Understanding Reproduction and heredity: Edexcel IGCSE Human Biology Explained

Reproduction and heredity brings together two large ideas: how humans produce offspring and how characteristics pass from one generation to the next. The edexcel igcse human biology reproduction and heredity section rewards logical thinking, and getting the logic right here pays off heavily on both Edexcel IGCSE Human Biology papers.

This section of the specification spans fertilisation, embryonic development, the reproductive systems, hormonal control of the menstrual cycle, contraception, IVF, meiosis, inheritance patterns and genetic conditions. It is dense, but every part follows a clear chain of reasoning. The edexcel igcse human biology revision notes below break the content into manageable blocks, each with the level of detail the exam expects.

Fertilisation and early development

Fertilisation is the fusion of a male gamete (sperm) and a female gamete (egg, or ovum) to produce a zygote. The zygote is diploid, meaning it contains the full complement of 46 chromosomes: 23 from the sperm and 23 from the egg. The zygote then divides repeatedly by mitosis to form a ball of cells, which develops into an embryo and eventually a fetus.

Every Edexcel IGCSE candidate should be able to state that fertilisation involves gamete fusion, that it produces a zygote, and that the zygote is diploid. These are foundational definitions that appear across both papers.

Meiosis

Meiosis is the type of cell division that produces gametes. Unlike mitosis, which produces two genetically identical diploid cells, meiosis produces four genetically different haploid cells, each with half the chromosome number (23 in humans). Two features of meiosis are particularly important for the exam:

• Halving the chromosome number - this ensures that when two gametes fuse at fertilisation, the resulting zygote has the correct diploid number (46). Without meiosis, the chromosome number would double with every generation.
• Producing genetic variation - meiosis shuffles alleles through independent assortment and crossing over. This means each gamete is genetically unique, which is why siblings (other than identical twins) differ from each other.

The male and female reproductive systems

The specification requires knowledge of the structure and function of both reproductive systems. Here are the key structures:

Specialised reproductive cells

Sperm and egg cells are adapted for their roles. The sperm has a streamlined head, an acrosome containing enzymes to penetrate the egg, many mitochondria in the midpiece to provide energy for swimming, and a long tail (flagellum) for propulsion. The egg is much larger, with a cytoplasm rich in nutrients to supply the early embryo, a cell membrane that changes after fertilisation to prevent entry of additional sperm, and a haploid nucleus carrying 23 chromosomes.

The menstrual cycle

The menstrual cycle is controlled by four hormones, and the Edexcel IGCSE exam expects you to know the role of each one:

1. FSH (follicle-stimulating hormone) - produced by the pituitary gland; stimulates a follicle to develop in the ovary and causes the egg to mature
2. Oestrogen - produced by the developing follicle in the ovary; causes the uterine lining to thicken and rebuild after menstruation; stimulates the release of LH
3. LH (luteinising hormone) - produced by the pituitary gland; triggers ovulation (the release of the mature egg from the ovary)
4. Progesterone - produced by the corpus luteum (the empty follicle after ovulation); maintains the thick uterine lining; if pregnancy does not occur, progesterone levels fall, and the lining breaks down (menstruation)

These hormones interact through feedback loops. Rising oestrogen levels stimulate LH release (a positive feedback loop), which causes ovulation around day 14 of a typical 28-day cycle. After ovulation, progesterone maintains the lining. If no embryo implants, progesterone falls, menstruation begins, and the cycle restarts.

Pregnancy

During pregnancy, progesterone levels remain high to maintain the uterine lining. The placenta, which develops from the embryo and the uterine wall, takes over progesterone production and serves several functions:

• Exchange of substances between maternal and fetal blood (oxygen and nutrients pass from mother to fetus; carbon dioxide and urea pass from fetus to mother)
• The maternal and fetal blood supplies do not mix directly, preventing blood pressure differences from damaging the fetus and reducing the risk of infection transfer

The umbilical cord connects the fetus to the placenta. It contains the umbilical artery (carrying deoxygenated blood from the fetus to the placenta) and the umbilical vein (carrying oxygenated blood from the placenta to the fetus). The amniotic fluid surrounds the fetus, cushioning it against physical damage and maintaining a stable temperature.

Birth and breastfeeding

The birth process involves strong contractions of the uterine wall muscles, dilation of the cervix, and delivery of the baby followed by the placenta (afterbirth). Breastfeeding offers several advantages: breast milk contains antibodies that provide passive immunity to the newborn, it provides the correct balance of nutrients, it is sterile and at the right temperature, and it promotes bonding between mother and child.

Growth and development

Growth curves for humans show a characteristic pattern. Growth is rapid in infancy, slows during childhood, accelerates again during puberty (the adolescent growth spurt), and levels off in adulthood. During puberty, the sex hormones oestrogen (in females) and testosterone (in males) trigger the development of secondary sexual characteristics:

• In males: deepening of the voice, growth of facial and body hair, increased muscle mass, broadening of the shoulders
• In females: breast development, widening of the hips, onset of menstruation, redistribution of body fat

Contraception

The specification covers several methods of contraception. The exam may ask you to describe a method, explain how it works, or compare advantages and disadvantages.

