Stimulates follicle growth in ovary.
LH (Luteinizing Hormone): Triggers ovulation and corpus luteum formation.
Oestradiol (a form of estrogen): Promotes growth of uterine lining; exerts positive feedback to increase LH before ovulation; later, negative feedback inhibits FSH and LH.
Progesterone: Maintains uterine lining; provides negative feedback on FSH and LH to prevent new follicle development.

🔍 Summary of Feedback Loops

Hormone	Feedback Type	Effect
Oestradiol	Positive (before ovulation)	Stimulates LH surge
Oestradiol	Negative (after ovulation)	Inhibits FSH and LH
Progesterone	Negative	Maintains uterine lining; inhibits FSH and LH

📌 Summary Box
The ovarian cycle produces and releases an egg, controlled by FSH, LH, and sex steroids.
The uterine cycle prepares the uterus for pregnancy.
Hormones work via positive and negative feedback to coordinate cycles.
Understanding these cycles is essential for grasping human reproduction and fertility.

D3.1.6 – Fertilization in Humans

🌿 Key Steps in Fertilization

Fusion of membranes: The sperm’s cell membrane fuses with the egg’s cell membrane, allowing sperm entry.
Entry of sperm nucleus: Only the sperm nucleus enters the egg; sperm tail and mitochondria are destroyed after entry.
Nuclear membrane dissolution: Nuclear membranes of sperm and egg break down, releasing chromosomes into egg cytoplasm.
Formation of diploid nucleus: Condensed chromosomes from both nuclei join and undergo mitosis, producing two diploid nuclei—beginning zygote’s first cell division.

🔍 Important Points

Fertilization restores the diploid chromosome number.
The sperm contributes genetic material but not mitochondria (maternal inheritance).
Fusion activates the egg to start development.

📌 Summary Box
Sperm and egg membranes fuse.
Sperm nucleus enters; tail and mitochondria destroyed.
Nuclear membranes dissolve; chromosomes merge.
Joint mitosis forms diploid nuclei, starting zygote development.

D3.1.7 – Use of Hormones in In Vitro Fertilization (IVF) Treatment

What is IVF?

IVF is a fertility treatment where eggs are fertilized by sperm outside the body.
Hormones are used to control and enhance the process.

Hormonal Control in IVF

Natural hormone secretion is temporarily stopped (suppressed) to control timing.
Artificial hormones are given to induce superovulation: stimulation of ovaries to produce multiple mature eggs in one cycle.
This increases the number of eggs available for fertilization.

Common Hormones Used

Hormone	Role in IVF
FSH (Follicle-Stimulating Hormone)	Stimulates development of multiple follicles
LH (Luteinizing Hormone) or hCG (human chorionic gonadotropin)	Triggers final maturation and release of eggs

🌿 Why Use Hormones in IVF?

To maximize the number of eggs collected in one cycle.
Increases chances of successful fertilization and pregnancy.
Allows precise timing of egg retrieval and fertilization.

📌 Summary Box
IVF involves hormone therapy to stop natural cycles and induce superovulation.
Artificial FSH and LH (or hCG) promote growth and release of multiple eggs.
Hormone control improves IVF success rates by providing more eggs for fertilization.

D3.1.8 – Sexual Reproduction in Flowering Plants

🌿 Key Features of Sexual Reproduction in Flowering Plants

Sexual reproduction involves fusion of male and female gametes.
Occurs inside pollen grains (male) and ovules (female).
Even hermaphroditic plants (both male and female parts in one flower) reproduce sexually.

🌿 Production of Gametes

Gamete Type	Location	Development
Male gametes (sperm cells)	Inside pollen grains	Pollen develops in anthers through meiosis
Female gametes (egg cells)	Inside ovules in ovary	Egg cells develop in ovules through meiosis

Pollination

Transfer of pollen grains from anther (male) to stigma (female).
Can be via wind, insects, or animals.
After pollination, pollen grain germinates on stigma.

