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Q3
short answer
2 marks
How is the number of chromosomes in the progeny of sexually reproducing organisms re-established? Explain.
Solution
The number of chromosomes in the progeny is re-established by meiosis and fertilisation. During meiosis, gametes are formed with half the number of chromosomes (haploid, $n$). When male and female gametes fuse during fertilisation, the diploid number ($2n$) is restored in the zygote. Thus, the chromosome number remains constant from generation to generation.
Q4
short answer
2 marks
Name the parts A, B and C of the diagram given below. Write a function of the part B.
Solution
The parts A, B and C are usually the main parts of the human brain diagram: A – Cerebrum, B – Cerebellum, C – Medulla oblongata.
Function of B (Cerebellum): It coordinates muscular movements and maintains balance and posture of the body.
Q5
short answer
2 marks
(a) Sex chromosomes in human males and females are $XY$ and $XX$ respectively. Statistical probability of getting either a male or a female child is 50%. Justify this statement giving reason.
Solution
Human female sex chromosomes are $XX$ and male sex chromosomes are $XY$. The mother produces only one type of egg carrying $X$, while the father produces two types of sperms, one carrying $X$ and the other carrying $Y$, in equal proportion. If an $X$-bearing sperm fertilises the egg, the child is female ($XX$); if a $Y$-bearing sperm fertilises the egg, the child is male ($XY$). Since both types of sperms are formed in equal number, the probability of getting a boy or a girl is 50% each.
Q5
short answer
2 marks
(b) If we cross a tall plant $(Tt)$ with a short plant $(tt)$, what ratio of $(Tt)$ and $(tt)$ plants shall we obtain in $F_2$ generation? Show with the help of a diagram (flow chart) only.
Solution
Cross: $Tt \times tt$
Gametes: $T, t$ and $t$
Flow chart:
$Tt \times tt$
$\Downarrow$
$T$ and $t$ from $Tt$; $t$ from $tt$
$\Downarrow$
Offspring: $Tt, tt$
So the ratio of $Tt : tt = 1 : 1$.
Q6
short answer
2 marks
(a) (i) A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? What does it indicate?
(ii) State Right hand thumb rule.
Solution
(i) If the current in the wire is increased, the deflection of the compass needle increases. This indicates that the magnetic field around the wire becomes stronger with increase in current.
(ii) Right-hand thumb rule: If a current-carrying conductor is held in the right hand with the thumb pointing in the direction of current, then the curled fingers give the direction of the magnetic field lines around the conductor.
Q6
short answer
2 marks
(b) List two factors on which the strength of magnetic field at a point due to a current carrying straight conductor depends. State the rule that determines the direction of magnetic field produced in this case.
Q7
short answer
2 marks
(a) (i) Why are crop fields considered as artificial ecosystems?
(ii) Write a common food chain of four steps operating in a terrestrial ecosystem.
Q7
short answer
2 marks
(b) (i) Why should biodegradable and non-biodegradable wastes be discarded separately?
(ii) What destructive effect do chlorofluorocarbons bring about in the atmosphere and to the mankind?
Q8
short answer
3 marks
(a) Write the basis of classification of Dobereiner's elements.
(b) ?
(c) In the chemical formulae (i) $X_2O_3$ and (ii) $YH_2$, the elements $X$ and $Y$ should be placed in which group in Mendeleev's periodic table?
Q9
short answer
3 marks
(a) (i) In the given below compound
$\begin{array}{ccc}
H & H \\
| & | \\
H & C\;C\;O\;H \\
| & | \\
H & H
\end{array}$
identify (I) the heteroatom, and (II) the functional group.
(ii) Write the structural formula of butane ($C_4H_{10}$).
(iii) Draw the complete structure of the molecule of (I) Cyclohexane, and (II) Benzene. Which of the two is unsaturated?
Q9
short answer
3 marks
(b) (i) A compound with molecular formula $C_3H_6O$ has two isomers.
(I) Draw the structures of both the isomers.
(II) Write their functional group.
(ii) Why are the compounds of carbon exceptionally stable?
Q10
short answer
3 marks
What is vegetative propagation? Name two methods in which this process is used to grow many plants. List any two advantages of vegetative propagation.
Q11
short answer
3 marks
(a) (i) In the following figure, three cylindrical conductors A, B and C are shown along with their lengths and areas of cross-section.
If these three conductors are made of the same material and $R_A$, $R_B$ and $R_C$ be their respective resistances, then find (I) $R_A / R_B$, and (II) $R_A / R_C$.
(ii) If the conductor A is made of copper and the conductor C is made of constantan (alloy of copper and nickel), then which one of the two will have more electrical resistance and why?
Q12
short answer
3 marks
Define the term electric power. An electric device of resistance $R$ when connected across an electric source of voltage $V$ draws a current $I$. Derive an expression for the power in terms of resistance $R$ and voltage $V$. What is the power of a device of resistance 400 operating at 200 V?
Q13
short answer
3 marks
Give reasons for the following:
(a) The existence of decomposers is essential in a biosphere.
(b) Flow of energy is unidirectional in a food chain.
(c) A food chain generally consists of only three or four trophic levels.
Q14
short answer
3 marks
(i) Write the formula for determining the equivalent resistance between A and B of the two combinations (I) and (II) of three resistors $R_1$, $R_2$ and $R_3$ arranged as follows:
(ii) If the equivalent resistance of the arrangements (I) and (II) are $R_s$ and $R_p$ respectively, then which one of the following VI graphs is correctly labelled? Justify your answer.
Q14
long answer
4 marks
A student crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds. He found that only one type of seeds were produced in the $F_1$ generation. When these $F_1$ generation pea plants were self-pollinated with each other, then in addition to the seed type of $F_1$ generation, some new types of seed combinations were also obtained in the $F_2$ generation.
(a) Mention the dominant traits observed in $F_1$ generation.
(b) What are the new possible combinations of seeds likely to be observed in $F_2$ generation?
(c) (i) Give reason why the traits which were not visible in the seeds of $F_1$ generation reappeared in the seeds of $F_2$ generation. Write the ratio of different types of seeds obtained in $F_2$ generation in this case.
OR
(ii) What is meant by the terms (I) dominant, and (II) recessive traits? Explain.
Q15
long answer
4 marks
Take two different coils of copper wire having large number of turns, say 50 and 100 turns respectively. Insert them over a non-conducting roll as shown in the given figure. Connect the Coil-1, having large number of turns, in series with a battery and a plug key. Also connect the other Coil-2 with a galvanometer.
(a) State what is observed in the galvanometer, when
(i) the key is closed.
(ii) the key is opened.
(b) Define the phenomenon involved in this case.
(c) (i) Explain the reason for the current which is responsible for the deflection in the galvanometer.
OR
(ii) A coil AB of copper wire is connected to a galvanometer as shown in the figure. What is observed when N-pole of a strong bar magnet is
(I) pushed into the coil, and
(II) held stationary inside the coil?
State the reason for each observation.
