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Principles of Inheritance and Variation Set-5

Practice Important MCQs for Principles of Inheritance and Variation — Biology, Class 12.

“Principles of Inheritance and Variation” is a core Class 12 Biology chapter because it builds the conceptual foundation for how traits are transmitted (Mendelian laws), how genes behave in linkage and crossing over (map distances, recombination, DCO/SCO), and how variation arises. These topics are frequently tested in CBSE exams and are also essential for competitive exams because they directly connect to probability-based genetics questions and gene-mapping logic.

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Q1. (In a test cross between two linked loci, $40$ recombinant progeny were observed among $200$ total offspring. Using the relation map distance $= \dfrac{\text{number of recombinants}}{\text{total}}\times 100$ , the map distance (in cM) between the loci is:)

(A)

$40\ \mathrm{cM}$

(B)

$10\ \mathrm{cM}$

(C)

$20\ \mathrm{cM}$

(D)

$5\ \mathrm{cM}$

Q2. (In pea plants two independently assorting genes $R/r$ (round dominant) and $Y/y$ (yellow dominant) are crossed: $RrYy\times RrYy$ . What is the probability that an offspring will show the dominant phenotype for both traits and be homozygous for at least one of the two genes?)

(A)

$\dfrac{5}{16}$

(B)

$\dfrac{9}{16}$

(C)

$\dfrac{1}{4}$

(D)

$\dfrac{7}{16}$

Q3. (A trihybrid heterozygote in coupling was test-crossed to a triple recessive tester and produced the following progeny counts: $ABC=320,\ abc=310,\ ABc=40,\ abC=34,\ Abc=36,\ aBC=38,\ AbC=10,\ aBc=12$ (total $800$ ). Determine the correct gene order and the map distances (in cM) between adjacent genes.)

(A)

Order $A\text{-}C\text{-}B$ ; $AC=24\ \mathrm{cM},\ CB=6\ \mathrm{cM}$

(B)

Order $B\text{-}A\text{-}C$ ; $BA=12\ \mathrm{cM},\ AC=12\ \mathrm{cM}$

(C)

Order $C\text{-}B\text{-}A$ ; $CB=12\ \mathrm{cM},\ BA=12\ \mathrm{cM}$

(D)

Order $A\text{-}B\text{-}C$ ; $AB=12\ \mathrm{cM},\ BC=12\ \mathrm{cM}$

Q4. (Assertion (A): In incomplete dominance the heterozygote exhibits a phenotype intermediate between the two homozygotes. Reason (R): Codominance occurs when both alleles in a heterozygote are fully expressed simultaneously (e.g., AB blood group).)

(A)

Both A and R are true and R is the correct explanation of A.

(B)

Both A and R are true but R is not the correct explanation of A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q5. (A woman is heterozygous for two X-linked loci in coupling as $AB/ab$ and the recombination frequency between the loci is $20\%$ . She mates with a male $ab/Y$ . What fraction of her sons are expected to inherit the recombinant genotype $Ab/Y$ ?)

(A)

$10\%$

(B)

$20\%$

(C)

$40\%$

(D)

$5\%$

Q6. In snapdragon (Antirrhinum) flower colour shows incomplete dominance: $RR$ = red, $rr$ = white and $Rr$ = pink. Two pink ( $Rr$ ) plants are crossed. What fraction of the offspring are expected to be homozygous (either $RR$ or $rr$ )?

(A)

$25\%$

(B)

$33\frac{1}{3}\%$

(C)

$50\%$

(D)

$75\%$

Q7. Genes $X$ , $Y$ and $Z$ lie on the same chromosome. In a testcross the observed recombination frequencies are $r(X,Y)=10\%$ , $r(Y,Z)=18\%$ , $r(X,Z)=28\%$ . Which gene order and map distances (in centiMorgans, cM) are most consistent with these data?

(A)

$X\;-\;10\ \text{cM}\;-\;Y\;-\;18\ \text{cM}\;-\;Z$

(B)

$Y\;-\;10\ \text{cM}\;-\;X\;-\;18\ \text{cM}\;-\;Z$

(C)

$X\;-\;28\ \text{cM}\;-\;Z\;-\;18\ \text{cM}\;-\;Y$

(D)

$X$ and $Z$ are on different chromosomes while $Y$ is linked to $X$

Q8. Two genes $A$ and $B$ are $20\%$ recombination apart and are in coupling phase in a heterozygous parent with genotype $AB/ab$ . This parent is testcrossed with $ab/ab$ . What proportions of offspring are expected for each phenotype class (parental phenotypes each; recombinant phenotypes each)?

