GCSE Biology: Family Trees, Mendel and Protein Synthesis

Knowing what dominant, recessive and heterozygous mean is only the start: GCSE Biology exams expect you to use genetics to reason about real families, real data and real decisions. This material builds on the inheritance material in this category, which introduces genetic terms, simple Punnett squares and sex determination, and moves on to the applied and extended content of the inheritance topic. Several points belong to separate Biology or to the Higher tier (for example, in the AQA specification the structure of DNA and the history of genetics are separate Biology only, and protein synthesis and mutations are also Higher tier); questions on these points say so.

The first part works with family histories described in words. You deduce that two unaffected parents with a child who has cystic fibrosis must both be carriers, work out the chance that their next child is affected, and show that a father with polydactyly who has an unaffected child must be heterozygous. You turn a cross into a ratio and an expected number of offspring, and remember that most characteristics are controlled by many genes rather than one. You then consider embryo screening and the economic, social and ethical issues it raises.

The second part follows the development of our understanding of genetics: Mendel's breeding experiments on pea plants and his idea of inherited 'units', why their importance was only recognised after his death, and how the observation of chromosomes, the link between units and chromosomes and the discovery of the structure of DNA led to the gene theory.

The third part looks inside DNA: a polymer of nucleotides, each made of a sugar, a phosphate group and one of four bases; complementary base pairing; the triplet code; protein synthesis on ribosomes, with carrier molecules bringing amino acids in the right order; and how a mutation can change the shape of a protein so that an enzyme no longer fits its substrate, while most mutations have little or no effect.

Four formats are offered. The quiz has twelve questions with explanations. The flashcards cover the key terms, from nucleotide and triplet code to carrier and embryo screening. The printable written sheet has eight longer questions, including Punnett squares for families described in words, a complementary strand and triplet calculation, an evaluation of embryo screening and an explanation of how a mutation can stop an enzyme working; it can be completed by hand and photographed for feedback. The oral exam lets you reason through genetics problems with an examiner who asks one question at a time and gives brief feedback at the end.

Any costs or numbers in the questions are illustrative.

• Deduce genotypes and probabilities from family histories of recessive and dominant disorders
• Express the outcome of a cross as a ratio and as an expected number of offspring
• Discuss the economic, social and ethical issues of embryo screening
• Describe Mendel's work and why its importance was recognised only after his death
• Describe DNA as a polymer of nucleotides and (Higher) apply complementary base pairing and the triplet code
• Higher: describe protein synthesis and explain how a mutation can change a protein's function

Practice material written by Zestly, based on the DfE GCSE biology subject content and exam-board specification content (for example, AQA GCSE Biology 8461: DNA structure; genetic inheritance; inherited disorders; the understanding of genetics). Costs and numbers in the questions are illustrative.