Structure of Atom in One Shot A to Z Class 9 Chemistry CBSE Class 9 Exams 2023 Crash Course 2.0

Heredity and Evolution: Mendel's Laws, Inheritance, and Natural Selection

Heredity is the transmission of traits from parents to offspring through generations, while evolution is the gradual change in inherited characteristics of a population over successive generations. Together, they explain why offspring resemble their parents but are not identical, and how the enormous diversity of life on Earth has arisen from common ancestors. This chapter in CBSE Class 10 Science connects the mechanism of inheritance at the molecular level with the grand narrative of evolutionary change, showing how genetics provides the raw material (variation) upon which natural selection acts.

Gregor Mendel (1822-1884), an Austrian monk, established the foundational principles of inheritance through careful experiments on pea plants in his monastery garden. Mendel chose pea plants because they had clearly visible contrasting traits (tall vs. short, round vs. wrinkled seeds, yellow vs. green pods), could be easily cross-pollinated, and had a short generation time. His first law, the Law of Dominance, states that when two different factors (now called alleles) for a trait are present, one is dominant and expresses itself while the other is recessive and remains hidden in the hybrid. The Law of Segregation states that during gamete formation, the two factors for a trait separate so that each gamete receives only one factor. This means a tall plant with genotype Tt produces gametes carrying either T or t, never both. In a monohybrid cross (Tt × Tt), the F₂ generation shows a phenotypic ratio of 3 tall : 1 short (genotypic ratio 1 TT : 2 Tt : 1 tt). Sex determination in humans is controlled by the sex chromosomes: females have XX and males have XY. During reproduction, the mother always contributes an X chromosome, while the father can contribute either X (producing a female XX) or Y (producing a male XY). Therefore, the sex of the child is determined by the father's sperm.

Evolution is the cumulative result of changes in inherited traits over many generations. Charles Darwin proposed the theory of natural selection in 1859 in "On the Origin of Species." The mechanism works as follows: within any population, individuals vary in their traits. Those individuals whose traits are better suited to the current environment survive longer and produce more offspring (differential reproduction). Over many generations, the advantageous traits become more common in the population. Genetic variation arises from several sources: mutations (random changes in DNA), recombination during sexual reproduction (new combinations of alleles), and gene flow (migration between populations). Speciation — the formation of new species — occurs when populations of the same species become reproductively isolated (by geography, behaviour, or other barriers) for long enough that they accumulate enough genetic differences to no longer interbreed successfully. Evidence for evolution comes from multiple sources: the fossil record (showing transitional forms like Archaeopteryx between reptiles and birds), comparative anatomy (homologous structures like the human arm, whale flipper, and bat wing share the same basic bone plan, indicating common ancestry), comparative embryology (vertebrate embryos look remarkably similar in early stages), and molecular evidence (DNA and protein sequence similarities between species). Evolution is not progress toward a goal — it is adaptation to current environmental conditions. Traits that are advantageous in one environment may be disadvantageous in another. Human evolution traces our lineage from ape-like ancestors through Australopithecus (about 3-4 million years ago), Homo habilis, Homo erectus, to modern Homo sapiens, with brain size increasing dramatically over this period.

• Mendel's laws: dominance (one factor masks the other) and segregation (factors separate during gamete formation); monohybrid cross gives 3:1 ratio.
• Sex determination in humans: XX (female) or XY (male); the father's sperm determines the sex of the child.
• Natural selection: individuals with advantageous traits survive and reproduce more, passing those traits to the next generation.
• Evidence for evolution: fossils, homologous structures, embryology, and molecular/DNA comparisons all support common ancestry.
• Speciation occurs when populations become reproductively isolated; evolution is adaptation, not progress toward a goal.