AIPMT 1991 Biology Question Paper with Answer and Solution

(D) Modern biological classification,often referred to as phylogenetic classification,relies on a comprehensive analysis of organisms.
While external features were used in early systems (like those of Aristotle),modern taxonomy emphasizes evolutionary relationships.
These relationships are determined by comparing anatomical (structural),physiological (functional),and biochemical characteristics,as well as genetic data.
Therefore,anatomical and physiological characteristics provide the most robust evidence for establishing evolutionary lineages and classifying organisms accurately.

(D) African sleeping sickness, also known as Human African Trypanosomiasis, is caused by protozoan parasites of the genus $Trypanosoma$.
Specifically, there are two subspecies that cause this disease in humans: $Trypanosoma \text{ } brucei \text{ } gambiense$ and $Trypanosoma \text{ } brucei \text{ } rhodesiense$.
Since both $T. \text{ } rhodesiense$ and $T. \text{ } gambiense$ are causative agents, and in many standard biology contexts, $T. \text{ } gambiense$ is the most frequently cited answer for the chronic form, but both are correct. Given the options, $T. \text{ } gambiense$ is the classic textbook answer for the primary causal organism.

(D) In $Spirogyra$,sexual reproduction occurs through a process known as conjugation.
During this process,two filaments come together,and their protoplasts fuse to form a diploid structure.
This resulting diploid structure is known as a $Zygospore$.
The $Zygospore$ is a thick-walled,resistant resting spore that allows the organism to survive unfavorable environmental conditions.

(B) The classification of sponges ($Phylum$ $Porifera$) is primarily based on the nature and composition of their endoskeleton,which consists of spicules or spongin fibers. Therefore,the presence and type of spicules are the key criteria for their classification.

(C) All sponges reproduce sexually or asexually.
Sex cells (sperm and ova) arise from undifferentiated cells known as archaeocytes,which are amoeboid cells capable of differentiating into various cell types,including gametes.

(B) In the earthworm, there are $4$ pairs of large, thick, muscular, and rhythmically contractile hearts.
These hearts are located in segments $7, 9, 12,$ and $13$.

(A) Cockroaches are unisexual and exhibit sexual dimorphism.
In males,the $9^{th}$ abdominal segment bears a pair of short,thread-like anal styles ventrally,which are absent in females.
Anal cerci are present in both sexes,so they cannot be used to distinguish between them.
Therefore,the presence of anal styles is a key external feature to distinguish males from females.

(C) The metamorphosis of insects is primarily regulated by the hormone $Ecdysone$.
$Ecdysone$ is a steroid hormone secreted by the prothoracic glands in insects.
It triggers the molting process and the transition between different developmental stages (larva to pupa to adult).
Therefore,the correct option is $C$.

(D) The order $Squamata$ is the largest order of reptiles,which includes lizards and snakes. These animals are characterized by their skin,which bears horny scales or shields that are shed periodically.

(C) An apocarpous gynoecium is one in which the carpels are free from each other. When there are multiple free carpels,it is referred to as a multicarpellary apocarpous gynoecium. Examples of plants exhibiting this condition include $Lotus$ and $Ranunculus$.

(C) The botanical name of cauliflower is $Brassica \ oleracea \ var. \ botrytis$.
It belongs to the family $Brassicaceae$ (formerly $Cruciferae$).
$Brassica \ oleracea \ var. \ capitata$ is the botanical name for cabbage,while $Brassica \ oleracea \ var. \ gemmifera$ is for Brussels sprouts.

(D) The correct answer is $D$.
$1$. Epipetalous stamens are those where the stamens are attached to the petals.
$2$. Syngenesious stamens are those where the anthers are fused together while the filaments remain free.
$3$. These conditions are characteristic features of the family $Asteraceae$ (also known as $Compositae$).
$4$. In $Solanaceae$, stamens are epipetalous but not syngenesious.
$5$. In $Brassicaceae$, the condition is tetradynamous.
$6$. In $Fabaceae$, the condition is usually diadelphous.

(A) The photosynthetic pigments (such as chlorophyll $a$,chlorophyll $b$,xanthophylls,and carotenoids) are fat-soluble molecules.
These pigments are embedded within the lipid bilayer of the thylakoid membranes in the chloroplast.
The thylakoid membranes are the sites where the light-dependent reactions of photosynthesis occur.