(A)

Parental each $45\%$ ; recombinants each $5\%$

(B)

Parental each $30\%$ ; recombinants each $20\%$

(C)

Parental each $35\%$ ; recombinants each $15\%$

(D)

Parental each $40\%$ ; recombinants each $10\%$

Q9. In an F2 dihybrid cross expected phenotypic ratio is $9:3:3:1$ . An experiment yielded observed counts $896,\;305,\;290,\;109$ (total $N=1600$ ). Using $\chi^2=\sum\frac{(O-E)^2}{E}$ and significance level $\alpha=0.05$ (critical value for $df=3$ is $7.815$ ), which conclusion is correct?

(A)

Reject the null hypothesis; $\chi^2\approx 20.6>7.815$ , so the observed distribution significantly deviates from $9:3:3:1$

(B)

Do not reject the null hypothesis; $\chi^2\approx 1.24<7.815$ , so the observed data are consistent with $9:3:3:1$

(C)

Reject the null hypothesis; $\chi^2\approx 8.5>7.815$ , but degrees of freedom were miscalculated

(D)

Do not reject the null hypothesis; $\chi^2\approx 0.12<7.815$ , indicating a near-perfect fit

Q10. Assertion (A): In three-point testcross analysis, the two least frequent offspring classes correspond to double crossover events. Reason (R): Double crossover frequency is approximately the product of the two single crossover probabilities (i.e. $\approx r_{12}\times r_{23}$ ), so it is much lower than single crossover frequencies; comparing double crossovers with parental classes identifies the gene in the middle.

(A)

Both A and R are true and R is the correct explanation of A

(B)

Both A and R are true but R is not the correct explanation of A

(C)

A is true but R is false

(D)

A is false but R is true

Q11. In a testcross of two linked loci a total of 1000 progeny were scored: parental types = 320 and 280, recombinant types = 90 and 110. Using the formula $r=\dfrac{N_{\text{recombinants}}}{N_{\text{total}}}\times100\%$ , the recombination frequency between the loci is:

(A)

$18\%$

(B)

$10\%$

(C)

$20\%$

(D)

$22\%$

Q12. In a three‑point testcross with 1000 progeny, recombinants between genes A–B were 100, between B–C were 80, and the observed double crossovers (DCO) were 6. Using

\text{expected DCO}=(r_{AB}\times r_{BC})\quad\text{(as fractions)},\quad c=\dfrac{\text{observed DCO frequency}}{\text{expected DCO frequency}},\quad I=1-c

the interference $I$ is closest to:

(A)

$0.25$

(B)

$0$

(C)

$0.75$

(D)

$0.125$

Q13. A couple has two children: one with blood group AB and another with group O. Which of the following parental genotype pairs (using $I^A,\;I^B,\;i$ notation) is consistent with these two children?

(A)

$I^A I^A \times I^B I^B$

(B)

$I^A I^B \times i i$

(C)

$I^A i \times I^A I^B$

(D)

$I^A i \times I^B i$

Q14. Assertion (A): A cross $AaBb\times AaBb$ where gene $a$ is epistatic and recessive over gene $B$ (i.e., $aa$ masks $B/b$ expression) gives an F2 phenotypic ratio $9:3:4$ (wild:other:epistatic).
Reason (R): If allele $a$ is also recessive lethal in homozygous state ( $aa$ lethal), the phenotypic proportions among surviving offspring will deviate from $9:3:4$ because classes containing $aa$ are removed.
Choose the correct option:

(A)

Both A and R are true and R correctly explains A.

(B)

Both A and R are true but R does not correctly explain A.

(C)

A is true but R is false.

(D)

A is false but R is true.

Q15. In a three‑point testcross for genes A, B and C the following progeny classes were recovered (counts):
$ABC:430,\;abc:425,\;aBC:3,\;Abc:2,\;aBc:48,\;AbC:47,\;ABc:23,\;abC:22.$
Using the standard method of identifying parental (largest) and double‑crossover (smallest) classes and comparing alleles to infer which locus is altered in DCOs, the gene in the middle position is:

(A)

Gene A

(B)

Gene B

(C)

Gene C

(D)

Data insufficient to decide

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