How genetic information is passed on, why organisms differ, and how species change over time
This topic explains how traits are inherited, why individuals vary, and how evolution shapes life on Earth. It builds directly on cell biology and homeostasis, and prepares students for ecology and biotechnology.
GCSE Exam Essentials
Students must be able to:
- Describe the structure of DNA and the role of genes and chromosomes
- Explain sexual and asexual reproduction
- Understand inherited characteristics and genetic diagrams (Punnett squares)
- Describe variation (genetic and environmental)
- Explain natural selection and evolution
- Understand selective breeding and genetic engineering
- Interpret genetic crosses, probability, and inheritance patterns
These points appear across AQA, Edexcel, and OCR GCSE Biology specifications.
1. DNA, Genes and Chromosomes
DNA
DNA is a long molecule that carries genetic information.
Genes
A gene is a section of DNA that codes for a specific protein.
Chromosomes
DNA is packaged into chromosomes. Humans have 46 chromosomes (23 pairs).
“DNA contains genetic information; genes control traits; chromosomes carry DNA.”
Alleles
Different versions of the same gene.
- Dominant alleles: show their effect even if only one copy is present
- Recessive alleles: need two copies to be expressed
2. Reproduction and Inheritance
Asexual Reproduction
- One parent
- Offspring are genetically identical (clones)
- No variation except for mutations
Examples: bacteria, some plants.
Sexual Reproduction
- Two parents
- Gametes (sperm and egg) fuse
- Offspring show genetic variation
- Involves meiosis, which halves the chromosome number
This variation is essential for evolution.
Genetic Crosses (Punnett Squares)
Used to predict inheritance patterns.
Key terms:
- Genotype: genetic makeup
- Phenotype: physical appearance
- Homozygous: two identical alleles
- Heterozygous: two different alleles
Students must interpret simple monohybrid crosses.
3. Variation
Variation is the difference between individuals.
Genetic Variation
Caused by:
- Different alleles
- Sexual reproduction
- Mutations
Environmental Variation
Caused by:
- Diet
- Climate
- Lifestyle
- Surroundings
Most traits are influenced by both.
4. Evolution and Natural Selection
Natural Selection
Proposed by Charles Darwin.
Steps:
- Variation exists in a population
- Some individuals are better adapted
- They survive and reproduce
- Useful traits become more common
This explains how species change over time.
Speciation
Over long periods, populations can become so different that they form new species.
5. Selective Breeding
Humans choose organisms with desirable traits and breed them.
Examples:
- High‑yield crops
- Disease‑resistant plants
- Animals with good temperament
Risks:
- Reduced gene pool
- Inherited health problems
6. Genetic Engineering
Genes are transferred from one organism to another.
Examples:
- Insulin‑producing bacteria
- GM crops
- Disease‑resistant plants
This links to biotechnology and ethics.
7. Common Misconceptions (GCSE‑specific)
Students often:
- Confuse genes and chromosomes
- Think evolution happens to individuals (it happens to populations)
- Believe dominant alleles are “better” (they’re not)
- Mix up genetic and environmental variation
- Think that selective breeding and genetic engineering are the same
“Confusing genes and chromosomes” “Thinking evolution happens in individuals”
8. Quick Check Questions
Use these for active recall:
- What is a gene?
- What is the difference between sexual and asexual reproduction?
- What is an allele?
- How does natural selection lead to evolution?
- Give one example of selective breeding and one example of genetic engineering.
9. Summary
Inheritance explains how traits are passed on. Variation explains why individuals differ. Evolution explains how species change over time.
Together, they form one of the most important and fascinating areas of GCSE Biology.