Inheritance and Variation - Biology Notes PDF

# Inheritance ## 17.1 Variation 1. **Variation:** Differences between individuals of the same species. 2. **Continuous Variation:** * Results in a range of phenotypes between two extremes. * Examples: Body length, body mass. * Caused by genes and the environment. 3. **Discont...

# Inheritance ## 17.1 Variation 1. **Variation:** Differences between individuals of the same species. 2. **Continuous Variation:** * Results in a range of phenotypes between two extremes. * Examples: Body length, body mass. * Caused by genes and the environment. 3. **Discontinuous Variation:** * Results in a limited number of phenotypes with no intermediates. * Examples: ABO blood groups, seed shape, seed color in peas. * Usually caused by genes only. 4. **Investigating Variation:** * Study and describe examples of both continuous and discontinuous variation. ## 17.2 DNA 1. **Structure of DNA:** * Two strands coiled into a double helix. * Each strand is made of nucleotides * Each nucleotide contains a base: A (Adenine), T (Thymine), C (Cytosine), G (Guanine). * Bases pair up: A with T, C with G. * Bonds between base pairs hold the strands together. 2. **Gene Definition:** * A length of DNA that codes for a protein. 3. **DNA Function:** * Controls cell function by controlling protein production, including enzymes. 4. **Protein Synthesis:** * The sequence of bases in a gene determines the sequence of amino acids in a protein. * Different amino acid sequences give proteins different shapes. ## 17.3 Inheritance 1. **Inheritance:** Transmission of genetic information from one generation to the next. 2. **Allele:** An alternative form of a gene. 3. **Key Terms:** * **Dominant:** Allele expressed in the phenotype even if only one copy is present. * **Recessive:** Allele only expressed if two copies are present. * **Phenotype:** Physical expression of a gene. * **Genotype:** Genetic makeup of an organism. * **Homozygous:** Two identical alleles for a gene (e.g., TT or tt). * **Heterozygous:** Two different alleles for a gene (e.g., Tt). 4. **Genetic Diagrams:** * Use Punnett squares to predict monohybrid cross outcomes. * Phenotypic ratios: 1:1 or 3:1. 5. **Observed vs. Expected Ratios:** * Differences occur due to small sample sizes (random chance). 6. **Pure-Breeding:** * Two identical homozygous individuals breeding together produce pure-breeding offspring. 7. **Codominance:** * Both alleles are expressed equally in the phenotype. * Example: ABO blood groups (IA, IB, Io) * Phenotypes: A, B, AB, O. 8. **Sex Determination:** * Humans: XX (female), XY (male). 9. **Gene Mutation:** * Random change in the base sequence of DNA. * Example: Sickle cell anaemia (caused by a mutation in the hemoglobin gene). 10. **Chromosome Mutation:** * Change in chromosome number or structure. * Example: Down's syndrome (47 chromosomes instead of 46). 11. **Sources of Genetic Variation:** * Mutation, meiosis, random mating, random fertilization. 12. **Mutation Rate:** * Increased by ionizing radiation and some chemicals. ## 17.4 Selection 1. **Natural Selection:** * Process by which species evolve over time: * Variation exists within populations. * Many offspring are produced. * Struggle for survival (competition for resources). * Better-adapted individuals survive and reproduce. * Their alleles are passed to the next generation. 2. **Evolution:** * Inherited features of a population change over time due to natural selection. 3. **Antibiotic Resistance:** * Example: MRSA (methicillin-resistant Staphylococcus aureus). * Bacteria with resistance survive and reproduce, passing on resistance genes. 4. **Artificial Selection (Selective Breeding):** * Humans select organisms with desirable traits. * Steps: * Select parents with desirable features. * Cross them to produce the next generation. * Select offspring with desirable features. * Repeat over many generations. 5. **Role of Artificial Selection:** * Used to produce economically important plants (e.g., high-yield crops) and animals (e.g., livestock with desirable traits).