Ans: Mendel studied inheritance using pea plants. He chose pure-breeding tall (TT) and dwarf (tt) plants. F1 cross: TT × tt → all Tt (tall) — dominant allele T is expressed. F1 self-cross: Tt × Tt → TT : Tt : tt = 1:2:1 (genotype ratio) = 3 Tall : 1 Dwarf (phenotype ratio). Laws: Law of Segregation — each organism has two alleles for each trait; they separate during gamete formation so each gamete has only one allele. Law of Dominance — when two different alleles are present, the dominant one is expressed. Law of Independent Assortment — alleles of different genes assort independently during gamete formation (shown in dihybrid cross — 9:3:3:1 ratio).

Ans: Homologous organs: structurally similar (same bones, same embryological origin) but perform different functions. Example: forelimbs of humans (grasping), whales (swimming), bats (flying), frogs (jumping) — all have same humerus, radius, ulna bones. Indicate divergent evolution — descended from a common ancestor. Analogous organs: structurally different (different origin) but perform the same function. Example: wings of birds (modified forelimb, bone + feather) and wings of insects (thin membrane). Indicate convergent evolution — unrelated organisms evolved similar features due to similar environments.

Ans: Humans have 23 pairs of chromosomes; the 23rd pair is the sex chromosome. Females: XX (two X chromosomes). Males: XY (one X, one Y chromosome). During reproduction: Mother's eggs all contain X chromosome. Father's sperm contain either X or Y. If X-sperm fertilises egg → XX → girl. If Y-sperm fertilises egg → XY → boy. Since the mother always contributes X, the sex of the child depends on which sperm (X or Y) fertilises the egg — therefore the father determines the sex of the child. Blaming or aborting female foetuses is scientifically wrong and socially harmful.

Ans: Acquired traits: characteristics developed during an organism's lifetime due to environment, injury, or use — NOT controlled by genes and cannot be passed to offspring. Examples: a bodybuilder's muscles, a scar from a wound, a farmer's tanned skin. Inherited traits: determined by genes and passed from parents to offspring through reproduction. Examples: eye colour, blood group, height, tongue rolling. Weismann's experiment proved this — cutting tails of mice for 20+ generations produced no tailless offspring. Only changes in the germ cells (sperm/eggs) can be inherited; changes in body (somatic) cells are not.

Ans: Darwin's theory (On the Origin of Species, 1859): (1) Overproduction — organisms produce more offspring than can survive. (2) Variation — individuals in a population differ from each other. (3) Struggle for existence — competition for food, space, mates. (4) Natural selection (Survival of the Fittest) — individuals with favourable variations survive and reproduce; unfavourable variations are eliminated. (5) Heredity — favourable variations are passed to offspring. (6) Over many generations, accumulated changes lead to new species (speciation). Example: antibiotic resistance in bacteria — bacteria with resistance gene survive antibiotic treatment and multiply.

Ans: A gene is a segment of DNA on a chromosome that carries the information for a particular trait and codes for a protein.

Ans: Genotype is the genetic make-up of an organism for a trait, e.g. TT, Tt or tt.

Ans: 46 chromosomes (23 pairs).

Ans: Gregor Johann Mendel.

Ans: Fossils are the preserved remains, impressions or traces of organisms that lived in the remote past, usually found in rocks.

Ans: Phenotypic ratio = 3 : 1 (3 tall : 1 short). Genotypic ratio = 1 : 2 : 1 (1 TT : 2 Tt : 1 tt).

Ans: The pea plant has a short life cycle, many easily visible contrasting traits, produces many seeds, can both self-pollinate and be cross-pollinated, and is easy to grow.

Ans: A dominant trait is the one that is expressed in the F1 hybrid (e.g. tallness). A recessive trait is masked in the F1 hybrid and expressed only when both alleles are recessive (e.g. shortness). Dominant is expressed with one allele; recessive requires two.

Ans: Vestigial organs are organs that are reduced in size and have lost their function during evolution. Examples: the appendix and the tail bone in humans.

Ans: Fossils show the forms of organisms in the past. Their age can be estimated and compared to give a sequence of how species changed over time, and some fossils (like Archaeopteryx) show links between groups.

Ans: F2 had round yellow, round green, wrinkled yellow and wrinkled green seeds in the ratio 9 : 3 : 3 : 1. The new combinations (round green and wrinkled yellow) show that seed shape and seed colour are inherited independently of each other.

Ans: Farmers selected and bred wild cabbage plants for different features: sterile flowers gave cauliflower, arrested flower development gave cabbage, swollen parts gave broccoli, swollen stem gave kohlrabi, and larger leaves gave kale. Thus one wild plant gave many vegetables.

Ans: Suppose a population of beetles lives on green bushes, and crows eat them. Beetles that are green are camouflaged, while red ones are easily seen and eaten. So red beetles die and decrease, and green beetles survive, breed and increase in number. After many generations, the population consists mainly of green beetles. This is natural selection, which acts on variations.

Ans: Parents RRYY × rryy; gametes RY and ry; F1: all RrYy (round, yellow). On self-pollination of F1, gametes RY, Ry, rY and ry combine in a 4 × 4 Punnett square. F2: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Conclusion: round and yellow are dominant; two new combinations appeared, so the traits are inherited independently (law of independent assortment).

Ans: Variations arise through small errors in DNA copying and through sexual reproduction, which shuffles genes from two parents. Natural selection: individuals with useful variations survive and reproduce more, e.g. green beetles on green bushes are not seen by crows, and so increase. Genetic drift: chance events change gene frequencies in small populations, e.g. if an elephant steps on a bush and kills most beetles, the few survivors, whatever their colour, determine the next generation. Both accumulate variations and can lead to new species.