(B) Ferredoxin is an iron-sulfur protein that acts as an electron carrier in the photosynthetic electron transport chain.
It is specifically associated with Photosystem-$I$ $(PS-I)$.
During the light reaction,electrons are transferred from the primary electron acceptor of $PS-I$ to ferredoxin,which then transfers them to $NADP^+$ reductase to reduce $NADP^+$ to $NADPH$.

(A) The $Krebs$ cycle,also known as the $Tricarboxylic$ $Acid$ $(TCA)$ cycle or $Citric$ $Acid$ cycle,begins with the condensation of an $Acetyl$ $CoA$ molecule $(2C)$ with $Oxaloacetic$ $acid$ ($OAA$,$4C$) in the presence of water to form $Citric$ $acid$ $(6C)$.
Since $Acetyl$ $CoA$ is derived from the oxidative decarboxylation of $Pyruvic$ $acid$,$Pyruvic$ $acid$ is the primary substrate that initiates the sequence leading into the cycle.

(D) $Florigen$ is a hypothetical hormone that is believed to be synthesized in the leaves and transported to the shoot apical meristem to induce flowering. Although it has not been isolated as a single chemical entity,it is considered a crucial signaling molecule in the process of photoperiodism.

(A) Tendrils are specialized structures in climbing plants that respond to touch or contact with a solid support.
When a tendril comes into contact with an object,the cells on the side touching the object grow more slowly than the cells on the opposite side.
This differential growth causes the tendril to coil or twine around the support.
This directional growth movement in response to touch or mechanical stimulus is known as $Thigmotropism$.

(C) The main sources of uric acid in humans are purines. Purines such as adenine and guanine are metabolized into xanthine,which is further oxidized to form uric acid.
$Adenine \to Xanthine \to Uric\ acid$

(B) The renal columns of Bertini are extensions of the renal cortex that project into the renal medulla between the renal pyramids. These columns contain blood vessels and connective tissue,serving as a structural support for the kidney.

(B) Peptide hormones are water-soluble and cannot cross the plasma membrane of the target cell.
They bind to specific receptors present on the surface of the plasma membrane.
This binding triggers the generation of second messengers,such as cyclic $AMP$ $(cAMP)$,$IP_3$,or $Ca^{2+}$,inside the cell.
These second messengers then regulate cellular metabolism and physiological responses.
Therefore,the action of peptide hormones is mediated by cyclic $AMP$.

(C) $ADH$ stands for Antidiuretic Hormone,also known as Vasopressin.
It is synthesized by the hypothalamus and stored and released by the posterior pituitary gland (neurohypophysis).
Its primary function is to act on the kidney tubules (specifically the distal convoluted tubules and collecting ducts) to promote the reabsorption of water from the glomerular filtrate,thereby reducing urine volume and preventing dehydration.

(C) The blood group of an individual is determined by the presence of specific antigens on the surface of red blood cells.
Individuals with blood group $AB$ possess both antigen $A$ and antigen $B$ on their red blood cells.
Because they have both antigens,their blood plasma does not contain anti-$A$ or anti-$B$ antibodies.
Therefore,a person with antigens $A$ and $B$ and no antibodies belongs to blood group $AB$.

(C) Nucleotides are the fundamental building blocks or monomeric units of nucleic acids ($DNA$ and $RNA$).
Each nucleotide consists of three distinct components:
$1$. $A$ nitrogenous base (Purines or Pyrimidines).
$2$. $A$ pentose sugar (Ribose or Deoxyribose).
$3$. $A$ phosphate group (derived from phosphoric acid).
Therefore,the correct composition is Base-sugar-phosphate.

(B) The correct answer is $B$.
Deep black soil is highly productive because it is rich in clay and humus.
Clay particles have a high water-holding capacity,and humus provides essential nutrients,making the soil fertile and suitable for agriculture.

(C) : The growth movement in response to touch,or contact of a foreign body,in plants is called thigmotropism or thigmotropic movement.
The stems and tendrils of the climbers are positively thigmotropic in their response.
The coiling of garden pea tendrils around any support is an example of thigmotropism.
Thigmonastic (haptonastic) movements are induced by some external stimuli,but they are non-directional. For example,tentacles of $Drosera$ leaf curve and the lamina lobes of $Dionaea$ fold on coming in contact with an insect.

