Heredity and Evolution

Genetic information can be compared to estimate evolutionary relationships. If two organisms share many similar DNA sequences or inherited features, they may have received them from a more recent common ancestor and therefore may be relatively closely related. Similarity is evidence that must be interpreted with other biological information; it does not mean the organisms are the same individual. All organisms used in such comparisons may have common ancestry, so options B, C and D are incorrect.

Evolutionary conclusions are stronger when independent lines of evidence agree. Fossils provide a record of organisms and forms that existed in the past, although that record is incomplete. Comparative anatomy reveals homologous structures, while genetic similarities can indicate shared ancestry and relative relatedness. Combining these sources reduces the limitations of relying on only one type of evidence. Therefore option A is correct; the other choices wrongly reject useful evidence or deny its relevance.

The ladder-like idea that evolution aims toward a perfect final organism is scientifically incorrect. Evolution is branching: populations share ancestors and change in different directions as environments and selection pressures differ. Humans are one present-day branch, not the endpoint or goal of all living organisms. Other organisms have also evolved successfully in their own environments, and evolution continues in many populations. Thus option A corrects both the direction and the goal misconception.

The governing concept is divergent evolution caused by variation and natural selection. Different environments create different selection pressures, so traits useful in one place may be less useful in another. Heritable variations that improve survival and reproduction are passed on and accumulate over many generations. Therefore, geographically separated groups can gradually become increasingly different. Options B, C, and D are incorrect because continued reproduction and inherited variation, not permanent similarity or disappearance of all changes, allow divergence.

The chapter connects heredity, variation, and evolution into one scientific framework. Genes carry hereditary information and influence traits, while variations create differences among individuals. Natural selection or other evolutionary processes can change the frequency of such inherited differences over many generations. Thus option A correctly combines the essential ideas. Option B confuses acquired changes with inherited traits, option C removes the necessary basis of evolution, and option D gives an incorrect general explanation of sex determination.

Let Y represent the dominant yellow allele and y the recessive green allele. Each heterozygous parent is Yy, so the cross is Yy × Yy. The four equally likely combinations are YY, Yy, Yy, and yy. Only yy produces green seeds because the recessive phenotype appears when both alleles are recessive. Thus 1 of 4 outcomes, or 25%, is green. The other three outcomes are yellow, so B, C, and D are incorrect.

For a single trait with complete dominance, the 3:1 phenotypic ratio is the expected result of crossing two heterozygotes. In Tt × Tt, the genotypes are TT, Tt, Tt, and tt in a 1:2:1 ratio. Three offspring show the dominant tall phenotype and one shows the recessive dwarf phenotype. TT × TT gives all tall, TT × tt gives all tall, and Tt × TT gives all tall phenotypically, so only option C fits.

A dihybrid cross follows two pairs of contrasting traits at the same time. When the allele pairs assort independently, a cross such as AaBb × AaBb produces the F2 phenotypic ratio 9:3:3:1: nine with both dominant traits, three with the first dominant and second recessive, three with the first recessive and second dominant, and one with both recessive. Thus C is correct; the ratio does not prove acquired inheritance, linkage, or only sex determination.

The governing concept is variation, which is necessary for evolution. DNA copying is highly accurate but not perfect, so small changes may arise. If such a change occurs in reproductive cells and is passed to offspring, it becomes heritable variation. Natural selection may then favour or eliminate it. Therefore, option C is correct; the other choices incorrectly describe reproduction, acquired traits, or organism survival.

In the usual human XX–XY system, the mother’s egg carries an X chromosome, whereas a father produces two types of sperm: X-bearing and Y-bearing. Fertilisation by an X sperm generally produces XX, while fertilisation by a Y sperm generally produces XY. Thus the paternal sperm determines whether the zygote receives X or Y from the father. Option B is correct; the other statements misrepresent the chromosomes in eggs or sperm.

Weismann repeatedly cut the tails of mice across generations, yet their offspring continued to develop normal tails. The governing principle is that a bodily change is not automatically inherited unless it alters genetic information in reproductive cells. Thus option A is correct. The experiment did not show that injuries are inherited, that tails lack hereditary control, or that reproductive cells are unrelated to traits.

Natural selection requires variation among individuals, and its evolutionary effect is strongest when the variation is heritable and affects survival or reproduction. Individuals with a favourable inherited trait are more likely to reproduce and pass that trait onward, so its frequency may increase. Option A contains both requirements. Learned-only, non-genetic and identical traits cannot be efficiently selected across generations in the stated sense.

