Heredity and Evolution

Use T for the dominant tall allele and t for the recessive dwarf allele. The heterozygous tall parent is Tt, while the dwarf parent is tt. The first parent produces T or t gametes, and the second produces only t gametes. The offspring are Tt and tt in equal expected numbers, so the phenotype ratio is one tall to one dwarf. Hence A is correct; 3:1 applies to a heterozygote × heterozygote cross.

Mendel selected pea plants because they possessed several easily recognisable contrasting traits, such as tall versus dwarf stems and round versus wrinkled seeds. Pea flowers normally self-pollinate, allowing pure lines to be maintained, but Mendel could remove anthers and transfer pollen to control cross-pollination. Their relatively short generation time also made repeated crosses practical. Therefore A is correct; the other statements are false.

In a heterozygous individual, the dominant allele determines the visible phenotype and masks the expression of the recessive allele. Masking does not remove or transform the recessive allele; it remains part of the genotype and can be transmitted to offspring. If two recessive alleles later come together, the recessive phenotype can reappear, often in the F2 generation. Thus A is correct, while B, C and D misunderstand inheritance.

The governing concept is Lamarck’s theory of use and disuse and inheritance of acquired characters. Lamarck proposed that giraffes repeatedly stretched their necks to reach leaves on tall trees; increased use was believed to lengthen the neck, and this acquired change was then thought to pass to offspring. Option A states this idea. The other options do not describe Lamarck’s explanation.

The governing concept is the distinction between inherited genetic information and changes acquired by body parts. Weismann removed the tails of mice over several generations, yet their offspring continued to develop normal tails. This observation weakened the claim that an acquired bodily change is automatically inherited. Therefore, option A is correct. The experiment did not disprove chromosomes, dominance, or variation in sexual reproduction.

Darwin’s natural selection is based on heritable variation, competition, and differential survival and reproduction. Individuals in a population differ, and a variation that improves survival or reproduction can give its bearer an advantage in a particular environment. Such individuals tend to leave more offspring, so the useful inherited trait becomes more common over generations. Thus, option A is correct; the other statements contradict natural selection.

The governing concept is homology. Homologous organs have a similar underlying structural plan because they were inherited from a common ancestor, although their functions may differ after adaptation to different environments. For example, the forelimbs of humans, whales, and bats share a basic skeletal pattern but serve different purposes. Option A is therefore correct. Analogous organs share function but not basic origin, while vestigial organs are reduced remnants.

The governing concept is analogy: organs are analogous when they perform a similar function but have different structural plans and evolutionary origins. Insect wings and bird wings both enable flight, yet an insect wing is an outgrowth of the exoskeleton while a bird wing is a modified vertebrate forelimb. Therefore, option A is correct. They are not homologous because their basic origins and structures differ.

The relevant principle is relative dating of sedimentary rock layers. When undisturbed layers accumulate, lower layers were deposited earlier and upper layers later. Consequently, fossils preserved in deeper layers are generally older than fossils in layers above them. Option A expresses this conclusion. The word “generally” is important because geological disturbance can overturn or mix layers, so “always” in option B is too absolute.

Fossils are preserved remains, impressions, or traces of organisms that lived in the past. By arranging fossils according to the relative age of their rock layers and comparing their structures, scientists can identify gradual changes and transitional features across time. This sequence supports the idea that organisms have changed through generations. Therefore, option A is correct; the other options make claims fossils cannot establish.

Variation means differences in characteristics among individuals of the same species. During sexual reproduction, one set of genetic material comes from each parent, and meiosis and fertilisation produce new combinations of alleles. These combinations can make offspring different from both parents and from one another. Therefore, option A identifies a major source of variation. Food color, injury, and aging do not by themselves create inherited genetic combinations.

Asexual reproduction generally involves a single parent and does not involve the fusion of two different gametes. The offspring therefore receive genetic material that is almost identical to the parent and to one another. Small differences can still arise through occasional DNA-copying errors or mutations, so “less variation” does not mean “no variation.” Hence option A is correct; genes are present and offspring are definitely formed.

An allele is one form of a gene, and a diploid organism usually has two alleles for a trait. If the pair is represented by one capital letter and one small letter, such as Tt, the two alleles are different. A genotype containing two unlike alleles is called heterozygous. Homozygous means that both alleles are alike, while asexual describes reproduction rather than a genotype. Therefore option A is correct.

Phenotype refers to the observable characteristics of an organism, such as seed shape, height, or flower color. Genotype refers to the allele combination underlying a trait, such as TT, Tt, or tt. Phenotype results from the interaction of genotype with environmental influences, so the two terms are related but not identical. Therefore option A is correct; the other choices misuse or falsely equate the terms.

Evolution is not a single, progressive ladder with one organism becoming universally superior. Populations change in different directions as variations are selected under particular environmental conditions. Lineages may branch from common ancestors, and each surviving form can be well adapted to its own surroundings. Therefore option A is correct; the other statements deny extinction patterns, variation or evolution in other organisms.

A complex organ can arise through cumulative evolutionary change rather than appearing fully formed at once. Slight inherited variations that improve light detection or image formation may provide an advantage, and natural selection can preserve them. Repeated changes over many generations can produce increasing complexity. Hence option A is correct; B is unsupported, C confuses acquired traits with inherited variation, and D ignores comparative and fossil evidence.

Bacterial populations may contain heritable variation, including occasional resistance. When an antibiotic is applied, susceptible bacteria are more likely to die, whereas resistant individuals survive. The survivors reproduce and pass the resistance trait to offspring, so resistance becomes more common in later generations. Thus option A correctly describes natural selection. The drug selects existing or newly arising variation; it does not make every bacterium equally strong.

Artificial selection occurs when humans choose organisms with preferred inherited traits and breed them so those traits become more frequent. Selecting cabbage plants for particular leaves, buds or stems is an example. Option A therefore directly describes human-directed breeding. A random mutation may provide variation but is not itself artificial selection, while mountain formation and fossil discovery are geological or evidential processes, not selective breeding.

Wild cabbage had heritable variation in different plant parts. Over generations, humans selected leaves, flower buds, stems or enlarged shoots according to their desired use. Continued selective breeding produced forms such as cabbage, cauliflower, broccoli and kohlrabi from a common ancestral stock. Therefore option A illustrates artificial selection and divergence of traits. The other options contradict genetics or the occurrence of evolution in plants.

Adaptation is an inherited structural, physiological, or behavioural feature that improves an organism’s chance of survival and reproduction in a particular environment. Natural selection can increase its frequency over generations. Therefore option A is correct. A learned skill need not be inherited, a colour change is only one possible example, and an adaptation is not necessarily harmful.

Similarity among organisms, especially in basic structures or inherited features, can indicate descent from a common ancestor. Variation creates differences within a population, and natural selection may favour some of those differences under changing conditions. Thus option A correctly combines evidence of ancestry with the raw material for evolutionary change. The other statements contradict heredity and selection.

Heredity transfers genetic information from both parents, but sexual reproduction does not give every offspring exactly the same combination. During meiosis and fertilisation, different alleles can enter gametes and unite in different pairs. These combinations influence the traits expressed by each offspring, while environment may also contribute to observable differences. Therefore option A is correct; chromosome absence and lack of parental contribution are biologically false, and food alone does not determine every trait.

A gene is a segment of DNA that carries hereditary information and can influence the production or functioning of a protein. A trait is an observable characteristic, such as height or seed colour, produced through gene expression along with environmental influence. Thus a gene and a trait are related but are not identical objects. Option A states this relationship correctly; B confuses information with its expression, while C and D contradict basic heredity.

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.