(B) Homeostasis is the ability of an organism to maintain a stable internal environment (such as body temperature,ionic balance,etc.) despite fluctuations in the external environment.
This process is essential for the survival of organisms under changing environmental conditions.

(B) Pedology is defined as the scientific study of soil in its natural environment. It involves the study of soil formation (pedogenesis),soil morphology,and the classification of soils. Therefore,the correct option is $B$.

(A) Alternation of generations involves the regular alternation between a haploid $(n)$ gametophyte phase and a diploid $(2n)$ sporophyte phase. This process requires the occurrence of syngamy (fusion of gametes) to restore the diploid state and meiosis (reduction division) to return to the haploid state. Bacteria are prokaryotic organisms that reproduce primarily through binary fission. They do not undergo true sexual reproduction involving the formation of gametes (syngamy) or reduction division (meiosis). Therefore,they cannot exhibit an alternation of generations.

(C) Osmoregulation is the process of maintaining the water and salt balance within an organism. In freshwater protozoans like $Amoeba$ and $Paramecium$,the contractile vacuole is the specialized organelle responsible for osmoregulation. It collects excess water from the cytoplasm and expels it out of the cell to prevent the organism from bursting due to the influx of water by osmosis.

(C) African sleeping sickness is a parasitic disease caused by the protozoan $Trypanosoma$ $gambiense$ (or $Trypanosoma$ $brucei$ $gambiense$).
It is transmitted to humans through the bite of the tsetse fly,which belongs to the genus $Glossina$ (specifically $Glossina$ $palpalis$ or $Glossina$ $morsitans$).
Therefore,the correct option is $C$.

(A) In $Spirogyra$,sexual reproduction occurs through conjugation,where two filaments fuse to form a diploid zygote. This zygote develops a thick wall to become a resting spore known as a $Zygospore$. Similarly,in $Chlamydomonas$,the fusion of gametes (fertilization) results in the formation of a diploid zygote,which also develops into a $Zygospore$ to survive unfavorable conditions. Therefore,the product of these processes is a $Zygospore$.

(A) Bryophytes are known as the amphibians of the plant kingdom because they can live in soil but are dependent on water for sexual reproduction. Specifically,the male gametes (antherozoids) require a film of water to swim to the female sex organ (archegonium) for fertilization. While they are found in moist and shady habitats,their classification as 'amphibians' is primarily due to this critical dependence on water for the completion of their life cycle.

(D) In the plant kingdom,the gametophyte generation shows a trend of reduction as evolution progresses.
In $Bryophytes$,the gametophyte is the dominant,independent,and photosynthetic phase,making it the largest among the given options.
In $Pteridophytes$ (like $Selaginella$) and $Gymnosperms$ (like $Cycas$),the gametophyte is reduced and dependent on the sporophyte.
In $Angiosperms$,the gametophyte is highly reduced,consisting of only a few cells (e.g.,pollen grain and embryo sac).

(A) In earthworms (Pheretima), there are $4$ pairs of specialized hearts. These are located in segments $7, 9, 12,$ and $13$. The lateral hearts are found in segments $7$ and $9$, while the latero-oesophageal hearts are found in segments $12$ and $13$.

(A) The $Cysticercus$ (bladder worm) is a larval stage in the life cycle of the tapeworm (specifically $Taenia$ $solium$).
It develops from the $Oncosphere$ larva after it is ingested by the intermediate host (pig).
This larva encysts in the muscles of the host,forming a fluid-filled bladder,hence the name 'bladder worm'.

(B) The phylum $Echinodermata$ is characterized by a unique feature where adults exhibit radial symmetry (specifically pentamerous radial symmetry),while their larvae exhibit bilateral symmetry. Among the given options,radial symmetry is the defining characteristic of adult echinoderms. Porous bodies are characteristic of $Porifera$,and bilateral symmetry is found in most other phyla like $Platyhelminthes$ to $Chordata$.

(A) In flatworms (Phylum Platyhelminthes),such as $Taenia$ (tapeworm),the specialized cells used for excretion and osmoregulation are known as flame cells or protonephridia.
Flame cells function by filtering metabolic waste from the body fluids and propelling it through the excretory duct system to the outside.
Malpighian tubules are found in insects,and green glands (antennary glands) are found in crustaceans.