The governing concept is natural selection acting on variation. A bacterial population may contain a few cells with inherited resistance before the drug is applied. The drug kills or inhibits susceptible cells, while resistant cells survive and reproduce, causing their proportion to increase. Therefore, option B is correct. This is not definite inheritance of an acquired trait, formation of homologous organs, or sex determination.

This is the classic industrial melanism example. The moth population already showed colour variation, and soot-darkened surroundings changed which colour provided better camouflage. Dark moths were less visible to predators, survived more often and reproduced more successfully. Their frequency therefore increased through natural selection. Option B is correct; artificial selection requires deliberate human choice, while test crosses and self-pollination are breeding concepts unrelated to this survival pattern.

Reproductive isolation means that individuals from separated groups cannot mate successfully or cannot produce fertile offspring with one another. The immediate population-level consequence is reduced exchange of genes, or reduced gene flow. This makes option A correct and can later support divergence and speciation. It does not equalize lifespan, convert acquired characteristics into inherited traits, or affect fossil formation as a direct consequence.

Analogous organs perform similar functions but arise from different basic structures and evolutionary origins. Insect wings and bird wings both enable flight, yet their anatomy and developmental history are different. They illustrate adaptation to a similar function through separate evolutionary pathways, often called convergent evolution. Therefore option B is correct. Same origin with different functions describes homologous organs, not analogous organs.

The human hand and horse forelimb contain corresponding bones arranged according to a similar basic structural plan, inherited from a common vertebrate ancestry. Their functions are different: the hand is suited for grasping and manipulation, whereas the horse forelimb supports weight and movement. Similar structure with divergent function identifies homologous organs, so option C is correct. They are not analogous, vestigial, or artificial organs.

Insect wings and bird wings perform the same broad function—flight—but they are built from different structures and arose independently in these unrelated animal groups. This combination of similar function and different origin is the definition of analogous organs and reflects convergent evolution. Option B is therefore correct. Similar basic structure would indicate homology, while these wings are neither fossils nor vestigial organs.

The governing concept is the law of superposition, which is used for relative dating. In an undisturbed sequence of sedimentary rocks, newer material is deposited above older material. Therefore, fossils in lower layers are generally older than fossils in upper layers. This does not give an exact numerical age, and folding or disturbance may alter the order; hence “always” makes option A incorrect.

A fossil sequence represents organisms that lived at different times. By comparing their preserved structures in chronological order, scientists can identify changes in body form and adaptations across generations. Thus, the sequence can provide evidence of gradual evolutionary change. It cannot prove that all organisms ended together, that ancient organisms lacked genes, or that inheritance was absent, so option A is correct.

Fossilisation requires preservation of remains for a long period. Rapid burial by mud, sand, or other sediment can protect remains from oxygen, scavengers, weathering, and physical damage, reducing decomposition. Hard parts such as bones and shells are especially likely to persist. Quick decay, burning, or prolonged exposure destroys evidence, so option B gives the most suitable condition.

Evolution is not a ranking system in which every organism moves toward a single superior endpoint. Populations share ancestors and may diverge when they experience different environments, resources, or selection pressures. Each lineage can follow its own adaptive pathway, producing branches. Therefore, option B correctly describes branching evolution; the other choices wrongly treat humans as the universal goal or reject variation.

In evolutionary biology, similarities in anatomy, development, and genetic material can indicate common ancestry. They do not mean that modern humans descended directly from modern chimpanzees; rather, both lineages diverged from an older ancestral population and evolved separately afterward. Thus option C is the scientifically suitable interpretation. Options A, B, and D misunderstand descent or genetics.

Artificial selection occurs when humans repeatedly choose and breed organisms that possess desired heritable traits. In wild cabbage, people selected different features, such as enlarged leaves, flower clusters, or stems, over many generations. This produced distinct cultivated forms including cabbage, cauliflower, and broccoli. A natural disaster, sex determination, or fossilisation does not create these selectively bred varieties, so option B is correct.

Both processes act on heritable variation, but the selecting force differs. In artificial selection, humans choose organisms with preferred traits and breed them. In natural selection, environmental conditions influence which inherited traits improve survival or reproduction, without deliberate human choice. Therefore option A states the main difference. The other options deny selection, wrongly restrict it to farms, or ignore genes.