Pollen Development & Fertilization

Pollen grain grows a pollen tube down the style toward the ovule.
Two sperm cells travel through the pollen tube.
One sperm fertilizes the egg cell → forms a zygote → develops into an embryo.
The other sperm often fuses with two polar nuclei to form endosperm (nutritive tissue).

🔍 Important Notes

Fertilization results in formation of a diploid embryo.
Sexual reproduction increases genetic variation.
Flowering plants ensure reproductive success even if self-fertilization occurs in hermaphrodites.

📌 Summary Box
Gametes formed in pollen grains (male) and ovules (female).
Pollination transfers pollen; pollen tube delivers sperm to ovule.
Fertilization forms embryo and endosperm.
Flowering plants reproduce sexually, promoting genetic diversity.

D3.1.9 – Features of an Insect-Pollinated Flower

🌿 Key Features and Their Functions

Feature	Description	Function
Brightly coloured petals	Often large and colorful (red, yellow, blue)	Attract insects visually
Scented flowers	Produce strong, sweet scents	Attract insects by smell
Nectar guides	Patterns on petals visible to insects	Guide insects to nectar
Nectar production	Nectar is produced at base of petals	Reward for visiting insects (food)
Sticky pollen grains	Pollen grains are large and sticky	Attach easily to insect bodies
Stigma	Often sticky	Ensures pollen grains stick when insect visits
Anthers	Positioned to brush pollen onto insects	Transfer pollen to insect bodies
Flower shape	Usually tubular or complex shapes	Encourage insect contact with reproductive parts

🔍 Note:

Flowers are often large and showy to be attractive.
Pollen is produced in moderate amounts, as insects transfer it efficiently.
Adaptations promote targeted pollination by specific insect species.

📌 Summary Box
Insect-pollinated flowers have bright petals, scent, nectar, and sticky pollen.
Structural adaptations maximize pollen transfer via insects.
These features increase the chances of successful fertilization.

D3.1.10 – Methods of Promoting Cross-Pollination

🌿 What is Cross-Pollination?

Transfer of pollen from the anther of one plant to the stigma of a different plant.
Promotes genetic diversity by mixing genes from different plants.

Methods to Promote Cross-Pollination

Method	Description	Purpose
Different maturation times (Dichogamy)	Pollen and stigma mature at different times in the same flower	Prevents self-pollination
Separate male and female flowers on the same plant (Monoecious)	Some flowers have only male parts, others only female parts	Forces pollen transfer between flowers
Separate male and female plants (Dioecious)	Individual plants produce only male or only female flowers	Ensures cross-pollination between plants
Pollination agents	Animals (insects, birds), wind	Transfer pollen between different plants

🌿 Role of Pollination Agents

Animals: Attracted by flower features (color, scent, nectar), carry pollen on bodies.
Wind: Carries lightweight pollen grains over distances to other plants.

🔍 Extra Info

These mechanisms reduce self-pollination, encouraging new genetic combinations.
Cross-pollination improves adaptability and survival of plant species.

📌 Summary Box
Plants use timing, flower sex separation, and pollinators to promote cross-pollination.
Animals and wind are main pollen carriers between plants.
These methods boost genetic diversity in plant populations.

D3.1.11 – Self-Incompatibility Mechanisms to Increase Genetic Variation

Why Self-Incompatibility?

Self-pollination = pollen fertilizes ovules of the same plant.
Leads to inbreeding → reduces genetic diversity and vigour.
Decreased diversity makes plants less adaptable to environmental changes and disease.

🌿 Self-Incompatibility (SI) Mechanisms

Genetic systems in many plants prevent fertilization by pollen from the same plant.
Ensure that male and female gametes come from different plants.
Helps maintain high genetic variation within the species.

🌿 How SI Works In Simple Way?

When pollen lands on the stigma, a genetic “recognition” system compares pollen and stigma alleles.
If pollen shares the same allele as the stigma, pollen tube growth is inhibited.
Prevents fertilization by genetically similar pollen.

