Heredity and Evolution

External appearance or function alone may be misleading because analogous organs can perform similar functions while having different basic structures and evolutionary origins. Reliable relationships are assessed using deeper anatomy, embryology, fossils and molecular evidence together. Therefore A expresses the relevant caution. B is too absolute, C wrongly dismisses function, and D is false because fossils provide important evidence of evolutionary history.

The governing concept is the relationship between heredity and variation. Offspring inherit genetic information from both parents, so many characteristics resemble those of the parents. During reproduction, chromosomes and genes are combined in new arrangements, and small changes may also arise, producing differences. Therefore option A is correct. The other choices deny inheritance or incorrectly claim that food alone determines every trait.

The governing concept is that a gene is a functional unit of heredity, not simply a label attached to an observable feature. A gene contains or represents information that contributes to the production or expression of a trait, often through proteins and cellular processes. Thus option A is correct. A gene is not a digestive gland, is not restricted to skin, and is normally transmitted rather than destroyed in every generation.

The governing concept is reduction division and maintenance of chromosome number. In humans, body cells normally contain 46 chromosomes, whereas each gamete contains 23. During fertilisation, one sperm and one egg unite, so 23 + 23 restores 46 in the zygote. If gametes carried the full number, chromosome number would double in each generation. Therefore option A is correct.

The governing concept is chromosome-number continuity through sexual reproduction. A human body cell has 46 chromosomes. If both sperm and egg also had 46, fertilisation would produce a zygote with 92 chromosomes. If that organism later produced full-number gametes, the next generation could have 184, so the number would continue rising rather than remaining stable. Hence option A is correct.

The governing concept is experimental suitability in Mendelian genetics. Pea plants offered several clear, contrasting traits, such as tall versus dwarf and round versus wrinkled seeds, making inheritance patterns easy to observe. They also self-pollinate naturally but can be cross-pollinated by hand, allowing Mendel to control the parent plants. Therefore option A is correct; the other choices contradict the useful features of peas.

The governing concept is dominance in a monohybrid cross. Represent pure tallness as TT and pure dwarfness as tt. Every first-generation offspring receives T from one parent and t from the other, producing Tt. Because T is dominant, all offspring appear tall even though the recessive t allele is present and can reappear in a later generation. Thus option A is correct.

The governing concept is the particulate, or discrete-unit, view of inheritance. If tall and dwarf traits blended completely into a permanent intermediate substance, the dwarf trait could not return unchanged in the second generation. Its reappearance shows that hereditary factors remain distinct, with the recessive factor hidden in the first generation. Therefore option A is the idea disproved by the observation.

The governing concept is a heterozygous genotype and phenotypic dominance. A plant with one dominant allele and one recessive allele has both forms in its genetic makeup, but the dominant allele controls the observable height under the usual Mendelian model. The recessive allele remains present but is not expressed in the phenotype. Therefore option A is correct; it does not mean the recessive allele has been destroyed.

The 3:1 phenotypic ratio in a typical Mendelian monohybrid F2 generation results from segregation of allele pairs. For example, crossing Aa with Aa gives AA, Aa, Aa and aa in a 1:2:1 genotypic ratio; dominance converts this into three dominant-phenotype offspring and one recessive-phenotype offspring. Thus option A expresses the principle of segregation, whereas B, C and D contradict heredity.

The 9:3:3:1 ratio is the typical phenotypic outcome of a dihybrid cross between two individuals heterozygous for two independently assorting traits, such as AaBb × AaBb. The four phenotype classes occur in the proportions 9, 3, 3 and 1 because allele pairs separate and recombine independently. Thus option A is correct; the other choices deny the presence of two inherited traits or invoke an unrelated idea.

The governing concept is that acquired changes in body cells are generally not inherited. Cutting a mouse’s tail removes or changes a body part, but it does not alter the DNA in its sperm or egg cells. Therefore, the genetic instructions for tail development can still pass to offspring, which may be born with tails. Option A is correct; B, C, and D contradict basic ideas about gametes, development, and heredity.

Natural selection acts across generations. A variation may help an organism survive or reproduce, but it can influence the population’s future only if the associated genetic information is transmitted to offspring. Inherited variation can therefore become more common when it improves reproductive success. Option A states this requirement. B is false because variation is not always acquired, while C and D misrepresent heredity and selection.

