WJEC GCSE Biology 2.3 DNA and Inheritance Exam Questions

About WJEC GCSE Biology 2.3 DNA and Inheritance

DNA and inheritance in Unit 2 links the structure of DNA to how features are passed on. You need to describe DNA as two sugar-phosphate strands joined by complementary bases in a double helix, and how the order of bases forms a triplet code for proteins. You must explain genetic profiling and its uses, define genetic terms such as allele, homozygous and phenotype, and use Punnett squares for monohybrid crosses, including F1 and F2 generations. Sex determination and artificial gene transfer complete the topic. Examiners regularly set genetic cross diagrams and ask for ratios or probabilities, so practise setting out crosses clearly.

What you need to know

• DNA structure DNA is two strands, each with a sugar-phosphate backbone, twisted into a double helix. The four bases are A, T, C and G; A pairs with T and C pairs with G (complementary base pairing).
• Genetic code A sequence of three bases (a triplet) codes for one amino acid. The order of bases in a gene determines the order of amino acids and so the protein made.
• Genetic profiling DNA is cut into fragments, which are separated into a pattern of bands that can be compared. Uses include criminal investigations, paternity testing, classifying species and identifying genes linked to disease.
• Genetic terms Learn gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, F1, F2 and selfing. Alleles are different forms of a gene and occur in pairs.
• Monohybrid crosses Use Punnett squares to predict ratios. A cross between two heterozygotes (Tt x Tt) gives a 3:1 ratio of dominant to recessive phenotypes. Most features are controlled by several genes, not one.
• Sex and gene transfer Females are XX and males XY, giving a 1:1 ratio of boys to girls. Artificial gene transfer moves a gene into another organism (e.g. insulin-producing bacteria or pest-resistant crops); consider advantages, disadvantages and ethical issues.

Common mistakes in 2.3 DNA and Inheritance

• Pairing bases wrongly, such as A with G. A always pairs with T and C always pairs with G.
• Writing genotypes with different letters, such as T and s. Use one letter, capital for the dominant allele and lower case for the recessive allele.
• Treating a ratio as a certainty. A 3:1 ratio means each offspring has a 75% chance of the dominant phenotype; it does not guarantee three out of every four.
• Confusing genotype and phenotype. Genotype is the alleles present (e.g. Tt); phenotype is the observed characteristic (e.g. tall).

Try three 2.3 DNA and Inheritance exam-style questions