Edexcel GCSE Biology Topic 3 Genetics: a complete overview of reproduction, meiosis, DNA, inheritance and variation

What Topic 3 actually demands

Genetics is one of the most heavily examined topics, and it rewards precise vocabulary and confident handling of genetic crosses. The examiners test the contrast between mitosis and meiosis, the structure of DNA, the mechanics of inheritance with Punnett squares and pedigrees, and the difference between genetic and environmental variation. This guide ties together the four dot-point pages, each with its own practice questions.

Reproduction and meiosis

Asexual reproduction needs one parent and gives genetically identical offspring quickly, but with no variation. Sexual reproduction needs two parents and gametes and gives variation, which aids survival but is slower. Meiosis makes the gametes: it produces four genetically different haploid cells, halving the chromosome number so that fertilisation restores it. This halving and doubling keeps the chromosome number constant each generation.

DNA and the genome

DNA is a polymer of two strands in a double helix, held together by complementary base pairs: A with T, C with G. The genome is all of an organism's DNA; a gene is a section that codes for a protein. The order of bases sets the order of amino acids (three bases per amino acid), and the protein folds into a specific shape. Protein synthesis (Biology only) happens by transcription (making mRNA) and translation (building the protein at a ribosome).

Inheritance and variation

A gene has versions called alleles, which can be dominant or recessive. The genotype is the alleles present; the phenotype is the characteristic shown. Punnett squares predict offspring ratios, and pedigrees track inheritance through families. Sex is determined by the sex chromosomes (XX female, XY male), giving a 1:1 ratio; codominance in the ABO blood groups means both alleles show.

Most features are controlled by many genes and show a continuous range. Variation is genetic, environmental, or both, and arises ultimately from mutations, most of which have little effect. The Human Genome Project mapped all human genes, with major uses in medicine.

How Topic 3 is examined

• Calculations. Genetic crosses with Punnett squares, ratios, percentages and probabilities, and chromosome numbers through meiosis and fertilisation.
• Definitions. The genetic terms (allele, dominant, homozygous, genotype, phenotype and so on), which mark schemes reward precisely.
• Comparisons. Mitosis versus meiosis, sexual versus asexual reproduction, genetic versus environmental variation.
• Application. Interpreting pedigrees and predicting blood groups with codominance.

Check your knowledge

A mix of recall and calculation questions covering Topic 3. Attempt them under timed conditions, then check against the solutions.

1. State two ways cells made by meiosis differ from those made by mitosis. (2 marks)
2. State which base pairs with adenine and which pairs with cytosine in DNA. (2 marks)
3. Define the terms genotype and phenotype. (2 marks)
4. In pea plants, tall (T) is dominant to short (t). A Tt plant is crossed with a tt plant. Give the ratio of tall to short offspring. (3 marks)
5. Two parents are both heterozygous (Aa) for a recessive disorder. Calculate the probability their child is affected. (2 marks)
6. Give one purely genetic feature and one purely environmental feature. (2 marks)
7. State the source of all genetic variation. (1 mark)
8. Explain why gametes are haploid. (2 marks)