Heredity and Evolution

In a Mendelian cross, a dominant allele can mask the expression of a recessive allele in the heterozygous first-generation offspring. The recessive allele is not destroyed; it remains part of the genotype and can pass into gametes. When two offspring contribute that recessive allele to a later generation, the dwarf phenotype can reappear. Therefore option A is correct. The other choices deny inheritance or describe biologically impossible events.

A tall plant may be heterozygous, represented as Tt, where T is dominant for tallness and t is recessive for dwarfness. Its tall appearance hides the effect of t, but the plant can pass t to some gametes. If the other parent also contributes t, the offspring genotype becomes tt and the dwarf phenotype is expressed. Hence option A is correct; no gamete formation would prevent sexual reproduction, and chromosome doubling is irrelevant.

The governing concept is dominance and recessiveness. A recessive phenotype appears only when an individual has two recessive alleles, written as aa, because no dominant allele is present to mask it. Thus observing the recessive trait provides evidence of a homozygous recessive genotype in the usual Mendelian model. A high amount of a dominant form, absence of hereditary information, or food alone cannot explain this inheritance pattern.

An acquired trait develops during an individual’s lifetime because of use, training, environment, injury, or other experiences. For evolution, a variation must be heritable, meaning that relevant information must pass through reproductive cells to offspring. Many acquired body changes do not alter the DNA of gametes, so they are not directly inherited. Therefore they cannot normally serve as a permanent evolutionary basis by themselves.

Regular training can enlarge or strengthen an athlete’s muscles through changes in body tissues, but this is an acquired somatic change. It does not normally alter the DNA carried by sperm or eggs. Since offspring arise from genetic information in gametes, the exercise-produced muscle development is not directly transmitted. The distractors are false because muscles contain cells, offspring do not form from food alone, and exercise does not destroy chromosomes.

Natural selection is context-dependent. A trait is useful when it improves survival or reproduction under the conditions an organism actually faces. The same feature may be advantageous in one environment, neutral in another, and harmful somewhere else. Thus its usefulness depends on current environmental pressures and living conditions, not on the trait’s name, subjective beauty, or the depth at which a fossil is found.

The governing concept is heredity in evolution. Evolution involves a change in the frequency of heritable traits in a population over successive generations. A useful trait may help one individual survive, but if it is not inherited, it cannot become more common through reproduction. Therefore, option A is correct. Survival alone is not enough; options B, C, and D incorrectly ignore inheritance or make false generalisations.

Natural selection occurs when heritable variation affects survival and reproduction. Green insects may be less visible on green leaves, so predators may capture fewer of them. If the colour is inherited, these survivors are more likely to reproduce and pass it on, increasing its frequency. Thus option A is correct. The other options deny colour vision, heredity, or wrongly claim that insects change colour instantly.

Fossils are preserved remains, impressions, or traces of organisms from the past. The fossil record is incomplete because preservation requires special conditions and many organisms leave no fossil. Nevertheless, fossils reveal ancient forms and their distribution in successive rock layers, allowing scientists to infer changes over time. Option A is correct; the other choices exaggerate, reverse, or misunderstand what fossils show.

The governing idea is the principle of relative dating in undisturbed sedimentary layers. In general, lower layers were deposited before the layers above them, so a fossil found deeper is likely to be older than fossils in upper layers. This gives relative, not exact numerical, age. Option A is correct. It says “may” because geological disturbance can alter layer order; B, C, and D are incorrect.

Homologous organs have a similar underlying structural plan because they were inherited from a common ancestor, although adaptation may give them different functions in different organisms. For example, the forelimbs of humans, whales, and bats have corresponding bones but serve different purposes. Therefore option A is correct. Colour and identical function are not required, and homologous organs are not limited to plants.

A human hand and a bat wing perform different functions, such as grasping and flying, but their forelimbs contain a corresponding basic arrangement of bones. This structural similarity is homology and suggests inheritance from a common ancestor, followed by adaptation for different uses. Thus option A is correct. They are not analogous merely because their functions differ, and the remaining options contradict observable structure.

