Heredity - Class 10 Science

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1. Introduction to Heredity & Variation Heredity: The process by which genetic information (traits) is passed from parents to their offspring. This ensures the continuity of life forms. (NCERT Pg 142) Genetics: The scientific study of heredity and variation. It explores how traits are inherited and why living organisms show differences. Variation: The differences in characters or traits among individuals of the same species or between parents and offspring. Sources of Variation: Mutations: Sudden inheritable changes in the DNA sequence. Recombination: Shuffling of genes during sexual reproduction (crossing over, independent assortment). Environmental Factors: Can influence the expression of traits. Significance of Variation: Survival Advantage: Variations enable some individuals to adapt better to changing environmental conditions, thus increasing their chances of survival and reproduction. Basis for Evolution: Variations are the raw material upon which natural selection acts, leading to the formation of new species over time. Diversity: Ensures biological diversity within a species and in the ecosystem. 2. Accumulation of Variation During Reproduction DNA Copying: The fundamental event in reproduction is the creation of a DNA copy. This process is not perfectly accurate, leading to small errors or mutations, which are a primary source of variation. (NCERT Pg 143) Asexual Reproduction: Involves a single parent producing genetically identical offspring (clones). Variations are generally fewer and arise mainly due to inaccuracies during DNA replication. Examples: Budding in Hydra, fission in Amoeba. Sexual Reproduction: Involves two parents contributing genetic material. Leads to significantly greater variation due to: Independent Assortment: Homologous chromosomes segregate independently during meiosis I. Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis I, creating new combinations of alleles on a chromatid. Random Fertilization: Random fusion of male and female gametes, each carrying unique combinations of genes. Combination of Parental DNA: Offspring inherit a mix of genes from both parents. This increased variation is crucial for the long-term survival and evolution of species, providing a wider pool of traits for natural selection to act upon. 3. Mendel's Contributions (Rules for the Inheritance of Traits) Gregor Johann Mendel (1822-1884): Austrian monk, known as the "Father of Genetics." (NCERT Pg 144) Pea Plant Experiments ( Pisum sativum ): Mendel chose pea plants due to several advantages: Easily cultivated and had a short life cycle. Showed several clearly distinguishable contrasting traits (e.g., tall/dwarf, round/wrinkled seeds, yellow/green seeds). Normally self-pollinating, but could be cross-pollinated artificially. Produced a large number of offspring, allowing for statistical analysis. Mendelian Terminology: Gene: A segment of DNA that codes for a particular protein, determining a specific trait. Allele: Alternative forms of a gene (e.g., 'T' for tallness and 't' for dwarfness are alleles of the gene for stem height). Dominant Trait/Allele: The allele that expresses its effect phenotypically in both homozygous and heterozygous conditions (e.g., Tallness 'T'). Recessive Trait/Allele: The allele that expresses its effect phenotypically only in the homozygous condition; its effect is masked in the presence of a dominant allele (e.g., Dwarfness 't'). Genotype: The genetic constitution of an organism (e.g., TT, Tt, tt). Phenotype: The observable characteristics or physical expression of the genotype (e.g., Tall, Dwarf). Homozygous (Purebred): An individual with two identical alleles for a trait (e.g., TT or tt). Heterozygous (Hybrid): An individual with two different alleles for a trait (e.g., Tt). $F_1$ Generation (First Filial): The first generation of offspring resulting from a cross between two parental (P) individuals. $F_2$ Generation (Second Filial): The offspring resulting from self-pollination or intercrossing of $F_1$ individuals. Monohybrid Cross: A cross between two parents differing in only one pair of contrasting characters. (NCERT Pg 145) Experiment: Mendel crossed pure Tall (TT) pea plants with pure Dwarf (tt) pea plants. P Generation: Tall (TT) $\times$ Dwarf (tt) $F_1$ Generation: All offspring were Tall (Tt). This showed that Tallness is dominant over Dwarfness. $F_2$ Generation: When $F_1$ plants (Tt) were self-pollinated, the offspring produced were Tall and Dwarf in a phenotypic ratio of 3:1. The genotypic ratio was 1 TT : 2 Tt : 1 tt. Conclusion $\rightarrow$ Law of Dominance and Law of Segregation. Dihybrid Cross: A cross involving two pairs of contrasting characters simultaneously. (NCERT Pg 146) Experiment: Mendel crossed pea plants with Round Yellow seeds (RRYY) with plants having Wrinkled Green seeds (rryy). P Generation: Round Yellow (RRYY) $\times$ Wrinkled Green (rryy) $F_1$ Generation: All offspring were Round Yellow (RrYy). $F_2$ Generation: When $F_1$ plants (RrYy) were self-pollinated, the $F_2$ generation showed a phenotypic ratio of 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green. Conclusion $\rightarrow$ Law of Independent Assortment. Mendel's Laws of Inheritance: Law of Dominance: In a cross between two pure individuals differing in one or more pairs of contrasting characters, the characters that appear in the $F_1$ generation are called dominant characters, and those that do not appear are called recessive characters. Law of Segregation (Law of Purity of Gametes): During gamete formation, the two alleles for a heritable character segregate (separate) from each other such that each gamete receives only one allele. These alleles do not blend or contaminate each other. Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters during gamete formation. This means that alleles for different genes are sorted into gametes independently of one another. 4. How do Traits get Expressed? Genes and Proteins: Genes are segments of DNA that carry the information for making proteins. Proteins are the workhorses of the cell, carrying out most cellular functions and determining the organism's traits. (NCERT Pg 147) Mechanism: A gene provides instructions for the synthesis of a specific protein (e.g., an enzyme). This protein (enzyme) then controls a particular biochemical reaction or cellular process. The outcome of this process determines the specific trait or phenotype. Example (Tall vs. Dwarf Pea Plants): The gene for stem height in pea plants codes for an enzyme that synthesizes a plant growth hormone. If the gene is 'T' (dominant), it produces a functional enzyme, leading to sufficient growth hormone and thus a Tall plant. If the gene is 't' (recessive), it produces a non-functional or less efficient enzyme, resulting in less growth hormone and thus a Dwarf plant. In a heterozygous plant (Tt), the dominant 'T' allele produces enough functional enzyme to make the plant tall, masking the effect of the 't' allele. 5. Sex Determination Sex Determination: The biological system that determines the development of sexual characteristics in an organism. (NCERT Pg 149) Factors influencing Sex Determination: Environmental Factors: In some reptiles (e.g., turtles), the temperature at which fertilized eggs are incubated determines the sex of the offspring. In some snails, individuals can change sex. Genetic Factors (Chromosomal): In many organisms, specific chromosomes determine sex. Human Sex Determination: Humans have 23 pairs of chromosomes. Out of these, 22 pairs are autosomes (non-sex chromosomes), and one pair is sex chromosomes. Females: Have two X chromosomes (XX). All eggs produced by a female will carry an X chromosome. Males: Have one X and one Y chromosome (XY). Males produce two types of sperm: 50% carry an X chromosome and 50% carry a Y chromosome. Fertilization: If an X-carrying sperm fertilizes an egg (always X), the zygote will be XX (develops into a female). If a Y-carrying sperm fertilizes an egg (always X), the zygote will be XY (develops into a male). Conclusion: The sex of the child is determined by the type of sperm (X or Y) contributed by the father. The mother always contributes an X chromosome. 6. Evolution Evolution: The process of gradual change in the heritable characteristics of biological populations over successive generations. It leads to the diversity of life on Earth. (NCERT Pg 151) Acquired Traits: Definition: Traits that an individual develops during its lifetime due to environmental influences, lifestyle, learning, or use/disuse of organs. Nature: These traits are somatic (affect body cells) and do not involve changes in the germ cells' DNA. Heritability: Not passed on to the next generation. (NCERT Pg 152) Examples: A person learning to play the piano, a bodybuilder developing large muscles, a person getting a tattoo, a scar from an injury. Lamarck's Theory of Inheritance of Acquired Characters: Proposed that acquired traits could be inherited, but this theory has been largely disproven. Inherited Traits: Definition: Traits that are passed from parents to offspring through their genes. These traits are encoded in the DNA of the germ (reproductive) cells. Nature: Are genetic and influence the offspring's characteristics from birth. Heritability: Passed on to the next generation. (NCERT Pg 152) Examples: Eye color, hair type, blood group, genetic diseases like color blindness. 7. Speciation Speciation: The evolutionary process by which new biological species arise from existing ones. (NCERT Pg 154) Factors leading to Speciation: Genetic Drift: Random changes in the frequency of alleles (gene variants) in a population. It has a more pronounced effect in smaller populations, as random events can significantly alter gene frequencies. It can lead to the loss of certain alleles or the fixation of others, even if they are not advantageous. Natural Selection: The process where organisms better adapted to their environment tend to survive and produce more offspring. Over time, this leads to an increase in the frequency of advantageous traits in a population, driving evolutionary change. Geographical Isolation: A physical barrier (e.g., mountain ranges, rivers, oceans) that separates populations, preventing gene flow between them. This isolation allows different mutations and selective pressures to act on each population independently, leading to genetic divergence. Reproductive Isolation: Mechanisms that prevent members of different species from interbreeding and producing fertile offspring. This can be pre-zygotic (preventing mating or fertilization) or post-zygotic (hybrid inviability or sterility). Once reproductive isolation is established, even if geographical barriers are removed, the populations cannot interbreed, confirming them as distinct species. Mutation: Introduce new alleles into a population, providing the raw material for evolution. 