1. Description of Variation:
- Definition: Variation refers to the differences observed between individuals of the same species.
- Sources: Can arise from genetic, environmental, or a combination of both factors.
2. Continuous Variation:
- Definition: Results in a range of phenotypes between two extremes.
- Examples: Body length, body mass.
- Cause: Influenced by both genetic and environmental factors.
3. Discontinuous Variation:
- Definition: Results in a limited number of distinct phenotypes with no intermediates.
- Examples: ABO blood groups, seed shape, seed color in peas.
- Cause: Primarily caused by genes; little influence from the environment.
4. Causes of Variation:
- Discontinuous Variation: Mainly caused by genetic factors.
- Continuous Variation: Influenced by a combination of genetic and environmental factors.
5. Examples of Continuous and Discontinuous Variation:
- Continuous Variation Examples:
- Human Height: Influenced by genetics, nutrition, and other environmental factors.
- Weight: Genetic predisposition and lifestyle choices contribute.
- Discontinuous Variation Examples:
- ABO Blood Groups: Determined by specific genes.
- Seed Shape and Color in Peas: Controlled by Mendelian genetics.
Understanding the nature of variation, whether continuous or discontinuous, provides insights into the underlying genetic and environmental factors shaping the diversity observed within a species.
DNA Structure and Function:
1. Structure of a DNA Molecule:
- (a) Double Helix: Two strands coiled together in a spiral.
- (b) Nucleotide Chain: Each strand consists of a chain of nucleotides.
- (c) Nucleotide Components: Each nucleotide contains a base (A, T, C, G).
- (d) Base Pairs: Bonds between bases hold the two strands together.
- (e) Base Pairing: Bases always pair up in a specific way – A with T, and C with G.
2. Gene Definition:
- Definition: A gene is a segment of DNA that codes for a protein.
3. DNA’s Control Over Cell Function:
- Explanation: DNA regulates cell functions by controlling protein production, including enzymes.
4. Gene Sequence and Protein Production:
- Statement: The sequence of bases in a gene determines the sequence of amino acids needed to produce a specific protein.
- Note: Detailed knowledge of nucleotide structure is not required at this level.
5. Protein Structure Determined by Amino Acid Sequences:
- Understanding: Different sequences of amino acids result in diverse shapes and functions of protein molecules.
Understanding the structure and function of DNA provides the foundation for comprehending how genetic information is stored, replicated, and utilized in the synthesis of proteins essential for cellular processes.
Inheritance:
1. Description of Inheritance:
- Definition: Inheritance is the transmission of genetic information from one generation to the next.
2. Allele Definition:
- Definition: An allele is an alternative form of a gene.
3. Key Terms:
- Dominant: Trait expressed when present; denoted by a capital letter (e.g., T).
- Recessive: Trait expressed only in the absence of the dominant allele; denoted by a lowercase letter (e.g., t).
- Phenotype: Observable characteristics resulting from the interaction of genes and the environment.
- Genotype: Genetic makeup of an individual.
- Homozygous: Two identical alleles (e.g., TT or tt).
- Heterozygous: Two different alleles (e.g., Tt).
4. Genetic Diagrams (Punnett Squares):
- Prediction: Use Punnett squares for monohybrid crosses; calculate phenotypic ratios (e.g., 1:1, 3:1).
5. Deviations from Expected Ratios:
- Explanation: Observed ratios may differ from expected ratios, especially with a small number of offspring, due to chance.
6. Pure-Breeding:
- Statement: Two identical homozygous individuals breeding together will produce pure-breeding offspring.
7. Codominance and ABO Blood Groups:
- Explanation: ABO blood groups demonstrate codominance (phenotypes A, B, AB, O; gene alleles IA, IB, Io).
8. Sex Determination in Humans:
- Definition: Sex is determined by XX (female) or XY (male) chromosomes.
9. Gene Mutation:
- Definition: Gene mutation is a random change in the DNA base sequence. Example: Sickle cell anemia.
10. Chromosome Mutation:
- Definition: Chromosome mutation is a change in chromosome number or structure. Example: Down’s syndrome (47 chromosomes).
11. Sources of Genetic Variation:
- Explanation: Genetic variation in populations comes from mutation, meiosis, random mating, and random fertilisation.
12. Factors Increasing Mutation Rate:
- Statement: Ionising radiation and certain chemicals increase the rate of mutation.
Understanding these inheritance principles provides insights into the mechanisms shaping genetic diversity in populations.
Selection
1. Natural Selection:
- (a) Variation within Populations: Natural selection acts on existing variation.
- (b) Production of Many Offspring: Overproduction of offspring increases competition.
- (c) Struggle for Survival: Individuals compete for limited resources.
- (d) Reproduction by Better Adapted Individuals: Those better adapted to the environment have higher reproductive success.
- (e) Passing on Alleles: Successful individuals pass on their advantageous alleles to the next generation.
2. Evolution via Natural Selection:
- Explanation: Inherited features of a population evolve over time through natural selection, favoring traits conferring better adaptation.
3. Antibiotic-Resistant Bacteria and Natural Selection:
- Example: Development of strains of antibiotic-resistant bacteria (e.g., MRSA) illustrates natural selection in response to selective pressure.
4. Artificial Selection (Selective Breeding)
- (a) Selection by Humans: Humans choose animals or plants with desirable features.
- (b) Crossing for the Next Generation: Selected individuals are bred to produce the next generation.
- (c) Selection of Offspring: Offspring showing desirable features are further selected.
- (d) Repetition Over Generations: This process is repeated over many generations.
5. Role in Economic Production:
- Explanation: Artificial selection is used by humans to enhance specific traits in plants and animals, contributing to the production of economically important species.
Understanding both natural and artificial selection is crucial for comprehending how traits are passed on and how populations can evolve over time in response to environmental pressures or human intervention.