Heredity and Evolution

The governing concept is stomatal regulation of transpiration. Stomata permit carbon dioxide entry and gas exchange, but open stomata also allow water vapour to diffuse out of leaves. During water shortage, failure to close them increases transpiration and can worsen dehydration, wilting, and loss of turgidity. Abscisic acid commonly promotes stomatal closure under water stress. Blood sugar, a spinal cord, and muscle contraction are irrelevant plant outcomes. Option A is correct.

Homology is determined mainly by common basic structural plan and evolutionary origin, not merely by outward appearance. Homologous organs may perform different functions, such as the forelimbs of humans, whales, and bats, yet share underlying structural features. Conversely, organs can look similar or perform similar functions while having different origins and structures. Therefore, option A is correct; colour, digestion, and occurrence only in humans are not criteria for homology.

Analogous organs perform similar functions but arise from different basic structures and evolutionary origins. Similar environmental demands can lead unrelated organisms to develop comparable adaptations, a process called convergent evolution. For example, a bird’s wing and an insect’s wing both help in flight, but their internal structures and origins differ. Hence, option A is correct. A shared ancestor, no function, or colour change does not explain analogy, so B, C, and D are incorrect.

Heredity is the transmission of genetic information and traits from parents to offspring, so it produces resemblance between generations. Variation means differences in inherited features among offspring, arising from recombination, mutation, and other biological processes. These differences provide the basis for natural selection and evolution. Hence option A correctly states the relationship; the other choices deny the meaning of heredity or variation.

An offspring resembles its parents because it inherits chromosomes and genes from them. However, sexual reproduction combines one set of genetic information from each parent in a new arrangement, and recombination can produce differences. Therefore resemblance and variation occur together, making option A the best answer. Chromosomes and gametes are essential, so options B and D are incorrect; food alone does not determine all traits.

A gene is a specific segment of DNA located on a chromosome. It carries hereditary information that contributes to the expression of a particular trait, often by directing the formation of a functional protein. This is why it is called a basic unit of heredity and traits. Option A is correct; genes do not independently make blood or digest food, and they are located within cells rather than outside them.

In humans, ordinary body cells are diploid and contain 46 chromosomes, arranged in 23 pairs. Gametes are produced by meiosis and are haploid, containing 23 chromosomes. During fertilisation, the egg and sperm nuclei fuse, restoring the diploid number of 46 in the zygote. Thus option A is correct; chromosomes are neither absent nor doubled in gametes, and they are not destroyed during fertilisation.

Let the allele for tallness be T and the allele for dwarfness be t. A pure tall plant has TT and a pure dwarf plant has tt. Their cross produces F1 offspring with genotype Tt. Since T is dominant, each heterozygous plant expresses tallness even though the recessive t allele is present. Thus option A is correct; the dwarf allele is masked, not destroyed.

In the F1 generation from a pure tall TT × pure dwarf tt cross, all plants are Tt. The dominant T allele masks the recessive t allele, so dwarfness is not expressed, but the t allele remains present. When F1 plants self-pollinate, the F2 combinations include TT, Tt, Tt, and tt. The tt plants express dwarfness, so option A is correct.

The governing concept is Mendel’s law of segregation. In a monohybrid cross such as Tt × Tt, the alleles separate during gamete formation, producing TT, Tt, Tt and tt in a 1:2:1 genotypic ratio. Because T is dominant, three offspring show the dominant phenotype and one shows the recessive phenotype. Thus option A is correct; the other choices incorrectly describe blending, absence of variation or chromosome formation.

The governing concept is independent assortment during gamete formation. In a dihybrid cross, each parent can produce different gamete types because the allele pair for one trait separates independently from the allele pair for the other. When these gametes fuse, alleles may recombine into combinations not seen in either parent, such as round-green or wrinkled-yellow. Thus A is correct; genes are not destroyed, parents do contribute genes, and food alone does not determine inherited traits.

The governing concept is the F2 phenotypic ratio of a standard Mendelian dihybrid cross with independent assortment and complete dominance. Each parent in AaBb × AaBb can form AB, Ab, aB and ab gametes. Combining these gives four phenotype classes in the ratio 9 showing both dominant traits, 3 showing the first dominant and second recessive, 3 showing the first recessive and second dominant, and 1 showing both recessive traits. Therefore B is correct.

