Heredity and Evolution

The governing concept is homology: structures inherited from a common ancestral body plan can retain similar bones even when their functions diverge. A human hand is mainly used for grasping, whereas a bird wing is adapted for flight, but both contain corresponding limb bones. Therefore option A is correct. Analogous organs share function without the same basic structure, and the other choices describe unrelated functions.

Analogous organs perform similar functions but arise from different basic structural plans and evolutionary origins. The wings of a bird and an insect both support flight, yet their internal construction is very different. Thus option A is correct. Homologous organs, represented by the distractor idea of identical structure and function, are defined by shared basic structure rather than merely similar use; the remaining choices are unrelated.

The governing idea is analogy, in which different structures independently perform a similar function. Both a bird wing and an insect wing enable flight, but a bird wing contains bones and is a modified forelimb, whereas an insect wing is an outgrowth of the body wall. Hence option A is correct. They are not homologous, respiratory, or secretory organs.

Homologous organs can indicate common ancestry because organisms may inherit a similar underlying structural plan from an ancestral form. Their present functions may differ as populations adapt to different conditions, but the corresponding arrangement of parts remains evidence of relationship. Therefore option A is correct. Shared colour, food, or location alone does not reliably establish ancestry and cannot replace structural or genetic evidence.

Fossils are preserved remains, impressions, or traces of organisms that lived in the past. Comparing fossils from different geological layers can reveal the appearance, disappearance, and gradual modification of life forms through time. This historical evidence helps scientists study evolution, so option A is correct. Fossils are not used to taste food, predict current weather, or measure a heartbeat.

In school-level biology, a species is a group of similar organisms that can generally interbreed with one another and produce fertile offspring. This reproductive connection helps distinguish one species from another and is important for classification and evolution. Therefore option A is correct. A food name, a geographical place, or a colour describes neither reproductive compatibility nor biological classification.

Genetic drift is a change in the frequency of inherited traits caused by chance rather than by a trait’s usefulness. Its effect is especially strong in a small population, where the accidental survival or reproduction of only a few individuals can alter the proportion of alleles in later generations. Thus, option A is correct. Options B, C, and D describe food movement, evaporation, and blood clotting, which are unrelated biological processes.

Natural selection operates through the interaction between inherited variation and environmental conditions such as food availability, temperature, disease, and predators. A trait that improves survival or reproduction in one environment may be less useful in another. Therefore, option A is correct: the environment influences which existing variations are advantageous. It does not make organisms identical, erase genes, or provide immortality, so B, C, and D are incorrect.

Charles Darwin is most closely associated with the theory of evolution by natural selection. He explained that individuals in a population show heritable variations, and those with variations better suited to their surroundings are more likely to survive and reproduce. Over generations, useful variations become more common. Thus, option A is correct. Blood circulation, electric current, and photosynthesis are important topics, but they are not Darwin’s central scientific contribution.

A dominant allele is expressed in the phenotype even when it is paired with a contrasting recessive allele. In pea plants, a plant with genotype Tt is tall because the tall allele masks the dwarf allele; the dwarf trait appears phenotypically only in tt plants. Therefore, option A is correct. Dominance does not mean that a trait is found only in leaves or that it makes food, so C and D are unrelated, while B is opposite to the observation.

A recessive trait is expressed when an individual has two recessive alleles, commonly represented by aa. If a dominant allele is present, it usually masks the recessive allele in the phenotype, although the recessive allele can still be inherited. Thus, option A is correct. Option B describes the situation in which the dominant trait is expressed, while C and D have no valid genetic connection with recessive expression.

Evolution is a population-level change in inherited characteristics across successive generations. It usually results from many small variations, selection, and reproductive differences accumulating over long periods, often far beyond a human lifetime. Consequently, a person generally cannot watch a complete evolutionary change directly, although fossils, comparative anatomy, embryology, and genetic evidence reveal it. Option A is correct; B, C, and D are scientifically irrelevant or contradictory.

A useful variation is an inherited difference that improves an organism’s suitability to particular environmental conditions. For example, better camouflage or resistance to a disease may increase the probability of surviving and reproducing. Natural selection can then make such a variation more common in later generations, though usefulness depends on the environment and is not guaranteed. Therefore, option A is correct. B, C, and D describe impossible or harmful effects, not adaptive variation.

