Heredity and Evolution

The mother’s egg contributes an X chromosome, while the father’s sperm may contribute either X or Y. In this case the combination is X + Y = XY. Under the usual human XX–XY system, an XY zygote generally develops as a male, so option A is correct. An XX combination is usually female, and the alternatives neither represent the chromosome result nor a biological outcome of this fertilisation.

Evolution is the gradual change in the inherited characteristics of populations over many generations. Small variations may accumulate, and helpful traits can become more common when they improve survival or reproduction. Therefore, option A correctly describes evolution. Option B is not a biological definition because a new organism does not normally arise in one day; options C and D are life processes, not evolutionary change.

Fossils are preserved remains, impressions, or traces of organisms that lived in the past. Comparing fossils from older and newer rock layers can show changes in body forms and relationships among organisms, so fossils provide evidence about past life and evolution. Thus option A is correct. Weather, blood sugar, and plant water content are not established by studying fossils.

When organisms die, their remains may be buried under sediments such as mud or sand. Over a very long period, pressure and mineral deposition can preserve these remains or impressions in sedimentary rock layers. Therefore option A is correct. Milk, human blood, and ordinary tree leaves are not the geological settings in which fossils are generally preserved; the word “always” also makes option D incorrect.

Homologous organs have a similar basic structural plan and common evolutionary origin, although their present functions may differ. The bones in a human arm and a whale forelimb follow the same basic arrangement, even though they are used differently. Hence option A is correct. Bird and insect wings are analogous, while the other pairs do not represent the standard homologous comparison.

Analogous organs perform similar functions but arise from different structural plans and evolutionary origins. For example, the wings of a bird and an insect both help in flight, but their internal structures and origins are different. Therefore option A is correct. Homologous organs share a basic origin even when their functions differ; acquired and reproductive organs do not describe this comparison.

Bird wings and insect wings have a similar function: they enable flight. However, the two structures developed from different body plans and do not share the same basic evolutionary origin. This combination of similar function and different origin defines analogous organs, so option A is correct. They are not homologous, recessive, or fossil organs.

Charles Darwin is chiefly associated with the theory of evolution by natural selection. He explained that individuals vary, organisms struggle for survival, and those with advantageous heritable traits are more likely to survive and reproduce. Therefore option A is correct. Mendel explained patterns of inheritance, Newton worked mainly in physics, and Pasteur made major contributions to microbiology and vaccination.

Natural selection favours heritable variations that help organisms cope with particular environmental conditions. Organisms possessing suitable traits may obtain resources, avoid dangers, survive longer, and reproduce more successfully. Thus option A is correct. Being the biggest is not always useful, and organisms must generally obtain food and reproduce for their traits to persist in later generations.

An adaptation is a structural, physiological, or behavioural characteristic that increases an organism’s ability to survive and reproduce in a particular environment. Examples include camouflage, thick fur, or specialised body parts. Therefore option A is correct. Adaptation does not mean that all organisms are identical, and changes in food colour or melting rocks are unrelated physical events.

Speciation is the evolutionary process through which one population becomes separated into distinct groups that gradually accumulate differences and eventually form a new species. Reproductive isolation often prevents these groups from producing fertile offspring with one another. Therefore option A is correct. Photosynthesis makes food, excretion removes wastes, and respiration releases energy; none of these processes creates a new species.

The governing concept is genetic drift, a change in the frequency of alleles or traits caused by chance rather than by a predictable advantage. Its effect is especially strong in a small, separated population because the accidental loss or reproduction of a few individuals can change the group’s genetic composition noticeably. Thus the described chance change is genetic drift. Photosynthesis, digestion and osmosis are biological processes but do not explain inherited trait frequencies.

Variation means differences among individuals of the same species. The governing evolutionary concept is that these differences provide the raw material for natural selection. If a variation helps an organism survive or reproduce in a particular environment, it may be passed to later generations and become more common. Therefore variations support adaptation and evolution. They do not automatically destroy organisms, stop reproduction or make food poisonous.

