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DNA is a polymer containing a sequence of bases. A gene is a section associated with a functional product, often a protein, and an allele is a version of a gene. Base sequence influences amino-acid sequence and hence protein structure and function. The genome is all an organism’s genetic material. Phenotype depends on genotype and environmental influences, so not every observable difference is inherited or controlled by a single gene.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain genes, alleles and protein information
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is an allele?
A version of a gene. Different alleles can influence the same inherited characteristic.
After watching 2: Which process halves chromosome number for gametes?
Meiosis. Fertilisation then restores the full number.
What you'll be able to do
DNA is a polymer containing a sequence of bases. A gene is a section associated with a functional product, often a protein, and an allele is a version of a gene. Base sequence influences amino-acid sequence and hence protein structure and function. The genome is all an organism’s genetic material. Phenotype depends on genotype and environmental influences, so not every observable difference is inherited or controlled by a single gene.
Meiosis produces gametes with half the usual chromosome number and creates genetic variation. Fertilisation joins two gamete nuclei to restore the full number. Sexual reproduction combines genetic information from two parents, whereas asexual reproduction normally produces genetically matching offspring apart from mutation. Mitosis produces matching cells for growth and repair and should not be used as the process that halves chromosome number.
Define allele symbols, write both parent genotypes and list their gametes. Combine one allele from each parent in each offspring square. A dominant allele affects phenotype in a heterozygote; recessive does not mean rare, weak or harmful. A Punnett square predicts probabilities, not a guaranteed order or exact number of children in a small family. Codominance, sex determination and inherited disorders require the particular model stated in the question.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: If the recessive phenotype probability is 1/4, must the fourth child show it?
No; every child has that probability independently.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.