Natural selection depends on the relationship between a trait and environmental conditions. A feature that improves survival in one setting may become neutral or disadvantageous after the surroundings change, while another feature may become useful. Individuals with the newly advantageous heritable trait may leave more offspring, shifting its frequency. Thus A is correct; B, C, and D deny environmental effects or biological variation.

Green colour can provide camouflage on green leaves, making those insects less visible to birds. If the colour is heritable, better survival can lead to greater reproduction, so the green variant may become more frequent in later generations. This differential survival and reproduction is natural selection, making A correct. The example does not show acquired-trait inheritance, blending, or chromosome loss.

The governing concept is the distinction between homologous and analogous organs. Analogous organs may perform a similar function because organisms face similar environmental demands, but their basic structure and evolutionary origin are different. Thus wings of insects and birds both help in flight, yet they do not by themselves indicate a common structural origin. Therefore option A is correct; colour, lack of function, and occurrence only in humans are unrelated claims.

Homologous organs share a basic structural plan and common evolutionary origin, even if their functions differ. A bird wing is a modified, bony forelimb, whereas an insect wing is an outgrowth of the body wall with a different construction and origin. Their shared function, flight, therefore represents analogy rather than homology. Option A is correct; the other choices deny flight or incorrectly claim a shared bone plan or organism.

Distance can reduce contact, but it does not by itself prove that two populations have become separate species. If separated groups meet again and produce fertile, viable offspring, gene flow can continue. Reproductive isolation—before mating, during fertilisation, or after offspring formation—prevents successful gene exchange and makes divergence biologically meaningful. Therefore option A is correct; the other statements incorrectly describe distance or isolation.

Classification groups organisms according to observable and, increasingly, structural, developmental, and molecular similarities. Shared detailed features can suggest common ancestry and help scientists construct evolutionary relationships. However, classification is an organised interpretation of similarities, not independent proof by itself. Fossils, comparative anatomy, embryology, and molecular evidence strengthen the conclusion. Option A is correct; the other choices wrongly deny its evolutionary value or oversimplify its basis.

External resemblance may result from analogous adaptation: unrelated organisms can develop similar shapes or parts because they perform similar functions in comparable environments. This is called convergent evolution and does not necessarily indicate close ancestry. Reliable evolutionary comparison also considers internal anatomy, development, fossils, and molecular evidence. Option A is correct because it identifies the difference between functional similarity and shared structural origin; the other choices are false or overgeneralised.

Evolution means a change in the inherited characteristics or allele frequencies of a population across generations. An individual can grow, learn, or acquire a change during its lifetime, but such an acquired change is not automatically inherited by offspring. When inherited variation affects survival and reproduction, its frequency may increase or decrease in the population. Therefore option A is correct; the distractors incorrectly deny individual changes, reproduction, or the timescale of evolution.

Darwin’s concept of struggle for existence follows from the fact that populations can produce more offspring than available food, space, shelter, and other resources can support. Because resources are limited, individuals compete, and those with heritable traits better suited to the environment are more likely to survive and reproduce. Their traits may become more common. Option A is correct; limited resources do not create identical organisms, erase genes instantly, or cause immortality.

Natural selection changes trait frequencies through differential reproductive success. A useful trait may help an organism avoid predators, obtain food or tolerate conditions, which supports survival. However, the trait can become more common only if the surviving organism reproduces and passes the relevant heritable information to offspring. Survival without reproduction does not increase the next generation’s frequency, and reproduction without survival cannot occur. Therefore option A correctly combines both requirements.

The governing concept is that heredity transmits genetic information from parents to offspring, while variation creates differences among individuals. Some of these variations are inherited and may become more common through natural selection, genetic drift, or isolation. Across many generations, such changes can produce evolution. Therefore, option A is correct; the other options deny the role of inheritance or confuse these biological processes with digestion.

A balanced conclusion must include both continuity and change. Genes transmit information for traits from one generation to the next, establishing heredity. At the same time, inherited variation produces differences in a population. Natural selection, genetic drift, and reproductive or geographical isolation can change the frequency of these differences over many generations, producing evolution. Thus option A is correct; the other choices contradict established genetic principles.