8. Evolution and Classification Classification: Grouping organisms based on similarities. Evolutionary relationships are a major basis for modern classification. The more characteristics two species share, the more closely they are related and the more recent their common ancestor. (NCERT Pg 155) Evidences of Evolution: Homologous Organs: Definition: Organs that have a similar basic structural plan and common embryonic origin, but are adapted to perform different functions. Significance: Indicate common ancestry and divergent evolution (evolution from a common ancestor into different forms adapted to different environments). Examples: Forelimbs of a human, bat, whale, and cat. All have a similar bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges) but are used for grasping, flying, swimming, and walking, respectively. Analogous Organs: Definition: Organs that have different basic structural plans and different embryonic origins but perform similar functions. Significance: Indicate convergent evolution (unrelated organisms evolving similar features due to similar environmental pressures or functional demands). Examples: Wings of a bird and wings of an insect (both used for flight but structurally very different); eyes of an octopus and a mammal. Fossils: Definition: The preserved remains or traces of organisms (plants, animals, microorganisms) that lived in the geological past. They are typically found in sedimentary rocks. (NCERT Pg 157) How Fossils are Formed: When an organism dies, its body parts may be buried in sediment, preventing decomposition. Over millions of years, the organic matter is replaced by minerals, or impressions are left in the rock. Evidence for Evolution: Provide a direct record of past life forms, showing how organisms have changed over time. Help in tracing evolutionary pathways and identifying common ancestors (e.g., Archaeopteryx, a fossil bird with reptilian features). Show extinction events and the appearance of new life forms. Dating of Fossils: Relative Dating: The simplest method. Fossils found in deeper geological strata are generally older than those found in shallower strata. It provides a sequence but not an absolute age. Absolute Dating (Radioactive Dating): More precise. Involves measuring the decay of radioactive isotopes (e.g., Carbon-14, Uranium-238) present in the rocks surrounding the fossil or in the fossil itself. The half-life of these isotopes allows for calculation of the fossil's age in years. 9. Human Evolution Evolutionary Path: Human evolution is a complex process involving various species of hominids, tracing back to a common ancestor shared with great apes. (NCERT Pg 159) Tools for Tracing Human Evolution: Excavating Fossils: Discovering skeletal remains provides direct evidence. Carbon Dating: Determining the age of these fossils. DNA Sequencing: Comparing the DNA of different human populations and other primates to establish evolutionary relationships (molecular phylogeny). Key Insights: All humans, regardless of geographical origin, belong to a single species, Homo sapiens . Apparent "racial" differences are superficial variations (e.g., skin color, hair texture) that have evolved over time due to adaptations to local environmental conditions and genetic drift, not distinct biological races. There is no "ladder" of evolution where some species are "more evolved" than others. Evolution is about adaptation to specific environments and the generation of diversity. The earliest members of the human species originated in Africa, and then migrated across the globe. 10. Important Questions & Answers (PYQ & NCERT Exemplar) Q1: List the advantages of using pea plants for experiments by Mendel. (2 marks) (PYQ) (NCERT Pg 144) A: Mendel chose pea plants for his experiments due to several advantages: They have a short life cycle, allowing for multiple generations to be studied quickly. They produce a large number of seeds (offspring), which facilitated statistical analysis of results. They exhibit several distinct and contrasting characters (e.g., tall/dwarf, round/wrinkled seeds) that are easy to observe. They are normally self-pollinating, making it easy to obtain pure lines, but can also be cross-pollinated artificially. Q2: State the Law of Segregation. How is it demonstrated in a monohybrid cross? (3 marks) (NCERT Pg 145) A: The Law of Segregation states that during the formation of gametes, the two alleles for a heritable character separate or segregate from each other such that each gamete receives only one allele. These alleles do not blend or contaminate each other. In a monohybrid cross, for example, between a pure tall (TT) and pure dwarf (tt) pea plant: The P generation parents produce gametes carrying either 'T' or 't' alleles respectively. In the $F_1$ generation (Tt), both 'T' and 't' alleles are present. When $F_1$ plants form gametes, the 'T' and 't' alleles segregate, so that 50% of the gametes carry 