Heredity and Evolution

The governing concept is independent assortment in a dihybrid cross. Two traits are examined together, and the alleles for one trait can enter gametes independently of the alleles for the other trait, provided the genes are not tightly linked. Their recombination can produce trait combinations that were not present in the original parents. Therefore option A is correct. New combinations do not mean recessive traits are destroyed, and offspring inherit genetic information from their parents.

The governing concept is dominance in heredity. An offspring receives genes or alleles for many traits from both parents. If the allele inherited from the father is dominant over the corresponding allele from the mother, the father-like characteristic can be expressed. This does not mean that the mother contributed nothing. Options B and C are biologically false, while food alone does not determine an inherited trait, so A is correct.

The governing concept is chromosome-number maintenance during sexual reproduction. Gametes are formed with half the chromosome number, or a haploid set. During fertilisation, the sperm and egg fuse and their chromosome sets combine, restoring the diploid number. If both gametes carried the full number, the chromosome number would double in every generation. Thus A is correct; the other options are unrelated biological claims.

The governing concept is that acquired somatic changes are different from changes in hereditary material. Removing a mouse’s tail changes its body after birth, but it does not normally alter the DNA in its sperm or eggs. The offspring receive hereditary information through those reproductive cells and therefore retain the genetic instructions for tail development. A is correct; B, C and D contradict basic genetics and development.

The governing concept is germ-line inheritance. During reproduction, hereditary information is transmitted through the reproductive cells, such as sperm and egg, and these cells contribute genetic material to the offspring. A change restricted to ordinary body cells is not normally copied into that reproductive-cell DNA, so it is not inherited. Option A states this principle; B, C and D reverse or deny the role of reproductive cells.

The governing concept is variation as the raw material for natural selection. Sexual reproduction combines genetic information from two parents through meiosis and fertilisation, producing new combinations of traits. If environmental conditions change, some variants may survive or reproduce more successfully than others, allowing populations to evolve over generations. Therefore A is correct. The other options describe outcomes opposite to the role of sexual reproduction.

The governing concept is the evolutionary importance of variation. Asexual reproduction usually produces genetically very similar offspring, so many members of a population may share the same weakness. If a disease, temperature shift or other environmental change challenges that weakness, most organisms may be affected at the same time. Thus A is correct. Low variation does not guarantee strength, immortality or instant formation of new species.

The governing concept is natural selection acting on variation already present in a population. Individuals may differ because of inherited genetic variation; if one difference improves survival or reproduction, that variant can become more common over generations. Nature does not produce a trait because an organism suddenly needs it. Therefore option A is correct. Options B and C deny the need for variation, while D confuses acquired characteristics with inherited traits.

Natural selection requires three linked conditions: variation among individuals, a difference in survival or reproduction, and inheritance of the advantageous feature. If better-coloured insects escape predators more often and leave more offspring, their colour may become more frequent in later generations. Thus option A shows selection clearly. B removes variation, C removes reproduction, and D provides no selective difference.

Evolution is defined as a heritable change in the characteristics or gene frequencies of a population across generations. A useful feature that is not passed to offspring can help one individual, but it cannot accumulate in the population. Therefore option A is correct. B is false because useful traits are not always harmful; C incorrectly says traits automatically become genes; D is unrelated because populations can reproduce.

In an undisturbed sequence of sedimentary rock, lower layers were generally deposited earlier than layers above them. Fossils found in successive layers therefore provide evidence about organisms living at different relative times. Comparing their structures can reveal a sequence of biological changes and support evolutionary explanations. Option A is correct; B reverses the usual order, while C and D ignore the historical information preserved by fossils.

The governing concept is that fossils provide historical evidence, but the fossil record is incomplete. A fossil forms only when suitable conditions preserve the remains or traces of an organism; many organisms decay, are eaten, or leave no preserved remains. Therefore option A is correct. Options B, C, and D are incorrect because fossils can be very old, are directly related to past organisms, and may reveal structure and habitat as well as feeding clues.

Homologous organs are structures inherited from a common ancestral body plan. Their internal arrangement is similar, although natural selection and different environments may modify their functions; for example, a forelimb can become an arm, wing, or flipper. Thus option A is correct. Option B confuses homology with identical function, while C states the opposite and D incorrectly describes inherited structural features as acquired traits.