(B) The kidneys in vertebrates are classified based on their embryonic development and structural complexity into three types: Pronephros,Mesonephros,and Metanephros.
$1$. Pronephros is the most primitive type,found in the embryos of all vertebrates and in adult cyclostomes.
$2$. Mesonephros is found in the embryos of higher vertebrates (amniotes) and in adult fish and amphibians.
$3$. Metanephros is the most advanced type of kidney,which is found in the adults of higher vertebrates,including reptiles,birds,and mammals (such as the rabbit).
Therefore,the kidney of an adult rabbit is Metanephros.

(B) The classification of the phylum $Porifera$ (sponges) is primarily based on the nature and composition of their endoskeleton,which consists of $spicules$ or $spongin$ fibers. These $spicules$ can be calcareous or siliceous,and their structure and arrangement are key taxonomic features used to classify sponges into different classes.

(C) Metamorphosis in insects is a biological process by which an animal physically develops after birth or hatching.
This process is primarily regulated by the hormone $Ecdysone$,which is secreted by the prothoracic glands.
$Ecdysone$ triggers the molting process and the transition between different developmental stages (larva to pupa to adult).
$Pheromones$ are chemical signals used for communication,and $Thyroxine$ is a hormone found in vertebrates,not insects.

(A) In cockroaches,sexual dimorphism is present.
Male cockroaches can be distinguished from females by the presence of a pair of short,thread-like anal styles on the $9^{th}$ sternum.
Anal cerci are present in both males and females on the $10^{th}$ segment.
Therefore,the presence of anal styles in males is the primary external feature used to distinguish them from females.

(D) Velamen is a specialized,spongy,multi-layered epidermis found in the aerial roots of epiphytic orchids.
It is composed of dead cells that are capable of absorbing moisture from the atmosphere.
This adaptation allows epiphytic plants to survive in environments where water is not directly available from the soil.
Therefore,the correct answer is the aerial roots of orchids.

(A) The botanical name of cabbage is $Brassica$ $oleracea$ $var.$ $capitata$.
$Brassica$ $campestris$ is the botanical name for mustard.
$Brassica$ $oleracea$ $var.$ $botrytis$ is the botanical name for cauliflower.
$Brassica$ $oleracea$ $var.$ $gemmifera$ is the botanical name for Brussels sprouts.

(D) $1$. Epipetalous stamens are those where the stamens are attached to the petals. This is a characteristic feature of the family $Solanaceae$.
$2$. Syngenesious stamens are those where the anthers are fused together while the filaments remain free. This is a characteristic feature of the family $Asteraceae$ (also known as $Compositae$).
$3$. The question asks for the family exhibiting both conditions. However,in standard botanical classification,$Asteraceae$ is primarily characterized by syngenesious stamens,while $Solanaceae$ is characterized by epipetalous stamens.
$4$. Re-evaluating the question: If the question implies a specific plant or a combination,it is important to note that $Asteraceae$ members often show syngenesious anthers. If the question is interpreted as asking for the family where these traits are observed,$Asteraceae$ is the correct answer for syngenesious,and $Solanaceae$ for epipetalous. Given the standard curriculum,$Asteraceae$ is the most common answer for syngenesious stamens.

(C) Sieve tube elements are specialized cells in the phloem of angiosperms.
During their maturation,they lose their nucleus,ribosomes,and vacuoles to facilitate the transport of food materials.
However,they retain a thin layer of cytoplasm and are associated with companion cells that regulate their metabolic activities.

(A) Collenchyma is a type of simple permanent tissue that provides mechanical support to the plant. Based on the deposition of pectin and cellulose,collenchyma is classified into three types:
$1$. Angular collenchyma: Thickening is present at the corners where cells meet. It is found in $Cucurbita$ (pumpkin/gourd family).
$2$. Lamellar collenchyma: Thickening is present on the tangential walls. It is found in $Althaea$.
$3$. Lacunar collenchyma: Thickening is present near the intercellular spaces. It is found in $Salvia$ or $Malva$.

(D) In plants,$Xylem$ is primarily responsible for the transport of water and minerals from roots to other parts of the plant (unidirectional). $Phloem$ is responsible for the translocation of organic solutes (mainly sucrose) from the source (leaves) to the sink (roots,fruits,storage organs,etc.). This transport is bidirectional,meaning it can move upwards and downwards depending on the plant's requirements.

