Heredity and Evolution

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.

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.

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.