The governing idea is the distinction between analogy and homology. Analogous organs perform similar functions, but their developmental origin and basic structural plan can be different because similar environmental demands can produce similar adaptations independently. Therefore option A is correct. Option B is false because these organs are functional; C describes homology rather than analogy, and D is false because analogous organs occur in living organisms too.

Homology is identified mainly by a shared underlying structural plan and origin, not by identical use. The human arm is adapted for grasping and manipulation, whereas the bat wing supports flight, yet both contain corresponding bones such as the humerus, radius, ulna, and hand bones. Therefore option A is correct. B is wrong because their functions differ; C is wrong because their outer shapes differ, and D is factually false.

Homologous organs share an underlying structural plan because of common ancestry, whereas analogous organs only share a function. Bird wings are modified vertebrate forelimbs with bones, while insect wings are extensions of the insect body wall and have a different origin and construction. Thus option A is correct. Options B, C, and D merely describe similarities that do not establish homology; similar function is precisely why they are called analogous.

External resemblance can result from convergent evolution, in which unrelated or distantly related organisms independently acquire similar features because they face similar environmental pressures or perform similar functions. Consequently, appearance alone may suggest a relationship that is not supported by internal structure, development, or molecular evidence. Option A is correct; B and D are nonsensical, while C incorrectly treats every similarity as proof of common ancestry.

Evolution is better represented as a branching tree than as a ladder. Populations diverge from common ancestors and adapt to different environments; evolution has no fixed goal of producing humans or increasing complexity in every lineage. Therefore option A is correct. B wrongly makes humans the destination, while C and D confuse simplicity or age with biological success. Existing organisms are not inferior merely because their body plans are simple or ancient.

Similarity between humans and chimpanzees indicates evolutionary relatedness, not direct descent from one present-day species to the other. Modern humans and modern chimpanzees are separate lineages that diverged from an earlier common ancestral population; both have continued evolving since that split. Therefore option B is correct. A and C incorrectly describe direct parent–offspring relationships, while D ignores substantial anatomical, genetic, and developmental evidence of relatedness.

The governing concept is that evolution measures adaptation and reproductive success in a particular environment, not progress toward greater complexity. A simple body plan may be highly effective and can persist for a very long time if it allows survival and reproduction. Thus option A is correct. B is false because simple organisms have genetic material; C is false because evolution does not always increase complexity, and D contradicts the survival of many simple organisms.

An evolutionary tree is a diagram of hypothesised relationships among lineages. When two branches join at a comparatively recent branching point, the organisms represented by them may share a more recent common ancestor than organisms whose branches meet farther back. Therefore option A is correct, although a tree does not mean the organisms are identical or that one is automatically superior. B, C, and D contradict the purpose of such a tree.

The governing concept is homology and divergent evolution. Homologous organs share a basic structural plan because of common ancestry, but they may perform different functions after adapting to different environments. The human arm and whale forelimb illustrate this clearly: their bones correspond, although one is suited for grasping and the other for swimming. Bird and insect wings are analogous, not homologous, while acquired muscles are not inherited.

The governing concept is speciation through variation, selection and reproductive isolation. Different environments can favour different colour variants, and long-term separation prevents regular interbreeding. As distinct genetic differences accumulate, the populations may become reproductively isolated and eventually form new species. This is a possibility over generations, not an immediate result. Isolation does not remove all genes or variation, so options B, C and D are incorrect.

Evolution is a change in inherited characteristics of populations over successive generations. A change in body cells, such as stronger muscles produced by exercise, may help that individual but normally does not alter the genes in its reproductive cells. Therefore it is not passed to offspring and has little direct importance in long-term evolution. A useful trait can spread evolutionarily only when its information is heritable and reproduction transmits it.

Genetic variation means that individuals in a population differ in their traits, including disease resistance. When variation is very low, most organisms may possess the same susceptible feature. A newly spreading pathogen can then affect many individuals at once, leaving fewer survivors to reproduce. In a more varied population, some individuals may carry resistance and survive. Thus low variation can increase vulnerability; it does not mean that the population lacks genes.