Topic B5: Genes, inheritance and selection — OCR GCSE Study Guide

 ## Overview Welcome to Topic B5: Genes, Inheritance and Selection. This topic forms the foundation of modern biology, explaining how characteristics are passed from one generation to the next and how species evolve over time. It is heavily tested in exams, particularly your ability to use Punnett squares, define genetic terms precisely, and explain natural selection without making common misconceptions. This topic connects strongly with cell biology (meiosis) and ecology (adaptation). Examiners love synoptic questions that link genetic variation to survival in different ecosystems. Throughout this guide, we will focus on the exact terminology and structures required by the mark scheme.  ## Key Concepts ### Concept 1: The Genetic Hierarchy To secure marks, candidates must clearly distinguish between the structures that carry genetic information. Inside a eukaryotic cell is a **nucleus**. The nucleus contains **chromosomes**, which are long, coiled molecules of DNA. A **gene** is a specific section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein. Think of it like a library: - The nucleus is the library building. - The chromosomes are the books. - The genes are the chapters in the books. - The DNA is the text itself.  ### Concept 2: Alleles and Inheritance Genes come in different versions called **alleles**. For example, the gene for eye colour has an allele for brown eyes and an allele for blue eyes. Because humans have pairs of chromosomes (one from each parent), we have two alleles for every gene. - **Dominant alleles** (represented by a capital letter, e.g., B) are always expressed in the phenotype, even if only one copy is present. - **Recessive alleles** (represented by a lowercase letter, e.g., b) are only expressed if two copies are present. If an individual has two identical alleles (BB or bb), they are **homozygous**. If they have two different alleles (Bb), they are **heterozygous**. ### Concept 3: Reproduction and Meiosis **Asexual reproduction** involves only one parent and no fusion of gametes. Cells divide by mitosis, producing genetically identical offspring (clones). This is rapid but produces no genetic variation. **Sexual reproduction** involves the fusion of male and female gametes (fertilisation). Gametes are formed by **meiosis**, a type of cell division that halves the number of chromosomes. In humans, meiosis produces sperm and egg cells with 23 chromosomes each. When they fuse, the resulting zygote has the full 46 chromosomes. Because meiosis shuffles genetic material, sexual reproduction leads to variation in the offspring. ### Concept 4: Predicting Inheritance with Punnett Squares A Punnett square is a model used to predict the outcome of a genetic cross. It shows all possible combinations of alleles from the parents.  **How to construct a Punnett square:** 1. Determine the genotypes of the parents. 2. Place the alleles of one parent across the top, and the other parent down the side. 3. Fill in the boxes by combining the alleles from the corresponding row and column. 4. Calculate the probabilities of different genotypes and phenotypes. ### Concept 5: Evolution by Natural Selection Evolution is the change in the inherited characteristics of a population over time through a process called natural selection. Charles Darwin and Alfred Russel Wallace independently proposed this theory.  **The 5-Step Examiner-Approved Sequence:** 1. **Variation**: Within a population, there is genetic variation due to random mutations. 2. **Competition**: Organisms overproduce offspring, leading to competition for limited resources. 3. **Advantage**: Some variants have characteristics better suited to the environment. 4. **Survival & Reproduction**: These better-adapted individuals are more likely to survive and reproduce. 5. **Inheritance**: They pass on their advantageous alleles to the next generation, increasing the frequency of these alleles over time. ## Mathematical/Scientific Relationships - **Probability in Genetics**: Often expressed as a fraction (1/4), a ratio (3:1), or a percentage (25%). - **Formula for percentage**: $rac{ ext{Number of specific phenotype}}{ ext{Total number of offspring}} imes 100$ - **Note**: Probabilities predict the *likelihood* of an outcome, not the guaranteed result. Each fertilization event is independent. ## Practical Applications Understanding inheritance is crucial for **selective breeding** (artificial selection), where humans breed plants and animals for particular genetic characteristics (e.g., disease resistance in crops, high milk yield in cows). It also underpins **genetic engineering**, where a gene from one organism is transferred to another.