# Inheritance

> Biology · Edexcel IGCSE Biology 4BI1
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s3-inheritance/

This guide covers all mandatory inheritance content for Edexcel IGCSE Biology (4BI1), including genetic diagrams, cell division, natural selection, and Biology-only Paper 2 topics such as protein synthesis and codominance.

**Prerequisites:** [Basic cell and nucleus structure](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-cell-structure/); [Asexual vs sexual reproduction basics](https://www.owlsprep.com/study/edexcel-igcse-biology-s3-reproduction/)

## Learning objectives

- Define core genetic terms including gene, allele, genotype, phenotype and mutation
- Construct and mark-perfect monohybrid genetic diagrams and sex determination crosses
- Compare mitosis and meiosis, and explain their roles in growth, reproduction and variation
- Describe protein synthesis, codominance and mutagen effects (Biology-only Paper 2 content)
- Explain Darwin’s theory of natural selection and the development of antibiotic resistance in bacteria

## Core Genetic & DNA Fundamentals

Genetic information is stored in the nucleus of cells, on structures called chromosomes. Each chromosome is made of DNA, and a **gene** is a short section of DNA that codes for one specific protein. The full set of DNA in an organism is called its genome.

**Gene & Genome** — A gene is a section of DNA coding for a specific protein. The genome is the entire DNA of an organism, stored on chromosomes in the cell nucleus.

> **Biology-only (Paper 2) Content**
>
> The following DNA, RNA and protein synthesis content is only assessed for single-award Biology students (4BI1 Paper 2). Double Award students can skip this section.

DNA has a double helix structure, made of two linked strands. The strands are joined by complementary base pairs: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). RNA is a single-stranded molecule that uses uracil (U) instead of thymine.

Protein synthesis has two key stages: *transcription* (mRNA copies a DNA sequence in the nucleus and moves to a ribosome) and *translation* (tRNA molecules bring amino acids to the ribosome, matching codons on mRNA to anticodons on tRNA to build a protein chain).

**Worked example:** A section of DNA has the base sequence GAT CCA. State the complementary DNA strand, the complementary mRNA strand, and name the two stages of protein synthesis.

1. 1. Complementary DNA follows A-T, C-G base pairing rules: CTA GGT
2. 2. mRNA replaces thymine (T) with uracil (U): CUA GGU
3. 3. The two stages of protein synthesis are transcription and translation

> **Exam tip:** For base pairing questions, always double check if the sequence is DNA or RNA before writing your answer: RNA never contains thymine (T).

*Calculator:* allowed

## Genetic Terminology & Monohybrid Crosses

Genes exist in alternative forms called alleles, which produce variation in inherited characteristics. Most phenotypic features are controlled by multiple genes (polygenic inheritance), but monohybrid crosses study characteristics controlled by a single gene pair.

**Key Genetic Terms** — Genotype = alleles present; Phenotype = observable characteristics; Homozygous = two identical alleles; Heterozygous = two different alleles; Dominant = expressed with one copy; Recessive = expressed only with two identical copies.

> **Biology-only (Paper 2) Content**
>
> Codominance, where both alleles are expressed in heterozygous organisms (e.g. AB blood group), is only assessed for single-award Biology students.

Monohybrid genetic diagrams must follow Edexcel’s mandatory structure to get full marks. You can also use pedigree diagrams to track inheritance of characteristics across family generations.

**Worked example:** Tongue rolling is a dominant trait (T) and non-rolling is recessive (t). Cross two heterozygous tongue-rolling parents, and state the probability of their child being a non-roller.

1. 1. Parental phenotypes: both tongue rollers; Parental genotypes: Tt × Tt
2. 2. Gametes (circled): T, t from each parent
3. 3. Punnett square: Offspring genotypes = TT, Tt, Tt, tt
4. 4. Phenotype ratio: 3 tongue rollers : 1 non-roller; Probability of non-roller = 25% (or 0.25, or 1 in 4)

Sex is controlled by one pair of chromosomes: females have XX chromosomes, males have XY chromosomes. A cross between a male and female always produces a 1:1 ratio of male to female offspring, and the father determines the sex of the child.

> **Exam tip:** Always circle gamete alleles in genetic diagrams: this is a mandatory mark point in all Edexcel inheritance questions.

*Calculator:* allowed

## Mitosis, Meiosis & Genetic Variation

**Ploidy** — Human diploid cells have 46 chromosomes (full set), human haploid gametes have 23 chromosomes (half the full set).

| Feature | Mitosis | Meiosis |
| --- | --- | --- |
| Number of daughter cells | 2 | 4 |
| Chromosome number | Diploid (same as parent) | Haploid (half parent) |
| Genetic similarity | Identical to parent cell | Genetically different from parent and each other |
| Uses | Growth, repair, cloning, asexual reproduction | Production of gametes for sexual reproduction |

Genetic variation in offspring comes from meiosis (which produces genetically unique gametes) and random fertilisation (any male gamete can fuse with any female gamete).

**Worked example:** Explain why meiosis is essential for sexual reproduction to maintain a consistent chromosome number across generations.

1. 1. Meiosis produces haploid gametes with 23 chromosomes, half the diploid number.
2. 2. When two gametes fuse at fertilisation, the resulting zygote has the full diploid number of 46 chromosomes.
3. 3. Without meiosis, the chromosome number would double every generation, which is not viable.

> **Exam tip:** Use the mnemonic GRAC to remember mitosis uses: Growth, Repair, Asexual reproduction, Cloning. Never state mitosis produces gametes.

*Calculator:* allowed

## Variation & Mutation

Variation within a species can be genetic, environmental, or a combination of both. A mutation is a rare, random change in genetic material that can be inherited.