The governing concept is that evolution means change in inherited characteristics over generations, not a guaranteed march toward greater complexity or superiority. Natural selection favours traits that work well in a particular environment, so a simple organism can remain successful without becoming more advanced in a general sense. Therefore option A is correct. Evolution may produce adaptation and diversity; it does not promise that every new organism will be more advanced than its ancestor.

The governing concept is heredity, the transmission of traits from parents to offspring. A family tree, or pedigree, records relationships across several generations and marks which individuals show a particular characteristic. By examining its pattern, one can infer how the trait may be inherited and whether it appears in successive generations. Hence option A is correct. Measuring weather, cooking food and breaking rocks have no connection with pedigree analysis or genetic study.

The governing concept is recessive inheritance. A recessive allele is usually masked when a dominant allele is present in a heterozygous individual. It is expressed when an individual receives the recessive allele from both parents, producing a homozygous recessive condition. Thus option A is correct, with “both copies” referring to the two recessive alleles. A dominant allele does not reveal the recessive phenotype, and fossils or absence of reproductive cells are irrelevant.

Use Mendel’s dominance principle. Let Y represent the dominant yellow allele and y represent the recessive green allele. Pure yellow has genotype YY, while pure green has genotype yy. Their cross is YY × yy, so every offspring receives Y from the first parent and y from the second, giving genotype Yy. Because Y is dominant, all first-generation seeds will appear yellow. Therefore option A is correct.

The governing concept is Mendelian inheritance. A recessive allele can be present in an organism without being expressed when a dominant allele is also present. During gamete formation and fertilisation, two recessive alleles may come together in the second generation, making the trait visible again. Thus, its reappearance proves that the trait was inherited and concealed, not destroyed. Options B, C and D contradict heredity principles.

The governing concept is homology and common ancestry. Homologous organs have the same basic structural plan but may perform different functions, such as the forelimbs of humans, bats and whales. This shared structure suggests descent from a common ancestor and therefore helps scientists infer evolutionary relationships. Body colour, food taste or breathing rate alone cannot reliably establish such relationships, so option A is correct.

A common ancestor is an earlier organism or ancestral population from which two or more related groups gradually evolved. It need not look exactly like any living descendant, because populations change over many generations. Similar structural features, fossils and genetic evidence can support this inference. Therefore option A gives the correct meaning; the other options either deny that the ancestor was living or incorrectly restrict the term to present organisms or seeds.

The governing concept is comparative evidence for evolution. Humans and chimpanzees share many anatomical, physiological and genetic features, so their similarities indicate that they are related through evolution and likely share a relatively recent common ancestor. This does not mean they are the same individual or that one is a plant. Both organisms possess chromosomes, so options B, C and D are scientifically incorrect.

The governing concept is adaptation and natural selection. A trait is advantageous only in relation to particular environmental conditions. Thick fur may help an animal in a cold region but be disadvantageous in a hot region; similarly, camouflage works only when it matches the surroundings. Thus option A is correct. The name of an organism, book colour or rock height does not by itself determine biological advantage.

The governing concept is adaptation, specifically protective colouration or camouflage. If an inherited colour pattern helps an organism blend with its surroundings, predators may find it harder to detect, increasing the organism’s chance of survival. Such a useful inherited feature is called an adaptation. Excretion removes wastes, digestion breaks down food and respiration releases energy; none of these describes hiding by colour. Therefore A is correct.

Evolution means heritable changes in populations across successive generations. Reproduction creates new generations and transfers genetic material, including inherited variations, from parents to offspring. Natural selection can act on those heritable differences only when they are passed on. Without reproduction, an individual might change during its lifetime, but that change would not form an evolutionary lineage. Hence A is correct; the other choices are unrelated.

The governing concept is genetic variation. During reproduction, DNA or other genetic material is copied, and an occasional copying error can alter a gene or its instruction. The resulting difference may produce a variation in a trait; it may be neutral, harmful or sometimes useful, so death is not inevitable. It also does not cause sameness or digestion. Therefore variation, option A, is the correct possible result.