Notes From Toppers
Genetics (NCERT Biology Part-II, Class-XII )##
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1. Mendelian Genetics:(Ch-5)_
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Laws of Inheritance (Mendel’s Laws)
- Law of Segregation: During gamete formation, the two alleles for each gene segregate (separate) and randomly unite with gametes from the other parent.
- Law of Independent Assortment: The alleles of different genes assort independently of one another during gamete formation.
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Dominance, Recessiveness, and Gene Interactions
- Dominant Allele: An allele that expresses its phenotype even in the presence of a recessive allele.
- Recessive Allele: An allele that expresses its phenotype only in the absence of the dominant allele.
- Incomplete Dominance: When heterozygous individuals show an intermediate phenotype between the phenotypes of the homozygous dominant and recessive individuals.
- Co-dominance: When both alleles of a gene are fully expressed in heterozygous individuals, resulting in two distinct phenotypes.
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Inheritance Patterns
- Monohybrid Cross: A cross between individuals that differ in one gene pair.
- Dihybrid Cross: A cross between individuals that differ in two gene pairs.
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Linked Genes and Genetic Mapping
- Genetic Linkage: The tendency of two or more genes to be inherited together more often than expected by chance due to their close physical proximity on the same chromosome.
- Genetic Mapping: Determining the relative positions of genes on a chromosome by measuring the frequency of recombination between them.
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2. Molecular Basis of Inheritance:_ (Ch-6:Ch-7 Class XII)_
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Structure and Replication of DNA
- DNA Structure: Double helix composed of deoxyribonucleotides (adenine, thymine, guanine, and cytosine) linked by phosphodiester bonds.
- Replication: Semi-conservative process by which DNA makes an identical copy of itself.
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Transcription and Translation
- Transcription: Synthesis of RNA from a DNA template.
- Translation: Synthesis of proteins from mRNA template using transfer RNA (tRNA) and ribosomes.
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Gene Expression and Regulation
- Gene Expression: The process by which the information encoded in a gene is used to direct the synthesis of a functional product, such as a protein.
- Gene Regulation: The control of gene expression to ensure that the right genes are expressed at the right time and in the right amount.
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3. Chromosomes and Cell Division:_ (Ch-5, 10 Class-XII)_
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Structure of Chromosomes
- Chromosomes: Thread-like structures in the nucleus that carry genetic information.
- Centromere: The region of the chromosome where sister chromatids are attached.
- Telomere: The repeating nucleotide sequences at the ends of chromosomes that protect them from degradation.
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Mitosis and Meiosis
- Mitosis: Cell division that results in two genetically identical daughter cells.
- Meiosis: Cell division that produces haploid gametes (eggs and sperm) with half the number of chromosomes as the parent cell.
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4. Genetic Variations:_ (Ch-8,10,11 Class XII)_
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Mutations
- Types of Mutations: Point mutations, deletions, insertions, inversions, and translocations.
- Causes of Mutations: Errors in DNA replication, environmental factors (radiation, chemicals), and transposable elements.
-Genetic Recombination
- Crossover: The exchange of genetic material between homologous chromosomes during meiosis.
- Independent Assortment: The random assortment of chromosomes during meiosis, leading to genetic diversity.
-Gene Flow, Genetic Drift, and Hardy-Weinberg Equilibrium - Gene Flow: The movement of alleles between populations. - Genetic Drift: The random change in allele frequencies in a population over time. -Hardy-Weinberg Equilibrium: The state in which the allele and genotype frequencies in a population remain constant from generation to generation.
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5. Evolution:(Ch-7 to Ch-11, Class-XII)
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Darwin’s Theory of Evolution by Natural Selection
- Variation: Differences in traits among individuals in a population. -Natural Selection: The process by which individuals that are better adapted to their environment are more likely to survive and reproduce. -Evolution: The change in the genetic composition of a population over time due to natural selection.
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Mechanisms of Evolution (Variation, Selection, Inheritance)
- Variation: Mutations, genetic recombination, and sexual reproduction.
- Selection: Natural selection, sexual selection, and artificial selection.
- Inheritance: The passing on of traits from parents to offspring through genes.
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Evidence for Evolution
- Morphological Evidence: Similarities and differences in the physical characteristics of organisms.
- Molecular Evidence: Comparison of DNA and protein sequences.
- Paleontological Evidence: The study of fossils.
- Biogeographical Evidence: The distribution of organisms around the world.
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6. Speciation and Patterns of Evolution:_ (Ch-10,11 Class-XII)_
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Isolating Mechanisms and Speciation
- Geographic Isolation: Physical barriers, such as mountains or rivers, that prevent gene flow between populations.
- Reproductive Isolation: Differences in reproductive behaviors or structures that prevent interbreeding between populations.
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Patterns of Evolution
- Divergent Evolution: Two or more species evolve from a common ancestor and become increasingly different over time.
- Convergent Evolution: Unrelated species evolve similar traits independently due to similar environmental pressures.
- Coevolution: The evolution of two or more species in response to each other.
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Phylogeny and the Tree of Life
- Phylogeny: The evolutionary history of a group of organisms.
- Tree of Life: A diagram that represents the evolutionary relationships among different groups of organisms.
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7. Human Genetics:(Ch-8 Class-XII)
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Pedigree Analysis
- A method of tracking the inheritance of traits within a family over several generations.
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Inheritance of Human Traits
- Single-Gene Inheritance: Traits controlled by a single gene.
- Polygenic Inheritance: Traits controlled by multiple genes.
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Genetic Disorders
- Autosomal Dominant Disorders: Disorders caused by a dominant allele located on an autosome (non-sex chromosome).
- Autosomal Recessive Disorders: Disorders caused by two recessive alleles located on an autosome.
- X-Linked Disorders: Disorders caused by a recessive allele located on the X chromosome.