The governing concept is heredity: acquired traits arise during an organism’s lifetime because of practice, use, injury, or environmental conditions, whereas inherited traits are associated with genetic information transmitted through gametes. Thus, a learned skill or developed muscle is not normally passed directly to offspring. Option A is correct; B ignores the distinction, C wrongly says all acquired traits are inherited, and D confuses nutrition with heredity.

The governing idea is the difference between acquired characteristics and inherited genetic information. Training enlarges an athlete’s muscles by changing body tissues and performance during life; it usually does not alter the DNA of sperm or eggs in a way that encodes those muscles. Therefore the developed muscles are not directly transmitted. Option A is correct, while B and C are biologically false and D describes no normal effect of exercise.

The governing concept is that a body modification is not automatically a hereditary change. Cutting a mouse’s tail alters a somatic body part, but it does not normally alter the genetic information carried by its reproductive cells. If tailless mice are bred, their offspring are generally born with tails, showing that the acquired loss was not inherited. Option A is correct; B and C contradict heredity, and D is false.

Evolution requires more than the appearance of differences. A variation must be heritable, and if it improves survival or reproduction in a particular environment, it may become more common over successive generations. Neutral or harmful variations may disappear, while a non-heritable variation cannot be reliably passed on. Option A captures this requirement. B and C are absolute and false, and D misunderstands evolution as an immediate event.

Natural selection is governed by differential survival and reproduction, not simply by size. A useful trait gives an organism an advantage under particular environmental conditions, such as better camouflage, disease resistance, or efficient feeding. The organism may then survive longer and leave more offspring, increasing the trait’s frequency in the population. Option A is correct; B is an unjustified absolute, while C and D are biologically incorrect.

Natural selection changes trait frequencies because environmental conditions give some heritable traits an advantage in survival or reproduction. Genetic drift changes allele frequencies through random events, such as the accidental survival or reproduction of particular individuals, and it is especially influential in small populations. Thus option A correctly contrasts selection with chance. B is irrelevant, C is false, and D denies the environmental role in selection.

Genetic drift is random change in allele frequency. In a small population, each individual represents a relatively large fraction of the gene pool, so the accidental loss, survival, or reproduction of only a few individuals can greatly change the proportion of alleles. Repeated chance events may even eliminate an allele. Option A is correct; B, C, and D are biologically false explanations.

Fossils are preserved remains, impressions, or traces of organisms that lived in the past. By comparing fossils from rock layers of different ages with living organisms, scientists can identify structural similarities, differences, and changes through geological time. This provides evidence for gradual biological change and evolutionary relationships. Option A is correct; the other choices assign fossils unrelated abilities such as controlling weather or changing blood groups.

The governing geological principle is relative dating: when rock layers have not been greatly disturbed, deeper layers were generally deposited earlier than layers above them. A fossil in such a deeper layer is therefore usually considered relatively older than one in a higher layer. This conclusion is comparative, not an exact numerical age. Option A is correct; B, C, and D contradict the meaning of a fossil and the ordering of strata.

Homologous organs have a similar basic structural plan because of common ancestry, although they may perform different functions. The human hand is adapted for grasping and the horse forelimb for support and locomotion. Their corresponding bones, such as the upper arm, forearm and hand-region bones, reveal the shared plan. Therefore option A is correct; the other choices confuse function, appearance or digestion with structural homology.

Analogous organs perform similar functions but arise from different basic structures and evolutionary histories. For example, wings of birds and insects both support flight, yet a bird wing is a modified bony forelimb while an insect wing develops from a different body structure. Similar environmental demands can produce similar functions without close ancestry. Thus option A is correct; the other choices are biologically false or irrelevant.

Analogous organs have a similar function but different structural plans and evolutionary origins. Both bird wings and insect wings enable flight, so their function is comparable. However, a bird wing is a bony forelimb with feathers, whereas an insect wing is an outgrowth associated with the insect body covering and lacks the bird’s limb-bone plan. Therefore option A is correct, not B, C or D.

An inherited trait is influenced by genetic information received from parents and can be transmitted through reproductive cells. Eye colour depends mainly on inherited genes that affect pigment production, although biological traits can involve several factors. A haircut changes the visible length of hair during an individual’s life but does not alter the genes in reproductive cells. Hence option A is correct; the other options confuse nutrition or acquired changes with heredity.