Heredity is the transmission of genetic characters from parents to offspring. Variations are differences between individuals of the same species. Variations arise from: sexual reproduction (new gene combinations), mutations (changes in DNA sequence), and environmental influences. Variations that are inherited (genetic) can be passed to the next generation. Those caused only by environment (e.g., a scar) are not inherited. Variations are the raw material for evolution — favourable variations are naturally selected.

Gregor Mendel (Father of Genetics) studied inheritance in pea plants. He chose 7 contrasting traits (tall/dwarf, round/wrinkled seeds, etc.). Key terms: Gene — segment of DNA controlling a trait. Allele — alternate forms of a gene. Dominant allele — expressed even in one copy (T). Recessive allele — expressed only in two copies (tt). Genotype — genetic makeup (TT, Tt, tt). Phenotype — observable trait (tall, dwarf). Law of Segregation: each organism has two alleles for each trait; they separate during gamete formation. Law of Independent Assortment: genes for different traits are inherited independently. Monohybrid cross: 3:1 phenotype ratio in F2. Dihybrid cross: 9:3:3:1 ratio in F2. Sex determination: XX = female, XY = male; father determines sex of child.

Evolution is the change in the heritable characteristics of populations over successive generations. Evidence for evolution: Homologous organs — same structure, different functions (e.g., forelimbs of whale, bat, human) — indicates common ancestry (divergent evolution). Analogous organs — different structure, same function (e.g., wings of bird and insect) — indicates convergent evolution. Fossils — preserved remains of ancient organisms. Embryological similarities — embryos of different vertebrates look similar in early stages. Molecular homology — similar DNA sequences in different organisms. Darwin's Theory of Natural Selection: individuals with favourable variations survive and reproduce more (survival of the fittest); unfavourable variations are eliminated. Lamarck's theory of use and disuse (inheritance of acquired characters) is no longer accepted.

Speciation is the formation of new species from existing ones. It occurs when populations are isolated geographically and undergo different genetic changes over many generations. Acquired traits: characteristics developed during an organism's lifetime due to environment or use — NOT inherited (e.g., muscle built by exercise, a missing limb). Inherited traits: determined by genes and passed from parents to offspring. Weismann's experiment — cutting tails of mice for many generations did not produce tailless offspring, proving acquired traits are not inherited. Human evolution: Homo sapiens evolved in Africa ~2 lakh years ago from earlier hominids. Modern humans and apes share a common ancestor but are not directly descended from each other.

Origin of life on Earth: about 3.5 billion years ago. Stanley Miller–Harold Urey experiment (1953): simulated early Earth conditions (methane, ammonia, hydrogen, water vapour + electric discharge) → produced amino acids, proving organic molecules can form from inorganic ones. Life likely began in the primordial soup (warm ocean) as simple chemical molecules that eventually became self-replicating. Evolution timeline: Simple prokaryotes (bacteria) → eukaryotes → multicellular organisms → invertebrates → fish → amphibians → reptiles → mammals → primates → Homo sapiens. Hominid evolution: Homo habilis → Homo erectus → Homo heidelbergensis → Homo neanderthalensis → Homo sapiens. Human evolution involved increase in brain size, bipedalism, use of tools, and development of language.

Traits are controlled by genes, which are segments of DNA located on chromosomes in the nucleus. Humans have 23 pairs (46) of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. One chromosome of each pair comes from the mother and one from the father, so each trait is controlled by two copies of a gene, which may be the same or different. A dominant trait is expressed when at least one dominant allele is present; a recessive trait is expressed only when both alleles are recessive. Genotype is the genetic make-up (e.g. Tt); phenotype is the observable trait (e.g. tall).

Mendel crossed pure tall pea plants (TT) with pure short plants (tt). F1 generation: all plants were tall (Tt), showing that tallness is dominant. When F1 plants were self-pollinated, the F2 generation had tall and short plants in the ratio 3 : 1 (phenotypic ratio). The genotypic ratio is 1 TT : 2 Tt : 1 tt. The short trait, which did not appear in F1, reappeared in F2, showing that traits are controlled independently and that the recessive factor was not lost. This leads to the law of dominance and the law of segregation.

Mendel crossed pure round yellow seeds (RRYY) with wrinkled green seeds (rryy). F1: all round yellow (RrYy), as round and yellow are dominant. On self-pollination of F1, F2 had four types of seeds in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. New combinations (round green and wrinkled yellow) appeared, showing that the traits of seed shape and seed colour are inherited independently of each other. This is the law of independent assortment.

Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair is the sex chromosomes. Females have two X chromosomes (XX); males have one X and one Y (XY). All eggs carry an X chromosome. Sperms are of two types, half carrying X and half carrying Y. If a sperm carrying X fertilises the egg, the child is a girl (XX); if a sperm carrying Y fertilises the egg, the child is a boy (XY). So the sex of a child depends on the father's sperm, and there is a 50% chance for each sex.

Homologous organs have the same basic structural design but different functions, e.g. forelimbs of a human, whale, bat and horse; they indicate a common ancestor. Analogous organs have different structures but perform similar functions, e.g. wings of a bird and an insect; they do not indicate a common ancestor. Vestigial organs are reduced organs that have lost their function, e.g. the appendix and the tail bone (coccyx) in humans; they suggest that the organs were fully functional in ancestors.

Fossils are the preserved remains or impressions of organisms that lived in the past, found in rocks. Deeper layers of rocks contain older fossils, while recent ones are near the surface. The age of fossils can be estimated by relative dating (depth of rock layer) and by radioactive dating, such as carbon-14 dating, which uses the ratio of different isotopes of the same element. Archaeopteryx is a fossil with feathered wings like a bird and teeth and a tail like a reptile, so it is a connecting link between reptiles and birds.

Natural selection (Charles Darwin): individuals with variations that help them survive in their environment are selected to survive and reproduce, and so their features are passed on. Example: in a population of green beetles on a green bush, crows can easily spot and eat red beetles; so red beetles decline. Genetic drift: random change in the frequency of genes in small populations by chance events, not by advantage; for instance, if an elephant accidentally steps on a bush and kills most beetles, the survivors' traits become common regardless of their usefulness.

Evolutionary relationships can be traced by comparing homologous organs, fossils and DNA sequences; the more similar the DNA of two species, the closer they are related. Human beings (Homo sapiens) evolved in Africa; some groups migrated to other continents. Skin colour differences arise from adaptation, genetic variation and migration; all humans are one species. Evolution should not be seen as a progress from lower to higher forms; it is a branching process, where species diversify by chance and selection.

Humans use artificial selection to produce varieties with desired traits. Wild cabbage was bred for different features: for sterile flowers to get cauliflower, for swollen parts to get broccoli, for arrested flower development to get cabbage, for swollen stem to get kohlrabi, and for larger leaves to get kale. Selecting and breeding the desired variants shows how changes accumulate over generations, which is similar to natural selection except that humans, not the environment, decide.

Heredity: transmission of traits from parents to offspring via genes. Mendel: F2 monohybrid ratio 3:1; dihybrid ratio 9:3:3:1. Sex determination: XX female, XY male; father decides. Evolution: gradual change in species over generations; evidence from homologous organs, fossils, embryology and DNA. Natural selection, genetic drift and geographical isolation lead to speciation. Acquired traits are not inherited because they do not change the DNA in germ cells.