🔍 Importance of SI

Promotes cross-pollination.
Increases offspring variation → better chance of survival and adaptation.
Found in many flowering plants as an evolutionary advantage.

📌 Summary Box
Self-pollination causes inbreeding and lowers genetic diversity.
Self-incompatibility mechanisms stop fertilization by same-plant pollen.
This boosts cross-pollination and genetic variation for healthier plant populations.

D3.1.12 – Dispersal and Germination of Seeds

🌿 Seed Dispersal vs Pollination

Process	Definition	Purpose
Pollination	Transfer of pollen to the stigma for fertilization	Leads to seed formation
Seed Dispersal	Movement of seeds away from the parent plant	Reduces competition and spreads species

🌿 Seed Dispersal

Seeds can be dispersed by:

Wind (e.g., dandelion seeds with parachutes)
Animals (seeds attach to fur or are eaten and excreted)
Water (seeds float to new locations)

Dispersal helps seeds reach suitable environments to grow.

🌿 Germination Process

Germination is the growth of a seed into a new plant.

Requires:

Suitable temperature
Adequate water
Oxygen for respiration

🌿 Growth and Development of the Embryo

The embryo in the seed begins to grow after water uptake.
Food reserves (stored in endosperm or cotyledons) are mobilized to provide energy and materials for growth.
Root (radicle) emerges first to anchor and absorb water.
Shoot (plumule) grows upwards toward light.

🔍 Real-World Example

Beans store food in cotyledons, which nourish the embryo during germination.
Dandelion seeds disperse by wind, allowing colonization of wide areas.

📌 Summary Box
Seed dispersal moves seeds away to reduce competition.
Germination is the start of growth, triggered by water, oxygen, and temperature.
Food reserves fuel embryo development until it can photosynthesize.

Additional Higher Level

D3.1.13 – Control of Developmental Changes in Puberty

Role of Gonadotropin-Releasing Hormone (GnRH)

During childhood, the hypothalamus releases low levels of GnRH.
At puberty onset, the hypothalamus increases GnRH release.
GnRH stimulates the pituitary gland to release:
- Luteinizing hormone (LH)
- Follicle-stimulating hormone (FSH)

Role of LH and FSH

LH and FSH act on the gonads (testes in males, ovaries in females).
They trigger the production of steroid sex hormones:
- Testosterone in males
- Oestrogen and progesterone in females

Effects of Increased Sex Hormones

Cause the physical and physiological changes of puberty:
- Growth of secondary sexual characteristics (e.g., body hair, breasts, voice deepening)
- Development of reproductive organs
- Onset of fertility

🔍 Summary of the Hormonal Control Pathway

Hypothalamus (↑ GnRH) → Pituitary (↑ LH & FSH) → Gonads (↑ sex hormones) → Puberty changes

📌 Summary Box
Puberty begins when the hypothalamus releases more GnRH.
GnRH causes LH and FSH release from the pituitary.
LH and FSH stimulate sex hormone production in gonads.
Sex hormones cause puberty’s physical changes.

D3.1.14 – Spermatogenesis and Oogenesis in Humans

🧫 What is Gametogenesis?

Process of producing gametes (sex cells).
Includes spermatogenesis in males and oogenesis in females.
Involves mitosis, cell growth, meiosis, and differentiation.

🧫 Spermatogenesis (Male Gamete Formation)

Occurs in testes.
Process:
- Mitosis: Spermatogonia divide to maintain the stem cell pool and produce primary spermatocytes.
- Cell growth: Primary spermatocytes grow before meiosis.
- Meiosis I & II: Primary spermatocytes → secondary spermatocytes → spermatids (4 haploid cells).
- Differentiation: Spermatids mature into sperm cells (spermatozoa).
Result: 4 functional sperm from each original spermatogonium.
Sperm have little cytoplasm for streamlined movement.