(D) Ribosomes were first observed by George $E$. Palade in $1955$ using an electron microscope.
He described them as dense particles in the cytoplasm.
Therefore,the discovery of ribosomes is attributed to George $E$. Palade.

(B) The $Fluid$ $Mosaic$ $Model$ of the cell membrane was proposed by $S.J.$ $Singer$ and $G.L.$ $Nicolson$ in $1972$.
According to this model,the cell membrane consists of a phospholipid bilayer with proteins embedded in it,which gives it a fluid-like appearance and dynamic nature.

(A) The cell wall is a rigid,non-living structure that surrounds the plasma membrane of plants,fungi,and bacteria. It is composed primarily of cellulose in plants. Because of its porous nature,it allows the free movement of water,solutes,and various molecules,making it fully permeable to most substances.

(A) Malignant tertian malaria is a severe form of malaria characterized by high fever and potential complications.
It is caused by the protozoan parasite $Plasmodium$ $falciparum$.
$P. vivax$ and $P. ovale$ typically cause benign tertian malaria,while $P. malariae$ causes quartan malaria.

(C) The housefly,belonging to the genus $Musca$,is considered the greatest mechanical carrier of diseases.
It carries pathogens on its legs,wings,and body parts from contaminated surfaces like garbage and excreta to human food.
Because of its habit of visiting both filth and food,it acts as a primary vector for diseases such as cholera,typhoid,and dysentery.

(D) ,$B$,and $C$ are parts of the ovule (megasporangium).
$D$ (Pollen grain) is the male gametophyte produced in the anther (microsporangium).
Therefore,the pollen grain is the odd one out.

(A) In an angiosperm embryo sac,the antipodal cells and the egg cell are both haploid $(n)$ structures.
$1$. Antipodal cells are part of the embryo sac and are formed after mitotic divisions of the functional megaspore.
$2$. The egg cell is the female gamete,also formed from the functional megaspore.
$3$. Nucellus and megaspore mother cells are diploid $(2n)$ structures.
$4$. The primary endosperm nucleus is triploid $(3n)$ as it is formed by the fusion of two polar nuclei and one male gamete.

(D) Syngamy is the process of fusion of male and female gametes to form a diploid zygote.
In flowering plants,it specifically refers to the fusion of one of the male gametes with the egg cell to produce a diploid zygote $(2n)$.

(A) Double fertilization is a characteristic feature of angiosperms. It involves two distinct fusion events:
$1$. Syngamy: One male gamete fuses with the egg cell to form a diploid zygote $(n + n = 2n)$.
$2$. Triple Fusion: The second male gamete fuses with the two polar nuclei (or the secondary nucleus) to form a triploid primary endosperm nucleus $(n + n + n = 3n)$.
Together,these two processes are known as double fertilization.

(A) After ejaculation,human sperms travel through the female reproductive tract to reach the ampulla of the fallopian tube for fertilization.
Experimental observations and physiological studies indicate that the average speed of sperm movement within the female reproductive tract is approximately $2$ to $4$ mm/minute.
This motility is essential for the sperm to navigate the cervical mucus and the uterine environment to reach the site of fertilization.

(D) The human female is homogametic,meaning she produces gametes with the sex chromosome constitution $22 + X$. When an egg is released from the ovary,it is a haploid cell containing $22$ autosomes and one $X$ chromosome. Therefore,the correct option is $(d)$.

(B) Human eggs are considered $Alecithal$ because they contain a negligible or extremely small amount of yolk.
Although they are technically $Microlecithal$ (containing very little yolk),in the context of comparative embryology,they are most accurately described as $Alecithal$ due to the near-absence of yolk.

(B) During cleavage,the zygote undergoes rapid mitotic divisions to form a multicellular structure called a morula.
Unlike ordinary mitosis,there is no growth phase ($G_1$ or $G_2$) between the divisions.
As a result,the total volume of the embryo remains constant,and the number of cells increases while their individual size decreases.
Therefore,the cells do not grow in size during this process.

(A) dihybrid condition refers to an organism that is heterozygous for two different traits.
In the genotype $Tt Rr$,the individual is heterozygous for both the height trait ($T$ and $t$) and the seed shape trait ($R$ and $r$).
This genotype produces $4$ types of gametes during meiosis: $TR$,$Tr$,$tR$,and $tr$.

