Heredity and Evolution

In a diploid organism, body cells contain two sets of chromosomes. Meiosis produces haploid gametes containing one set, or half the somatic chromosome number. During fertilisation, one male and one female gamete fuse, so their two haploid sets restore the normal diploid number. If gametes were diploid, the chromosome number would double in each generation. Therefore option A is correct; options B, C, and D do not maintain genetic continuity.

Fertilisation combines the chromosome sets carried by two gametes. Normally, meiosis makes each gamete haploid, so two haploid sets combine to restore the stable diploid number. If gametes carried the full diploid number, fusion would produce twice as many chromosomes, and repeated generations would cause progressive chromosome-number increase and genetic imbalance. Thus option A is correct. The other choices do not follow from chromosome-number change and are unrelated to the mechanism.

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.

The ratio 9:3:3:1 is not an isolated number to recite. In the usual Mendelian dihybrid cross, it arises when two heterozygous parents produce gametes independently and the traits show complete dominance. The classes represent two parental-type and two new combinations in the F2 generation. Understanding this reasoning is more meaningful than memorising the ratio without its genetic basis.

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.

The governing distinction is the cause of change in allele or trait frequency. In natural selection, environmental conditions favour some heritable variations because they improve survival or reproduction. In genetic drift, frequencies change randomly, especially in small populations, without regard to usefulness. Therefore option A is correct. Food may be one environmental factor, but it does not define both processes, and drift is not restricted to large organisms.

Genetic drift is a random change in the frequency of inherited variants. In a small population, each individual represents a relatively large fraction of the gene pool. Therefore, the chance death, survival, or reproduction of only a few individuals can greatly alter the proportion of variants in the next generation. Option A is correct; small populations still have chromosomes, reproduce, and contain variation, so B, C, and D are false.

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.

Analogous organs perform similar functions but arise from different basic structural plans and do not necessarily indicate close common ancestry. Bird wings and insect wings both enable flight, yet one is a bony vertebrate forelimb and the other is an external insect structure. Therefore option A is correct. Similar function alone is insufficient evidence; B, C, and D are false.

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 geographical isolation followed by divergence. When a physical barrier separates two populations, their gene flow and interbreeding decrease. Mutations, recombination, natural selection, and chance may then change allele frequencies differently in the two groups. If these differences eventually cause reproductive isolation, the separated populations may form new species. Therefore, option A is correct; the other options incorrectly claim that isolation makes organisms identical, removes genes, or stops evolution.