AIIMS 1983 Biology Question Paper with Answer and Solution

(B) The contractile vacuole is an organelle primarily involved in osmoregulation in fresh water organisms to pump out excess water that enters the cell via osmosis.
In fresh water,the environment is hypotonic to the cell,so water enters the cell,necessitating a contractile vacuole.
Species $X$ developed a contractile vacuole in fresh water,indicating it is adapted to a fresh water environment.
Species $Y$ did not develop a contractile vacuole in fresh water,suggesting it is naturally adapted to a marine (hypertonic/isotonic) environment where osmoregulation via a contractile vacuole is not required.
When transferred to sea water,both lost their contractile vacuoles because sea water is hypertonic,preventing the influx of water and rendering the contractile vacuole unnecessary.
Therefore,$X$ is a fresh water species and $Y$ is a marine species.

(B) The defining characteristic of the phylum $Arthropoda$ is the presence of jointed appendages ($arthros$ = joint,$poda$ = appendages). While triploblastic,coelomate,and segmented body organization are features of arthropods,these are also found in other phyla like $Annelida$ (segmented,coelomate) or $Chordata$. Therefore,to specifically identify an animal as an arthropod,the presence of jointed appendages is the most critical diagnostic feature that must be verified.

(C) Adrenaline (epinephrine) is a hormone secreted by the adrenal medulla. It acts on the heart by binding to $\beta_1$-adrenergic receptors. This binding increases the heart rate (positive chronotropic effect) and the force of contraction (positive inotropic effect). When a physiological concentration is injected,it leads to a sustained increase in the heart rate to support the body's 'fight-or-flight' response.

(A) The marriage between an $Rh^+$ male and an $Rh^-$ female should be avoided because it can lead to Erythroblastosis fetalis in the offspring.
If the mother is $Rh^-$ and the fetus is $Rh^+$,the mother's immune system may develop antibodies against the $Rh$ antigen during the first pregnancy.
In subsequent pregnancies,these maternal antibodies can cross the placenta and destroy the fetal red blood cells,causing severe anemia and jaundice in the newborn.

(B) The condition where the testes fail to descend into the scrotum during development is known as $Cryptorchidism$. Since the temperature of the scrotum is lower than the body temperature,which is essential for spermatogenesis,the failure of descent leads to sterility in the individual.

(C) Gastrulation is a phase in early embryonic development during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula.
The most fundamental and defining characteristic of gastrulation is the morphogenetic movement of cells.
During this process,cells migrate and rearrange themselves to occupy their specific,definite positions,which eventually leads to the formation of the three germ layers (ectoderm,mesoderm,and endoderm).
While the formation of the archenteron and the transition from a single-layered to a multi-layered structure are consequences of gastrulation,the active movement of cells is the primary mechanism that defines the process.

(D) The development of the neural tube is determined by the process of embryonic induction. The ectoderm in the neural tissue area is 'determined' to form the neural tube. When this tissue is transplanted to the presumptive belly region (which is normally destined to form the epidermis),it retains its original fate due to its determination. Therefore,the transplanted tissue will form a second neural tube in the belly region,resulting in an embryo with two neural tubes.

(C) In humans,males are $XY$ and females are $XX$.
Since males have only one $X$ chromosome,any recessive gene present on the $X$ chromosome will be expressed because there is no corresponding allele on the $Y$ chromosome to mask its effect.
This condition is known as hemizygous.
Therefore,a single recessive gene on the $X$ chromosome of a male is sufficient to express the trait.

(D) Let the colour-blind man be $X^cY$ and the daughter of a colour-blind person be $X^cX$ (carrier).
When they marry,the cross is $X^cY \times X^cX$.
The possible genotypes of the offspring are $X^cX^c$ (colour-blind daughter),$X^cX$ (carrier daughter),$X^cY$ (colour-blind son),and $XY$ (normal son).
Thus,half of their sons will be colour-blind.

(D) To determine if variations in a character are genetically controlled,one must observe whether these traits are heritable.
By crossing individuals representing the two extreme phenotypes of a population,we can observe the inheritance pattern in the offspring.
If the offspring exhibit the same range of variations as the parents,it indicates that the trait is genetically determined and passed down through generations.
This method of breeding and observation is a fundamental approach in classical genetics to distinguish between genetic variation and environmental influence.

(D) The given base sequence is $CAG, AGG, CGA, CCA$.
In nucleic acids,$DNA$ contains the nitrogenous base Thymine $(T)$,while $RNA$ ($mRNA$,$tRNA$,$rRNA$) contains Uracil $(U)$ instead of Thymine.
The provided sequence contains only Cytosine $(C)$,Adenine $(A)$,and Guanine $(G)$.
Since neither Thymine $(T)$ nor Uracil $(U)$ is present in the sequence,it is impossible to distinguish whether this segment belongs to $DNA$ or $RNA$.
Therefore,the data is not sufficient to conclude the nature of the nucleic acid strand.

(B) Silk is produced by the larva of the silkworm. The larva secretes a proteinaceous fluid from its salivary glands,which hardens upon contact with air to form the silk thread used to construct the cocoon.