🧫 Oogenesis (Female Gamete Formation)

Occurs in ovaries.
Process:
- Mitosis: Oogonia divide during fetal development to produce primary oocytes.
- Cell growth: Primary oocytes grow and begin meiosis but pause until puberty.
- Meiosis I: Each primary oocyte completes meiosis I each menstrual cycle → produces:
  - One large secondary oocyte (most cytoplasm)
  - One small polar body (discarded)
- Meiosis II: Secondary oocyte completes meiosis II only if fertilization occurs → produces:
  - One mature ovum
  - One polar body
Result: 1 large ovum with most cytoplasm, 2-3 polar bodies (non-functional).
Large cytoplasm supports embryo development after fertilization.

Feature	Spermatogenesis	Oogenesis
Number of gametes	4 sperm per original cell	1 ovum per original cell
Cytoplasm amount	Very little (streamlined sperm)	Large amount in ovum
Timing	Continuous from puberty onwards	Starts before birth, pauses, resumes at puberty
Gamete size	Small and motile	Large and nutrient-rich

📌 Summary Box
Spermatogenesis produces many small sperm with little cytoplasm.
Oogenesis produces one large, nutrient-rich egg and small polar bodies.
Both start with mitosis, followed by growth, meiosis, and differentiation.
Differences support sperm mobility and egg nourishment.

D3.1.15 – Mechanisms to Prevent Polyspermy

🧬 What is Polyspermy?

Polyspermy is the fertilization of an egg by more than one sperm.
It causes abnormal chromosome numbers and usually prevents normal embryo development.
Therefore, mechanisms exist to block polyspermy.

🧬 Acrosome Reaction

The acrosome is a cap-like structure on the head of a sperm.
During fertilization, the acrosome releases enzymes.
These enzymes digest a path through the zona pellucida (a protective layer around the egg).
This allows one sperm to reach and fuse with the egg membrane.

🧬 Cortical Reaction

After one sperm fuses with the egg membrane, the egg releases chemicals from cortical granules.
These chemicals cause the zona pellucida to harden and chemically change.
This prevents other sperm from penetrating the zona pellucida.
The cortical reaction is a fast and effective block to polyspermy.

📌 Summary Box
Polyspermy is prevented by two key reactions:
• Acrosome reaction enables one sperm to penetrate.
• Cortical reaction blocks all other sperm.
These ensure normal chromosome number and healthy embryo development.

D3.1.16 – Development of a Blastocyst and Implantation in the Endometrium

🌱 Blastocyst Development

After fertilization, the zygote undergoes mitotic divisions as it moves down the fallopian tube.
These divisions produce a hollow ball of cells called the blastocyst.
The blastocyst has two main parts:
- Inner cell mass: will develop into the embryo.
- Outer layer (trophoblast): will form part of the placenta.

🌱 Implantation in the Endometrium

The blastocyst reaches the uterus about 5–7 days after fertilization.
It attaches to the thickened, nutrient-rich lining of the uterus called the endometrium.
The trophoblast cells secrete enzymes that help the blastocyst embed into the endometrium.
Successful implantation allows the blastocyst to receive nutrients and oxygen from the mother.

🔍 Key Points

Implantation is essential for establishing pregnancy.
The endometrium must be ready (thick and well-supplied with blood) for implantation to occur.
Only the blastocyst stage implants, not earlier embryonic stages.

📌 Summary Box

The blastocyst is a hollow ball of cells with an inner cell mass and outer trophoblast.
It implants into the uterus’s endometrium around 5–7 days after fertilization.
Implantation allows nutrient exchange critical for embryo development.

D3.1.17 – Pregnancy Testing by Detection of Human Chorionic Gonadotropin (hCG)

🌟 What is hCG?

Human chorionic gonadotropin (hCG) is a hormone produced by the embryo soon after fertilization.
Later, hCG is produced by the developing placenta.
Its presence in body fluids signals pregnancy.

🌟 How Pregnancy Tests Work

Pregnancy tests detect hCG in urine or blood.
They use monoclonal antibodies that specifically bind to hCG.
When hCG binds to these antibodies, it triggers a visible reaction (like a colored line or symbol).
This confirms the presence of pregnancy.