'T' and 50% carry 't'. This independent segregation leads to the reappearance of the recessive dwarf trait in the $F_2$ generation, demonstrating that the alleles remained distinct and separated during gamete formation. Q3: What is the difference between homologous and analogous organs? Give one example of each. What is the significance of these organs in understanding evolution? (5 marks) (PYQ) (NCERT Pg 156) A: Feature Homologous Organs Analogous Organs Definition Organs with similar basic structure and embryonic origin but different functions. Organs with different basic structure and embryonic origin but similar functions. Origin Common ancestry. Different ancestry. Evolution Divergent evolution (from a common ancestor, adapting to different environments). Convergent evolution (evolving similar features due to similar environmental pressures). Example Forelimbs of human, bat, and whale (all have humerus, radius, ulna but for different functions). Wings of a bird and wings of an insect (both for flight but structurally different). Significance in understanding evolution: Homologous organs indicate that different species share a common ancestor and have diversified over time to adapt to different niches. They provide strong evidence for evolutionary relationships and common descent. Analogous organs show that similar environmental pressures can lead to the evolution of similar adaptations in unrelated species. They demonstrate the power of natural selection in shaping organisms to fit their environment, but do not imply close evolutionary kinship. Q4: How do fossils help in the study of evolution? Explain any two methods of determining the age of fossils. (3 marks) (NCERT Pg 157) A: Fossils are crucial evidence for the study of evolution by: Providing a historical record: They show the forms of life that existed in the past and how they have changed over geological time. Revealing evolutionary links: Some fossils act as connecting links between different groups of organisms, indicating shared ancestry (e.g., Archaeopteryx linking birds and reptiles). Showing extinction: They provide evidence of species that once existed but are now extinct. Two methods of determining the age of fossils are: Relative Dating: This method estimates a fossil's age by comparing its position in rock layers (strata). Fossils found deeper in sedimentary rock layers are generally older than those found in upper layers. It gives a sequence of events but not an exact age. Absolute Dating (Radiometric Dating): This method uses the decay rate of radioactive isotopes (like Carbon-14 or Uranium-238) present in the fossil or the surrounding rocks. By measuring the ratio of the radioactive isotope to its stable decay product, the precise age of the fossil in years can be calculated. Q5: A cross is made between a tall pea plant (T) and a dwarf pea plant (t). In the $F_1$ generation, all plants were tall. When $F_1$ plants were self-pollinated, the $F_2$ generation produced 75% tall plants and 25% dwarf plants. Explain the genetic basis of these results. (3 marks) (PYQ) (NCERT Pg 145) A: $F_1$ Generation: When a pure tall pea plant (genotype TT) is crossed with a pure dwarf pea plant (genotype tt), all the $F_1$ offspring are tall plants with genotype Tt. This demonstrates the Law of Dominance, where the allele for tallness (T) is dominant over the allele for dwarfness (t), masking its expression. $F_2$ Generation: When the $F_1$ hybrid tall plants (Tt) are self-pollinated, they produce two types of gametes (T and t) in equal proportions. Random fusion of these gametes results in the $F_2$ generation with the following genotypes and phenotypes: TT (Tall): 25% Tt (Tall): 50% tt (Dwarf): 25% This results in a phenotypic ratio of 3 Tall : 1 Dwarf (75% tall, 25% dwarf), demonstrating the Law of Segregation, where the alleles for height separate during gamete formation and then recombine randomly. Q6: "Variations that confer an advantage to an individual organism in a given population would tend to be passed on to successive generations." Justify this statement with an example. (3 marks) (NCERT Pg 151) A: This statement describes the core principle of natural selection. If a variation arises in an individual that provides a survival or reproductive advantage in a particular environment, that individual is more likely to survive, reproduce, and pass on that advantageous trait to its offspring. Over generations, this trait will become more common in the population. Example: Consider a population of beetles living on green leaves. Initial Population: Most beetles are green, blending with the leaves, and a few are red due to a random genetic variation. Predation Pressure: Birds feed on these beetles. The red beetles are more conspicuous against the green leaves and are therefore more easily spotted and eaten by birds. The green beetles, being camouflaged, survive better. Survival Advantage: The green coloration provides a survival advantage. More green beetles survive to reproduce. Inheritance: The gene for green coloration is passed on to their offspring. Outcome: Over several generations, the proportion of green beetles in the population increases, while the proportion of red beetles decreases, demonstrating how advantageous variations are selected and passed on.