Understanding Inheritance: IGCSE Biology Explained

Ever wondered why you have your mum's eyes but your dad's nose?

That question sits right at the heart of inheritance in IGCSE Biology. It's also one of the topics that comes up in nearly every Cambridge IGCSE exam sitting, so getting comfortable with it now will pay off big time. The good news? Once you understand a few core ideas, the rest clicks into place like puzzle pieces.

This guide walks you through chromosomes, genes, and alleles, then moves into cell division (mitosis and meiosis), and finishes with the bit students either love or dread: genetic diagrams and Punnett squares. By the end, you'll know exactly how to tackle inheritance questions with confidence.

Chromosomes, genes and DNA

Your body is made up of trillions of cells, and almost every one of them contains a nucleus. Inside that nucleus, you'll find thread-like structures called chromosomes. Chromosomes are made of a molecule called DNA (deoxyribonucleic acid), and it's the DNA that carries your genetic information.

Here's the key chain to remember: DNA makes up chromosomes. Sections of DNA that code for specific proteins are called genes. And different versions of the same gene are called alleles.

Human body cells contain 46 chromosomes, arranged in 23 pairs. One chromosome in each pair comes from your mother, the other from your father. That's why you inherit characteristics from both parents.

Key genetic terminology

Sex determination

Of your 23 chromosome pairs, 22 are autosomes (non-sex chromosomes). The 23rd pair determines your biological sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

Because a mother always passes on an X chromosome, it's the father's contribution that determines the sex of the child. If the sperm carries an X, the child is female. If it carries a Y, the child is male. This gives a 50:50 chance of either sex with each pregnancy.

Mitosis: making identical copies

Your body grows and repairs itself through a type of cell division called mitosis. Before a cell divides, it copies all of its DNA so that each new cell gets an exact replica. The result? Two genetically identical daughter cells, each with the full set of 46 chromosomes.

Mitosis happens everywhere in your body: in your skin when a cut heals, in your bones as you grow taller, in your gut lining as old cells are replaced. It's also how organisms like strawberry plants reproduce asexually, producing clones of themselves.

Meiosis: making sex cells

If mitosis is about copying, meiosis is about halving. Meiosis only happens in the reproductive organs (ovaries and testes in humans). It produces gametes (egg cells and sperm cells) that contain half the normal chromosome number, so 23 chromosomes instead of 46.

Why halve them? Because when two gametes fuse during fertilisation, the full number is restored: 23 from mum + 23 from dad = 46 in the new organism. If gametes had 46 chromosomes, the offspring would end up with 92, and the number would double with every generation.

Meiosis also introduces genetic variation in two ways. First, the chromosome pairs are shuffled randomly during division (independent assortment). Second, sections of DNA can swap between partner chromosomes (crossing over). That's why siblings from the same parents look different from each other.

Mitosis vs meiosis at a glance

Monohybrid inheritance

This is the section that earns you the most marks in the exam, so it's worth spending extra time here. Monohybrid inheritance looks at how one gene (with two alleles) is passed from parents to offspring.

Setting up a genetic diagram: step by step

Let's work through an example. In a species of plant, the allele for tall stems (T) is dominant over the allele for short stems (t). Two heterozygous tall plants are crossed.

1. Write down the parental phenotypes: Tall x Tall
2. Write down the parental genotypes: Tt x Tt
3. Identify the gametes each parent can produce: Parent 1 can produce T or t. Parent 2 can produce T or t.
4. Draw a Punnett square:

5. Read the offspring genotypes: TT, Tt, Tt, tt
6. State the offspring phenotypes and ratio: 3 tall : 1 short (genotypic ratio 1 TT : 2 Tt : 1 tt)

Another worked example: sickle cell trait

Sickle cell anaemia is controlled by a single gene with two alleles. The normal haemoglobin allele (H^A) and the sickle cell allele (H^S) show codominance, which means neither allele is dominant over the other. When a person is heterozygous (H^AH^S), they have sickle cell trait: they produce both normal and sickle-shaped red blood cells.

Cross: carrier father (H^AH^S) x carrier mother (H^AH^S)

Offspring: 1 normal (H^AH^A) : 2 sickle cell trait/carrier (H^AH^S) : 1 sickle cell anaemia (H^SH^S)

With codominance, you can't use capital and lowercase letters for the same base letter. Instead, use a shared letter with superscripts (like H^A and H^S) to show that both alleles contribute to the phenotype.

Why inheritance matters beyond the exam

Understanding inheritance isn't just about passing your IGCSE Biology exam. It's the foundation for genetics at A-Level and beyond, and it connects to real-world applications you encounter every day. Genetic testing, selective breeding in agriculture, understanding inherited medical conditions, and even forensic science all rely on the principles you're learning right now. Medical professionals use Punnett squares to counsel families about the probability of inherited conditions. Agricultural scientists use knowledge of dominant and recessive alleles to breed crops with desirable traits like disease resistance or higher yield. If you're considering a career in medicine, veterinary science, agriculture, or biotechnology, this topic is where your journey starts.

Common exam mistakes

These are the errors that crop up again and again in examiner reports. Avoid them and you're already ahead of most candidates.

• Confusing mitosis and meiosis. Mitosis = two identical cells, same chromosome number. Meiosis = four varied cells, half the chromosome number. Don't mix them up. If the question says "gametes," the answer is meiosis. If it says "growth," the answer is mitosis.
• Messy Punnett squares. Label the gametes across the top and down the side, not the genotypes. Each box should contain exactly two alleles. Putting the full parental genotype in the margin is a common source of confusion.
• Using the wrong letter convention for codominance. Standard dominant/recessive uses a capital and lowercase of the same letter (B and b). Codominance requires a shared base letter with superscript variants (C^R and C^W). Using Bb notation for a codominance question will lose you marks even if the ratios are correct.
• Forgetting to state the ratio. Drawing a perfect Punnett square but not writing out the phenotypic ratio is like running a race and stopping one metre before the finish line. Always state it: "3 tall : 1 short" or "1 : 2 : 1" for codominance.
• Saying meiosis produces "different" cells without explaining why. Examiners want you to mention independent assortment and crossing over as the sources of genetic variation. Just saying "they're different" isn't enough.
• Mixing up genotype and phenotype. Genotype is the letters (Bb). Phenotype is the physical trait (brown fur). If a question asks for phenotype, don't write "Bb" as your answer.

Self-check questions

Test yourself before moving on. Try answering these without looking back at your notes first.

1. What is the difference between a gene and an allele?
2. A cell with 46 chromosomes undergoes meiosis. How many chromosomes does each daughter cell contain?
3. Two heterozygous brown mice (Bb) are crossed. What ratio of brown to white offspring would you expect? Draw the Punnett square to support your answer.
4. Explain why siblings from the same two parents are not genetically identical.
5. In snapdragon flowers, red (C^RC^R) and white (C^WC^W) show codominance. The heterozygous phenotype is pink. Cross two pink flowers (C^RC^W) and state the expected phenotypic ratio.

Inheritance can feel like a lot of new vocabulary at first, but the underlying logic in IGCSE Biology is surprisingly straightforward. Learn the terminology, practise your Punnett squares until they're automatic, and always label every step in your genetic diagrams. Do that, and you'll pick up marks that many other IGCSE candidates leave on the table.