🔍 Key Points

hCG levels rise rapidly in early pregnancy.
Monoclonal antibodies provide high specificity for hCG, reducing false positives.
Early detection helps confirm pregnancy before symptoms appear.

📌 Summary Box

hCG is produced by the embryo and placenta during early pregnancy.
Pregnancy tests detect hCG using monoclonal antibodies.
A positive test indicates presence of hCG → confirms pregnancy.

D3.1.18 – Role of the Placenta in Foetal Development Inside the Uterus

Key Features of the Placenta

The placenta connects the developing foetus to the mother’s uterus.
It has a large surface area due to structures called placental villi.
This large surface area maximizes exchange between maternal and foetal blood.

Exchange Processes in the Placenta

Oxygen and nutrients (glucose, amino acids, vitamins) pass from mother’s blood to the foetus by diffusion and active transport.
Carbon dioxide and waste products (e.g., urea) move from the foetus to the mother for removal.
The placenta acts as a barrier, protecting the foetus from some harmful substances and pathogens, though some can cross.

🌟 Importance of the Placenta

Allows the foetus to develop inside the uterus for a longer time compared to mammals without a placenta.
This longer development results in a more fully developed foetus at birth, increasing survival chances.

📌 Summary Box

Placenta’s large surface area enables efficient nutrient, gas, and waste exchange.
Supports longer in-utero development than non-placental mammals.
Acts as a selective barrier between mother and foetus.

D3.1.19 – Hormonal Control of Pregnancy and Childbirth

🤰Hormonal Control During Pregnancy

Progesterone is key to maintaining pregnancy.
Early in pregnancy, progesterone is secreted by the corpus luteum (in the ovary).
Later, the placenta takes over progesterone production.
Progesterone keeps the uterine lining thick and stable, preventing contractions.

🧬Hormonal Changes Triggering Childbirth

As childbirth approaches, progesterone levels drop sharply.
This reduction allows secretion of oxytocin to increase.
Oxytocin stimulates uterine contractions (labour).
Uterine contractions cause more oxytocin to be released – a positive feedback loop that intensifies contractions until birth.

🔍 Key Points

Progesterone supports pregnancy by stopping uterine contractions.
Drop in progesterone triggers oxytocin release to start labour.
Positive feedback with oxytocin ensures strong, regular contractions.

Hormone	Role in Pregnancy and Childbirth
Progesterone	Maintains uterine lining, prevents contractions
Oxytocin	Stimulates and maintains uterine contractions during labour

D3.1.20 – Hormone Replacement Therapy (HRT) and the Risk of Coronary Heart Disease (CHD)

🧬Background: Early Observations

Early epidemiological studies showed that women on HRT had a lower incidence of CHD.
This was initially thought to mean HRT protected against heart disease.
Suggested a cause-and-effect relationship between HRT and reduced CHD risk.

🌿 Later Findings: Randomized Controlled Trials

Later randomized controlled trials found that HRT actually increased the risk of CHD slightly.
This contradicted the early observational studies.

🌿 Why the Difference?

Initial correlation was not causal.
Women using HRT often had higher socioeconomic status.
Higher socioeconomic status linked to lower CHD risk due to lifestyle, healthcare, diet.
Lower CHD incidence was due to socioeconomic factors, not HRT itself.

Study Type	Outcome on HRT and CHD Risk
Epidemiological studies	Observed lower CHD risk in HRT users (correlation)
Randomized controlled trials	Found slight increase in CHD risk from HRT (causal)

🔍 Key Concepts

Correlation ≠ causation: linked factors don’t always imply one causes the other.
Socioeconomic status can confound observational studies results.

📌 Summary Box
Early studies suggested HRT reduced CHD risk, but later trials showed a small increase.
Initial correlation was due to socioeconomic status, not a direct effect of HRT.
Careful experimental design is needed to distinguish correlation from causation.

IB DP Biology Reproduction Study Notes | New Syllabus

IB DP Biology Reproduction Study Notes - New Syllabus -2025