Unit 6: Inheritance, variation and evolution
AQA GCSE Biology Unit 6 common mistakes: genetics without the panic
Unit 6 can feel heavy because it contains genetics diagrams, evolution, selective breeding, GM crops, fossils, and classification. It is a lot. The good news is that many lost marks come from the same few mix-ups.
Quick takeaways
- Learn the difference between DNA, chromosome, gene, allele, genotype, and phenotype.
- Punnett square marks depend on symbols, gametes, genotypes, and giving probability in the exact form the question asks for (fraction, ratio, percentage, or decimal).
- Natural selection answers need variation, advantage, survival, reproduction, and inheritance.
- Do not confuse selective breeding with genetic engineering.
Mistake 1: using DNA words as if they are interchangeable
A chromosome is a long molecule of DNA. A gene is a section of DNA that codes for a protein or characteristic. An allele is a version of a gene. A genotype is the combination of alleles. A phenotype is the characteristic shown.
Tiny test
If you can explain those six words without looking, Unit 6 immediately becomes more manageable. Still genetics, obviously, but less of a guessing game.
AQA evidence
AQA genetics questions often lose students marks because symbols, phenotypes, probabilities or key words are missing. The biology may be understood, but the method has to be visible on the page.
Mistake 2: muddling mitosis and meiosis
Mitosis makes genetically identical body cells for growth and repair. Meiosis makes gametes with half the number of chromosomes. Fertilisation restores the full number.
- Mitosis: two identical cells, same chromosome number, growth and repair.
- Meiosis: four gametes, half chromosome number, genetically different cells.
- Fertilisation: gametes fuse, combining genetic information from two parents.
Mistake 3: Punnett squares with mystery symbols
A Punnett square is not just a grid. You need to define the alleles, put the parent genotypes in correctly, work out the gametes, complete the offspring genotypes, then give the answer in the form asked for.
- Use the same letter for both alleles, with capital for dominant and lower-case for recessive.
- Do not write two different letters for one gene unless the question has told you to.
- If the question asks for probability, give your answer in the exact form it asks for — a fraction, ratio, percentage, or decimal.
Mistake 4: natural selection without inheritance
Many students write that organisms "adapt because they need to". That is not natural selection. The exam wants inherited variation already present in the population, then differential survival and reproduction.
- There is variation in the population.
- Some individuals have a characteristic that gives an advantage in that environment.
- They are more likely to survive and reproduce.
- The advantageous allele is passed on to offspring.
- Over generations, the proportion with that characteristic increases.
Mistake 5: selective breeding versus genetic engineering
Selective breeding chooses parents with desired characteristics and breeds them over many generations. Genetic engineering changes the genome by inserting a gene, often using enzymes and a vector. Similar goal, different method.
Exam comparison
If asked to compare them, give both sides. Selective breeding can increase desired traits but may reduce the gene pool. Genetic engineering can be faster and more specific, but may raise concerns about long-term effects.
Mistake 6: fossils and extinction as vague history chat
Fossils are evidence for evolution because they show organisms have changed over time. The fossil record is incomplete because soft-bodied organisms decay, conditions for fossilisation are rare, and many fossils have not been found.
Extinction needs a cause linked to survival or reproduction: environmental change, new predators, new diseases, competition, catastrophic events, or failure to adapt quickly enough.
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