Revision: Heredity and Evolution Science and Technology 2 SSC (English Medium) 10th Standard Maharashtra State Board

The term heredity may be defined as "transmission of genetically based characteristics from parents to offspring".

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A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a triplet codon.

Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes.

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Mutation is a phenomenon which results in an alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

Evolution is a slow and continuous process whereby complex forms of life have emerged from simpler forms through millions of years.

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Differences between individuals of the same species that arise naturally and can be passed to offspring are called variations.

The fittest individuals in nature are most likely to reproduce and pass on their good qualities to their offspring. It is called natural selection.

The origin of new species by gradual modification is called speciation.

The process of formation of a new species from the pre-existing species is called speciation.

According to A.E. Emerson, speciation is the evolutionary process by which new species arise due to genetic divergence and isolation.

• Mendel is the father of modern genetics; he discovered the basic principles of heredity.
• Hugo de Vries (1901) proposed the mutation theory explaining sudden genetic changes.
• Walter Sutton (1902) linked chromosomes with heredity through his study on grasshoppers.
• Avery, McCarty & McLeod (1944) proved DNA is the genetic material in living organisms.
• Jacob & Monod (1961) developed a model of protein synthesis, leading to recombinant DNA technology.

• Transcription is the process by which genetic information from one DNA strand (the template strand) is copied into RNA using RNA polymerase.
• It occurs in the nucleoid in prokaryotes and in the nucleus in eukaryotes; mRNA then moves to the cytoplasm for translation.
• A transcription unit has three parts: promoter (start site), structural gene, and terminator (stop site).
• The process occurs in three stages: initiation (RNA polymerase binds the promoter), elongation (the RNA chain is formed), and termination (RNA polymerase detaches).
• Only one DNA strand acts as a template (3′→5′), while the other is the coding strand (5′→3′).
• In eukaryotes, primary RNA (hnRNA) is processed by capping, tailing, and splicing to form mature mRNA.

• Translation is the process by which the codon sequence on mRNA is decoded with the help of tRNA at the ribosome to form a specific sequence of amino acids in a protein.
• It needs mRNA (template), tRNA (adapter), ribosome (with A, P, and E sites), amino acids, aminoacyl-tRNA synthetase, ATP/GTP for energy, and Mg²⁺ ions.
• Before translation, amino acids are activated and linked to their specific tRNAs (charging) by aminoacyl-tRNA synthetase.
• Initiation: the small ribosomal subunit binds mRNA at the start codon (AUG), the initiator tRNA carrying methionine attaches, and the large subunit joins to form the initiation complex.
• Elongation: amino acids are added one by one through codon–anticodon pairing; peptide bonds form between them, and the ribosome moves forward by one codon at a time (translocation).
• Termination occurs at a stop codon (UAA, UAG, UGA), where release factors free the polypeptide and the ribosomal subunits separate.

• A mutation is a sudden heritable change in DNA sequences that leads to changes in the genotype and phenotype of an organism.
• Loss of DNA segment = deletion; gain of DNA segment = insertion/duplication; both cause chromosomal aberrations, commonly seen in cancer cells.
• Frame-shift mutation - caused by loss or gain of a DNA segment; Point mutation - change in a single base pair (e.g., sickle cell anaemia).
• Physical mutagens that cause mutation include UV radiation, X-rays, alpha, beta and gamma rays; Chemical mutagens include mustard gas, phenol and formalin.
• Mutation is an important source of genetic variation in organisms, alongside recombination.

• Evolution is the gradual and progressive development of living organisms over a long period due to natural selection.
• Life originated around 3.5 billion years ago from simple inorganic and organic compounds, eventually forming primitive cells.
• The diversity of life on Earth today, from unicellular organisms to complex plants and animals, is a result of continuous evolutionary processes.
• According to the theory of evolution, all complex organisms evolved slowly from unicellular ancestors over 300 crore (3 billion) years.
• Evolution is considered an organizational and multi-dimensional process, reflecting structural and functional advancement in living organisms.

• Structures like human hand, ox foreleg, bat’s patagium, and whale flipper differ in function but have similar bone structure.
• This similarity in bones and joints across different animals suggests a common ancestry.
• Such internal structural similarities serve as strong anatomical evidence for evolution.

• Charles Darwin developed the idea of natural selection based on extensive data collected during his voyage on the HMS Beagle.
• His views on organic evolution were heavily influenced by the essays of Thomas R. Malthus and Charles Lyell.
• Alfred Russel Wallace independently reached the exact same conclusions while studying the fauna of the East Indies, resulting in a joint presentation in 1858.
• Darwin officially detailed his theory in 1859 by publishing his famous book, Origin of Species by Natural Selection.
• He identified variations, the struggle for existence, and heredity as the three chief factors driving the complex process of evolution.

• Organisms evolve by acquiring traits during their lifetime to adapt to environmental changes and passing these traits to their offspring.
• New organs develop in an organism as a direct result of a continuous "new need" created by their environment.
• Continuous use of an organ strengthens and develops it, while continuous disuse leads to its degeneration.
• Lamarck supported this with examples like giraffes developing long necks from stretching and snakes losing limbs from disuse.
• The theory was rejected after Weismann's rat-tail experiments proved that acquired changes only affect body cells and cannot be inherited.

• Speciation is the evolutionary process responsible for the formation of new, distinct species.
• A species is defined as an interbreeding population that shares a specific gene pool and is reproductively isolated from other distinct populations.
• While members within a species exhibit continuous variation, there is a complete discontinuity of variation between different species.
• A species can be divided into geographical races with unique gene frequencies due to environmental differences, yet these races can still interbreed successfully.
• Reproductive isolation, which is the inability of individuals from different populations to interbreed, constitutes the most definitive boundary between different species.

• Modern humans (Homo sapiens sapiens) evolved from Cro-Magnons around 50,000 to 11,000 years ago, mainly near the Caspian and Mediterranean regions.
• They have a fully upright posture, binocular vision, a mobile neck, and spinal curves that help with walking and balance.
• Their brain capacity is about 1450–1600 cm³, with a large cerebrum, well-developed forehead and chin, and the ability to speak and think clearly.
• They have proportionate limbs, flat nails, less body hair, and a broad chest, which are all features that support advanced physical and mental tasks.
• Modern humans made metal tools, started agriculture, formed permanent homes, and developed writing, science, and industries, leading to today's civilizations.