(C) Crossing over is a biological process that occurs during the pachytene stage of prophase-$I$ of meiosis.
During this process,there is an exchange of genetic material between non-sister chromatids of homologous chromosomes.
This exchange leads to the formation of new combinations of alleles,which is known as genetic recombination.
Therefore,crossing over results in recombination between linked genes,increasing genetic diversity.

(A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition affects the autosomes,not the sex chromosomes.
Since the individual is a male child,the sex chromosomes remain normal,which is $XY$.
Therefore,the karyotype of a male with Down's syndrome is $47, XY, +21$.

(C) $Down's$ syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ ($trisomy$ $21$).
This condition occurs due to the failure of homologous chromosomes to separate during meiosis,a process known as non-disjunction.
As a result,one gamete receives an extra chromosome,and upon fertilization,the zygote develops the $trisomic$ condition.

(A) The sex of a foetus can be determined by amniocentesis,a procedure where amniotic fluid is sampled.
In female cells,one of the two $X$ chromosomes is inactivated and condensed into a structure known as a $Barr$ body,which can be observed in the interphase nucleus.
Since males have only one $X$ chromosome,they do not possess $Barr$ bodies.
Therefore,the presence or absence of $Barr$ bodies in the cells obtained from the amniotic fluid is used to determine the sex of the foetus.

(A) In genetics, when two different alleles of a gene are present in a heterozygous individual, the allele that expresses its phenotype is called the $Dominant$ allele. The allele that is masked or fails to express its phenotype in the presence of the dominant allele is known as the $Recessive$ allele. Therefore, the allele that fails to express itself is termed $Recessive$.

(B) Let the allele for brown eyes be $B$ (dominant) and blue eyes be $b$ (recessive).
$1$. The woman is blue-eyed, so her genotype must be $bb$.
$2$. The man is brown-eyed, so he must have at least one $B$ allele. Since his mother was blue-eyed $(bb)$, he must have inherited one $b$ allele from her. Therefore, his genotype is $Bb$.
$3$. The cross is $bb$ (woman) $\times$ $Bb$ (man).
$4$. The possible offspring genotypes are $Bb$ (brown-eyed) and $bb$ (blue-eyed) in a $1:1$ ratio.
$5$. Thus, $50\%$ of the children are expected to be brown-eyed and $50\%$ are expected to be blue-eyed.

(D) Palaeontology is the branch of biology and geology concerned with the study of fossils of animals and plants.
$A$. Ecology is the study of organisms and their interactions with the environment.
$B$. Ornithology is the scientific study of birds.
$C$. Ethology is the scientific study of animal behavior.
Therefore,the correct option is $D$.

(A) $Leprosy$ is a chronic infectious disease caused by the bacterium $Mycobacterium$ $leprae$.
It primarily affects the skin, peripheral nerves, mucosal surfaces of the upper respiratory tract, and the eyes.
Since the causative agent belongs to the genus $Mycobacterium$, option $A$ is the correct answer.

(D) Thurioside (often referred to as $Bt$ toxin) is a proteinaceous insecticidal toxin produced by the bacterium $Bacillus$ $thuringiensis$.
This bacterium produces crystal proteins during a particular phase of its growth.
These crystals contain a toxic insecticidal protein.
Therefore,it is of bacterial origin.

(B) The correct sequence is $(b)$ Phytoplankton $\rightarrow$ zooplankton $\rightarrow$ fish.
In an aquatic ecosystem,phytoplankton acts as the producer (primary trophic level).
Zooplankton feeds on phytoplankton (primary consumer).
Fish feeds on zooplankton (secondary consumer).
Option $(a)$ represents a detritus food chain,but the sequence is biologically inaccurate as birds do not typically consume insect larvae that feed on bacteria.
Options $(c)$ and $(d)$ are incorrect because the trophic levels are reversed or illogical (e.g.,foxes do not eat grasses,and chameleons are secondary consumers that eat insects,not primary producers).

(D) The greenhouse effect is a natural phenomenon that warms the earth's surface.
It is primarily caused by greenhouse gases that trap heat in the atmosphere.
Among these,$CO_2$ (carbon dioxide) is the most significant contributor,accounting for approximately $50\%$ of the increase in global temperature.
Other gases like chlorofluorocarbons (CFCs) contribute about $20\%$,while methane and nitrous oxide also play roles.
Therefore,the presence of a $CO_2$ layer in the atmosphere is the primary driver of the greenhouse effect.