In vitro fertilisation (IVF)

IVF involves collecting eggs from the ovaries (after hormone treatment to stimulate multiple egg production), fertilising them with sperm in a laboratory, and transferring one or more embryos into the uterus. IVF can help couples who have difficulty conceiving naturally, for example due to blocked oviducts, low sperm count or unexplained infertility. The process is expensive, emotionally demanding, and not always successful. Multiple embryo transfer can lead to multiple pregnancies, which carry higher health risks.

Heredity and genetics

This is where the subject becomes highly logical, and the edexcel igcse human biology practice questions in this area reward systematic working. The key terminology must be precise:

• Gene - a length of DNA that codes for a specific protein
• Allele - an alternative form of a gene
• Dominant - an allele that is expressed in the phenotype when one or two copies are present
• Recessive - an allele that is only expressed when two copies are present (homozygous recessive)
• Homozygous - having two identical alleles for a gene (e.g. AA or aa)
• Heterozygous - having two different alleles for a gene (e.g. Aa)
• Genotype - the alleles present in an organism (e.g. Aa)
• Phenotype - the physical expression of the genotype (e.g. brown eyes)
• Co-dominance - both alleles are expressed in the phenotype of a heterozygous individual
• Diploid - a cell with two complete sets of chromosomes (2n = 46 in humans)
• Haploid - a cell with one set of chromosomes (n = 23 in humans)

Sex determination

Sex is determined by the 23rd pair of chromosomes. Females have XX and males have XY. At fertilisation, every egg carries an X chromosome, while a sperm carries either X or Y. If an X-bearing sperm fertilises the egg, the offspring is female (XX). If a Y-bearing sperm fertilises the egg, the offspring is male (XY). A simple genetic diagram shows that the probability of a boy or a girl is 1 in 2 (50:50) for each pregnancy.

Monohybrid inheritance: worked example

Question: In humans, brown eyes (B) is dominant over blue eyes (b). Two heterozygous parents have children. Draw a genetic diagram to show the possible genotypes and phenotypes of the offspring, and give the expected ratio. (4 marks)

Model answer:

Parental genotypes: Bb x Bb

Parental gametes: B or b from each parent

Genotype ratio: 1 BB : 2 Bb : 1 bb

Phenotype ratio: 3 brown eyes : 1 blue eyes

ABO blood groups: multiple alleles and co-dominance

The ABO blood group system involves three alleles: I^A, I^B and I^O. I^A and I^B are co-dominant with each other, and both are dominant over I^O. This means:

• I^AI^A or I^AI^O produces blood group A
• I^BI^B or I^BI^O produces blood group B
• I^AI^B produces blood group AB (both alleles expressed)
• I^OI^O produces blood group O

Questions on blood groups test your ability to construct genetic diagrams with three alleles and to identify possible offspring genotypes from given parental blood groups.

Sex-linked inheritance

Some genes are located on the X chromosome and have no corresponding allele on the shorter Y chromosome. Haemophilia and red-green colour blindness are sex-linked conditions caused by recessive alleles on the X chromosome. Because males have only one X chromosome, a single recessive allele will be expressed. Females need two recessive alleles to show the condition; with one dominant and one recessive, they are carriers.

Notation: X^H = normal allele, X^h = haemophilia allele. A carrier female is X^HX^h. An affected male is X^hY.

Genetic conditions

Gene therapy for cystic fibrosis involves using a virus as a vector to deliver a normal copy of the defective gene into the lung cells. The approach is still being developed and the effects are not permanent, so treatment must be repeated.

Family pedigrees

Pedigree diagrams use circles (females) and squares (males), with shaded symbols for affected individuals. The edexcel igcse human biology explained approach to pedigrees is to work backwards: determine whether the condition is dominant or recessive (if two unaffected parents have an affected child, the condition must be recessive), then assign genotypes starting with the individuals whose genotype is certain.

Variation

Variation can be genetic (caused by differences in DNA, inherited from parents), environmental (caused by living conditions), or a combination of both. Height, for example, is influenced by genes but also by nutrition during growth. Eye colour is almost entirely genetic. The distinction matters because the exam expects you to identify which type of variation applies in a given scenario.

Random fertilisation contributes to genetic variation because any sperm can fuse with the egg, producing a unique combination of alleles each time. Combined with meiosis, this ensures enormous diversity within a population.

Self-check questions

Use these to test your knowledge of reproduction and heredity edexcel igcse content before working through full papers. These cover IGCSE 4HB1 reproduction and heredity material at exam standard.

1. Define the term "fertilisation" and state the chromosome number of the resulting zygote.
2. Explain two ways in which meiosis differs from mitosis.
3. Name the four hormones that control the menstrual cycle and state the role of each.
4. Draw a genetic diagram to show the inheritance of cystic fibrosis when both parents are carriers. What proportion of offspring would you expect to be affected?
5. A man with blood group A (genotype I^AI^O) and a woman with blood group B (genotype I^BI^O) have children. Use a Punnett square to determine the possible blood groups of their children.
6. Explain why haemophilia is more common in males than in females.
7. State two advantages and two disadvantages of IVF.
8. A couple's first three children are all girls. What is the probability that their fourth child will be a boy? Explain your answer.

Thorough practice with these edexcel igcse human biology notes builds the systematic thinking that genetics questions demand. Work through each genetic diagram on paper, not in your head, and check your ratios against the mark scheme. For additional topic-organised questions and performance tracking, explore the Green Bridge CBT platform's dedicated edexcel igcse human biology explained resources.