GCSE Biology: Inheritance & Evolution — Deep Revision Workbook

Inside GCSE Biology: Inheritance & Evolution — Deep Revision Workbook

revise reproduction, meiosis, DNA and the genome, secure the genetics vocabulary and work monohybrid crosses to their ratios and probabilities, tackle inherited disorders, sex determination and carrier crosses, revise variation, natural selection, selective breeding, genetic engineering, evolution evidence and classification, then plan and write an extended exam-style 6-mark answer. About 45 minutes.

Learning objectives

• reproduction and the genome — sexual versus asexual reproduction, meiosis, haploid gametes, fertilisation, and DNA, nucleotides, genes and the genome;
• genetics and crosses — homozygous, heterozygous, genotype, phenotype, dominant and recessive, and monohybrid Punnett squares giving 1:2:1, 3:1 and 1:1 ratios with their fractions and percentages;
• inherited disorders and sex — recessive cystic fibrosis and carriers, dominant polydactyly, probability from carrier crosses, and XX/XY sex determination;
• variation and evolution — genetic and environmental variation, natural selection, the peppered moth, selective breeding, genetic engineering, fossils, and classification by Linnaeus and the binomial system;
• exam technique — planning and writing an extended 6-mark answer, including a genetic diagram, using correct scientific vocabulary.

Activities in this book

1. Section 1 — Reproduction, Meiosis, DNA & the Genome

   sexual reproduction fuses a gamete from each parent at fertilisation, mixing genes to give variation; asexual reproduction uses one parent and mitosis to make identical clones. Gametes are made by meiosis, which halves the chromosome number so gametes are haploid (23 chromosomes in humans) and the full number is restored at fertilisation.

2. Section 2 — Genetic Terms, Crosses & Punnett Squares

   two identical alleles = homozygous; two different alleles = heterozygous. The alleles are the genotype; the characteristic they produce is the phenotype. For a Tt × Tt cross the genotypes are 1 TT : 2 Tt : 1 tt (a 3:1 phenotype ratio). A Tt × tt test cross gives a 1:1 ratio. Take T = tall (dominant) and t = short (recessive).

3. Section 3 — Inherited Disorders, Sex & Probability

   cystic fibrosis is recessive, so an affected child (cc) needs two copies, one from each carrier parent (Cc). A carrier has one copy but is unaffected. Polydactyly is dominant, so one affected allele (D) is enough. Sex is set by the sex chromosomes: XX female, XY male. Use C = healthy (dominant), c = cystic fibrosis (recessive); D = polydactyly (dominant), d = unaffected.

4. Section 4 — Variation, Selection, Engineering & Classification

   variation comes from genes, the environment or both. Only inherited (genetic) variation drives evolution. Natural selection (Darwin): the best-adapted survive and breed, passing on their alleles. Selective breeding and genetic engineering are ways humans change organisms deliberately. Fossils and examples like the peppered moth give evidence for evolution, and Linnaeus organised life using the binomial system.

5. Section 5 — Extended Exam-Style 6-Marker

   a 6-mark question rewards a clear, logical chain of reasoning, not a list of facts. Plan four or five linked points, use correct terms (allele, gamete, dominant, recessive, mutation, natural selection) and, where useful, draw a genetic diagram. Link each idea to the next so a marker can follow you step by step.

Suggestions for teaching

• Fix the vocabulary first. Learners cannot do crosses until gamete, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype are secure — drill these before Section 2.
• Model every Punnett square. Work Tt × Tt (1:2:1 genotype, 3:1 phenotype) and the Tt × tt test cross (1:1) on paper, then convert each result to fractions and percentages.
• Contrast the two disorders. Use recessive cystic fibrosis (two copies needed, carriers matter) against dominant polydactyly (one copy is enough) so the difference is explicit.
• Tell the natural-selection story. Walk the peppered moth or finch beak step by step — variation, mutation, survival, reproduction, allele spreads — as a model 6-mark answer.