(B) Domestic waste refers to the garbage generated from households. This waste consists mainly of organic materials such as vegetable peels,food scraps,paper,and garden waste. Since these materials can be broken down by microorganisms into simpler substances,they are classified as biodegradable pollutants.

(C) Pedology is defined as the scientific study of soil in its natural environment.
It involves the study of soil formation (pedogenesis),soil morphology,and the classification of soil types.
Therefore,the correct option is $C$.

(B) Gregor Johann Mendel is known as the 'Father of Genetics'.
He performed pioneering experiments on pea plants ($Pisum$ $sativum$) to understand the principles of inheritance.
His work laid the foundation for modern genetics,making him the first great geneticist.

(B) The correct answer is $B$.
Rice is the principal cereal crop in India.
Approximately $50\%$ of the total area under cultivation of cereal crops in India is dedicated to rice.
It serves as the main staple food for a vast majority of the Indian population.

(B) West Bengal is the largest producer of jute in India.
It accounts for more than $70\%$ of the total jute production in the country due to its favorable alluvial soil and humid climate,which are ideal for the cultivation of this fiber crop.

(A) Groundnut $(Arachis \, hypogaea)$ is a major oilseed crop in India.
Among all Indian states, Gujarat consistently ranks first in the production and cultivation area of groundnut due to its favorable climatic conditions and soil types.

(D) The correct answer is $D$.
Sugarcane is cultivated in various states of India,including Andhra Pradesh,Karnataka,Punjab,Tamil Nadu,and Maharashtra.
However,Uttar Pradesh is the largest producer of sugarcane in India.

(C) Vegetative propagation is a form of asexual reproduction. In this process,the offspring are genetically identical to the parent plant (clones). Therefore,the desirable traits of the parent,such as the quality,taste,and texture of the pulp,are preserved and maintained in the next generation without any variation.

(D) good quality fodder must possess several essential characteristics to ensure the health and productivity of livestock.
$1$. It should contain high dry matter content to provide sufficient energy.
$2$. It must be free from diseases,pests,and fungal infections to prevent health issues in animals.
$3$. It should be rich in essential nutrients (proteins,vitamins,minerals) and completely free from any toxic substances or anti-nutritional factors.
Therefore,all the given options are correct.

(A) Malignant tertian malaria is a severe form of malaria caused by the protozoan parasite $Plasmodium$ $falciparum$.
It is characterized by high fever and can lead to serious complications like cerebral malaria,which is why it is termed 'malignant'.

(C) In $1880$,the French army surgeon Charles Louis Alphonse Laveran discovered the presence of the parasite $Plasmodium$ in the red blood cells of patients suffering from malaria. This discovery was a landmark in the study of protozoan diseases and established that malaria is caused by a parasite.

(C) The housefly,belonging to the genus $Musca$ (specifically $Musca$ $domestica$),is considered the greatest mechanical carrier of diseases.
It carries pathogens on its hairy legs and body parts from contaminated surfaces like garbage and excreta to human food and water.
It is responsible for transmitting diseases such as cholera,typhoid,dysentery,and tuberculosis.

(B) Ascaris lumbricoides is a common roundworm parasite in humans.
Its eggs are excreted in the feces of infected individuals.
These eggs contaminate soil, water, and vegetables.
When a healthy person consumes these contaminated items, the eggs enter the digestive tract, leading to infection.
Therefore, the primary mode of transmission is through contaminated food and water.

(C) In $Agave$,vegetative propagation occurs through specialized fleshy buds known as bulbils.
These bulbils develop in the inflorescence instead of flowers.
When these bulbils fall on the ground,they germinate to form new $Agave$ plants.
Therefore,the correct option is $C$.

(C) Industrial wood,often referred to as timber or lumber,is primarily obtained from trees belonging to the genus $Quercus$ (Oak).
$Quercus$ species are widely valued for their hard,durable wood,which is extensively used in construction,furniture making,flooring,and barrel production.
While other trees like $Salix$ (Willow) and $Betula$ (Birch) are used for specific purposes (like cricket bats or plywood),$Quercus$ is the most prominent source of high-quality industrial wood.