> **Biology-only (Paper 2) Content**
>
> Mutation effects and mutagens are only assessed for single-award Biology students (4BI1 Paper 2).

A change in DNA sequence can alter the amino acid sequence of the protein it codes for, changing the phenotype. Most mutations have no effect on phenotype, some have a small effect, and rarely they have a large effect. Mutation rates can be increased by exposure to ionising radiation (gamma rays, x-rays, UV light) and chemical mutagens (e.g. chemicals in tobacco).

**Worked example:** Explain why regular use of sunbeds increases the risk of skin cancer.

1. 1. UV radiation from sunbeds is an ionising mutagen that increases mutation rate.
2. 2. A rare mutation may occur in a skin cell that controls cell division.
3. 3. This mutation leads to uncontrolled cell growth, resulting in a cancerous tumour.

> **Exam tip:** Never state that all mutations are harmful: most have no effect on the phenotype of an organism.

*Calculator:* allowed

## Natural Selection & Antibiotic Resistance

**Natural Selection (Darwin’s Theory)** — 1. Variation exists in a population, 2. There is a struggle for survival, 3. Organisms with advantageous characteristics survive (survival of the fittest), 4. Survivors reproduce and pass on their advantageous alleles, 5. Over generations, the favourable characteristic becomes more common in the population.

Antibiotic resistance in bacteria is a common example of natural selection that is regularly assessed in exams.

**Worked example:** Explain how antibiotic-resistant strains of bacteria develop, and why they make infections harder to treat.

1. 1. A random, pre-existing mutation in a bacterial population produces an individual with antibiotic resistance.
2. 2. Antibiotics kill all non-resistant bacteria, leaving resistant bacteria with no competition for resources.
3. 3. Resistant bacteria reproduce rapidly, passing the resistance allele to all their offspring.
4. 4. Over time, the entire population becomes resistant, so antibiotics no longer work to treat the infection.

> **Exam tip:** Never state that bacteria 'try' or 'choose' to become resistant: mutations are random and occur before exposure to antibiotics.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Forgetting to circle gametes in genetic diagrams
  - Why it fails: Circled gametes are a mandatory mark point in all Edexcel inheritance questions, missing them loses marks even if the rest of the diagram is correct.
  - Correct: Draw a clear circle around every gamete allele in your genetic diagram.
- **Wrong:** Stating that the mother determines the sex of offspring
  - Why it fails: Females only produce X gametes, while males produce X and Y gametes, so the father’s gamete determines the sex of the child.
  - Correct: Always explicitly state that the male parent determines offspring sex, with a 50% chance of each sex.
- **Wrong:** Confusing mitosis and meiosis functions, stating meiosis is used for growth
  - Why it fails: Meiosis only produces haploid gametes for sexual reproduction, growth is a function of mitosis.
  - Correct: Use the GRAC mnemonic for mitosis uses: Growth, Repair, Asexual reproduction, Cloning.
- **Wrong:** Writing that bacteria adapt to antibiotics by mutating on purpose
  - Why it fails: Mutations are random, and occur before exposure to antibiotics. Antibiotics only select for pre-existing resistant bacteria.
  - Correct: Explain that random pre-existing mutations give some bacteria resistance, which is selected for when antibiotics are used.
- **Wrong:** Using thymine (T) in RNA sequences (Biology-only)
  - Why it fails: RNA uses uracil (U) instead of thymine, so writing T in an RNA sequence is a mark-loss error.
  - Correct: Double check base pairing questions: replace T with U for all RNA sequences.
- **Wrong:** Giving a 9:3:3:1 ratio for monohybrid crosses
  - Why it fails: Dihybrid crosses are out of scope for this specification, so 9:3:3:1 ratios are never correct for monohybrid questions.
  - Correct: Use 3:1 for heterozygous × heterozygous monohybrid crosses, and 1:1 for heterozygous × homozygous recessive crosses.

## Cheatsheet

| Concept | Key Fact | Exam Mark Tip |
| --- | --- | --- |
| Genome & Gene | Genome = full DNA of organism; Gene = section of DNA coding for 1 protein | Do not mix these terms up in definition questions |
| Monohybrid Cross Ratio | Heterozygous × Heterozygous = 3 dominant : 1 recessive phenotype | Circle all gametes and state the ratio clearly for full marks |
| Sex Determination | Female = XX, Male = XY; 50% chance of each sex | Father determines sex of offspring, never state mother does |
| Mitosis | 2 identical diploid daughter cells | Used for growth, repair, cloning, asexual reproduction only |
| Meiosis | 4 genetically different haploid gametes | Halves chromosome number to maintain diploid number at fertilisation |
| Natural Selection | Variation → struggle for survival → survival of fittest → pass on alleles → trait common | Write all 5 steps for 4+ mark explanation questions |
| Antibiotic Resistance | Random mutation → non-resistant killed → resistant reproduce | Never state bacteria 'choose' to become resistant |
| Biology-only: Base Pairs | DNA: A-T, C-G; RNA: A-U, C-G, single-stranded | Do not use thymine (T) in RNA sequences |

## What's next

Now that you have mastered inheritance concepts, you can apply this foundational knowledge to related Edexcel IGCSE Biology topics including cloning, genetic modification, and ecology, where genetic variation and adaptation drive ecosystem stability and species survival. Inheritance questions make up ~15% of both Paper 1 and Paper 2 exams, so practice past paper genetic diagram questions to perfect your structure and avoid common mark pitfalls, especially for Biology-only Paper 2 content such as protein synthesis and codominance. Make sure you also review related content on reproduction to contextualise meiosis and gamete formation.

- [Reproduction Basics](https://www.owlsprep.com/study/edexcel-igcse-biology-s3-reproduction/)

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