Analogy is identified by similar function combined with different evolutionary and structural origins. Both bird and insect wings enable flight, but a bird wing is a modified, bony vertebrate forelimb, whereas an insect wing is a different external structure without the same bone plan. Thus option A is correct. Options B, C, and D contradict these anatomical facts.

The governing concept is the struggle for existence. Organisms produce more offspring than the environment can support, while food, space, shelter, and mates are limited. Competition therefore increases, and individuals with heritable traits that improve survival or reproduction are more likely to leave offspring. Across generations, these traits may become more common through natural selection. Hence A is correct; B, C, and D have no biological basis.

The governing concept is differential survival and reproduction. A population contains heritable variation before selection acts. If a particular inherited trait improves survival or reproductive success in a given environment, its carriers are more likely to leave offspring. The trait can therefore increase in frequency over several generations, producing adaptation at the population level. Option A states this process; B, C, and D incorrectly invoke will, guaranteed inheritance, or no environmental effect.

The governing concept is the environment-dependent nature of fitness. A trait is not universally beneficial; its value depends on factors such as climate, food, predators, disease, and competition. When these conditions change, a trait that once improved survival may provide little advantage or even become harmful. Selection may then favor different variants. Therefore A is correct; B and C deny environmental effects, while D wrongly claims that selection stops.

The governing concept is that evolution is supported by multiple, independent lines of evidence. Fossils provide a historical sequence, showing organisms and structural changes through geological time. Homologous organs, such as the forelimbs of humans and whales, share a basic structural plan despite different functions, suggesting common ancestry. Thus, option A is correct; the other options confuse evolutionary evidence with weather, sex determination, or acquired traits.

The governing concept is molecular evidence for evolutionary relationship. Hereditary material stores biological information, and substantial similarities in its chemical composition or sequences can indicate that organisms inherited some information from a common ancestor. Such evidence does not prove that the organisms are the same species, nor does it eliminate variation or show that evolution has stopped. Therefore option A is the most accurate conclusion.

The governing concept is branching evolution rather than a linear ladder of progress. Populations that share an ancestor can diverge, adapt to different environments, and form separate lineages. Some lineages continue while others become extinct, and organisms living today are not simply unfinished stages leading toward humans. Therefore option A is correct; the other choices express the misconception that evolution has one goal or lacks variation.

The governing concept is that evolution describes descent, variation, and adaptation to particular environments, not a universal ranking from inferior to superior. A feature useful in one environment may be less useful in another, and every surviving organism represents adaptation to its circumstances. Humans also have an evolutionary history, but they are not the goal of all evolution. Hence option A is correct.

The governing concept is that evolution is an ongoing population-level process, not a journey toward a predetermined final organism. Environmental conditions can change, and new heritable variations can arise. A trait that is advantageous today may become less useful under different conditions. A successful organism can also reproduce and continue evolving through its population. Therefore option A is correct; success does not remove genes or stop reproduction.

The governing concept is natural selection acting through differential survival and reproduction. Survival allows an organism carrying a useful heritable trait to remain alive, but reproduction is needed to copy and transmit that trait to offspring. If those offspring also survive and reproduce, the trait can increase in frequency across generations. Thus option A is correct; survival alone cannot spread a trait without descendants.

The governing concept is that heredity transmits genetic information, while variation produces differences among individuals. Only variations that are genetically inherited can be passed to future generations and become more or less common through natural selection, genetic drift, or isolation. Therefore, inherited variation directly connects heredity with evolution. Acquired habits, food preference, and cleanliness are not, by themselves, inherited genetic changes.

The governing concept is dominance in Mendelian inheritance. Let tallness be T and dwarfness be t. A pure tall parent is TT and a pure dwarf parent is tt, so every first-generation offspring receives T from one parent and t from the other, giving Tt. Because T is dominant, all plants look tall, while the t allele remains present but is not expressed. Thus option B is correct; the dwarf trait is hidden, not destroyed.

The governing concept is segregation of alleles in a monohybrid cross. If tall hybrids are Tt × Tt, each parent can pass either T or t. The possible offspring combinations are TT, Tt, Tt, and tt. Only tt expresses dwarfness, so approximately one quarter of the offspring may be dwarf under the usual assumptions. Option A is correct because the dwarf form appears when a recessive allele is inherited from both parents; the other choices contradict genetics.