(D) The $Leydig$ cells,also known as interstitial cells,are located in the connective tissue spaces between the seminiferous tubules of the testis.
These cells are responsible for the synthesis and secretion of a group of hormones called androgens,primarily $Testosterone$.
$Testosterone$ is the main male sex hormone responsible for the development of secondary sexual characteristics in males.

(C) In $Agave$,vegetative propagation occurs through specialized structures known as $Bulbils$.
$Bulbils$ are modified floral buds that develop into new plants when they fall on the ground.
These are distinct from other vegetative structures like $Rhizomes$ (e.g.,ginger),$Suckers$ (e.g.,banana),or $Runners$ (e.g.,grass).

(A) Syngamy is the process of fusion of two haploid gametes to form a diploid zygote. This is a fundamental event in sexual reproduction. Therefore,the correct definition is the fusion of gametes.

(B) In asexual reproduction,the offspring produced are genetically and morphologically identical to their parents and to each other. Such genetically identical individuals are collectively referred to as a $Clone$.

(C) Pollination is the process of transfer of pollen grains from the anther to the stigma of a pistil. This process is a characteristic feature of seed-bearing plants that produce pollen. Both $Angiosperms$ (flowering plants) and $Gymnosperms$ (naked-seed plants) produce pollen grains and undergo pollination to facilitate fertilization. Therefore,pollination occurs in $Angiosperms$ and $Gymnosperms$.

(C) The embryo sac,also known as the female gametophyte,develops within the nucellus of the ovule.
During the process of megasporogenesis,a single megaspore mother cell undergoes meiosis to produce four megaspores.
Usually,three of these degenerate,and the remaining functional megaspore develops into the embryo sac through the process of megagametogenesis.
Therefore,the embryo sac is formed inside the ovule.

(D) The correct answer is $D$ (Pollen grain).
$A$ (Nucellus),$B$ (Embryo sac),and $C$ (Micropyle) are all components of the ovule (megasporangium) in angiosperms.
$D$ (Pollen grain) is the male gametophyte produced in the microsporangium (anther),whereas the others are parts of the female reproductive structure.

(D) Double fertilization is a characteristic feature of angiosperms.
It involves two fusion events:
$1$. Syngamy: One male gamete fuses with the egg cell to form a diploid zygote $(n + n = 2n)$.
$2$. Triple Fusion: The second male gamete fuses with the two polar nuclei (or the secondary nucleus) to form a triploid primary endosperm nucleus $(n + 2n = 3n)$.
Thus,double fertilization is the fusion of one male gamete with the egg cell and another male gamete with the secondary nucleus.

(C) Leydig cells,also known as interstitial cells,are located in the interstitial spaces between the seminiferous tubules of the testis.
These cells are responsible for the synthesis and secretion of androgens,primarily testosterone,in response to stimulation by Luteinizing Hormone $(LH)$.

(C) In humans,the sex chromosome constitution of a female is $44 + XX$. During oogenesis,the primary oocyte undergoes meiosis to form a secondary oocyte and a polar body. The secondary oocyte is released during ovulation. Since the female has only $X$ chromosomes,the ovum produced after meiosis will always contain $22$ autosomes and one $X$ chromosome. Therefore,a freshly released human ovum contains one $X$ chromosome.

(B) Cleavage is a series of rapid mitotic divisions of the zygote that converts a single-celled zygote into a multicellular structure called a morula.
During cleavage,the total volume of the embryo remains the same as that of the zygote,meaning there is no increase in the overall size of the embryo.
As the cells (blastomeres) divide,the cytoplasm is partitioned into smaller cells,leading to an increase in the nuclear-cytoplasmic ratio.
Oxygen consumption increases significantly during cleavage due to high metabolic activity.
Therefore,the correct statement is that there is no increase in size.

(B) Fertilizin is a chemical substance (a glycoprotein) secreted by the mature egg (ovum) to attract sperm. It interacts with the antifertilizin present on the surface of the sperm,facilitating the recognition and binding of the sperm to the egg during the process of fertilization.

(B) The human sperm is composed of a head,neck,middle piece,and tail.
The middle piece contains numerous mitochondria,which are arranged in a spiral pattern (often called the $Nebelkern$).
These mitochondria provide the energy required for the movement of the tail,which facilitates sperm motility.
While the neck contains the proximal and distal centrioles,the middle piece is specifically characterized by the presence of mitochondria.