The chapter connects two governing ideas: heredity and evolution. Heredity explains how genetic information and traits are transmitted from parents to offspring. Reproduction can also introduce heritable variations, so members of a population are not always identical. If useful variations are preserved over many generations by natural selection, populations may gradually change and evolve. Therefore option A gives the complete message, while the other options deny heredity, variation, or the role of inherited traits.

During reproduction, genetic material must be copied and passed to the next generation. Copying is generally accurate, but small errors or changes can occur, producing heritable variation. Sexual reproduction can also combine genetic material from two parents, increasing differences among offspring. These variations may later be acted upon by natural selection. Therefore option A explains the difference; the other options either contradict observed variation or describe unrelated environmental conditions.

Genes are the basic units of heredity. A gene is a specific segment of DNA that carries information related to a characteristic and helps direct the production of functional molecules, often proteins. Because genes are transmitted from parents to offspring, they influence inherited traits. Saliva, sweat, and bile have important physiological functions, but they do not store or transmit the hereditary instructions controlling such traits. Thus option A is correct.

In a typical eukaryotic cell, most genetic material is present as DNA inside the nucleus, where it is organised into chromosomes. The nucleus protects and regulates access to this hereditary information. Some DNA is also present in mitochondria, and in plants in chloroplasts, but the main location remains the nucleus. The cell wall provides support, the vacuole stores materials, and the space outside the cell is not the principal location of genetic material. Hence option A is correct.

Chromosomes are thread-like structures in the nucleus made primarily of DNA, a nucleic acid, associated with specialised proteins called histones. DNA stores hereditary information, while the proteins help package and organise the long DNA molecule. Therefore the scientifically correct form of option A is nucleic acid and proteins. Water, fats, and starch may occur in cells for other purposes, but none of them alone forms chromosomes or carries their complete hereditary organisation.

Gregor Mendel is called the father of genetics because his controlled pea-plant crosses revealed regular patterns in the inheritance of traits. By analysing traits across parent, first-generation, and later-generation plants, he proposed principles such as segregation and independent assortment. These principles became foundations of classical genetics. Plant nutrition, respiration, and excretion are different biological processes and were not the subject for which Mendel became famous. Thus option A is correct.

The governing concept is dominance and recessiveness in Mendelian inheritance. When two contrasting alleles are present, a dominant allele masks the expression of a recessive allele in the first generation. Thus, a trait that remains hidden is called a recessive trait. It can reappear in a later generation when the offspring receives two recessive alleles. Therefore, option A is correct; a dominant trait is expressed, while acquired and fossil traits are different concepts.

Heredity is the transmission of genetic information from parents to offspring through reproductive cells. A trait whose information is encoded in DNA and passed through gametes is called an inherited trait. Therefore option A is correct. Learned behaviour, a scar, or a body part removed during life is generally acquired after birth and does not normally alter the genetic information of the gametes, so these are not inherited in the usual sense.

Inheritance depends on genetic information carried in reproductive cells, such as sperm and eggs. A change acquired during life, such as a scar or increased muscle size from exercise, usually affects body cells but does not alter the DNA of the reproductive cells. It therefore is not normally transmitted to offspring. Option A states this principle; dominance, plant restriction, and chromosome doubling are unrelated explanations.

The governing principle is that only heritable genetic changes in reproductive cells are normally transmitted to the next generation. Cutting a mouse’s tail changes its body after birth, but it does not change the DNA in its sperm or eggs. Consequently, the offspring inherit the genetic instructions for developing a tail and usually have tails. Option A is correct; dominance, plant identity, and food alone do not explain inheritance.

Humans normally have one pair of sex chromosomes. In the standard chromosomal system, a human female has two X chromosomes, written as XX. Each egg carries an X chromosome, and an X-bearing sperm produces the XX combination after fertilisation. Therefore option A is correct. XY is the usual male combination, YY is not a viable standard human combination, and XO represents a chromosomal condition rather than the usual female pattern.

In the usual human XX–XY system, a male has two unlike sex chromosomes: one X and one Y. The combination is therefore written as XY. The mother contributes an X chromosome, while a Y-bearing sperm from the father produces the XY combination. Option A is correct. XX is the usual female combination, YY is not a standard viable human combination, and XXX is a sex-chromosome variation rather than the usual male pattern.