Class 12/Question Bank/Competency Based
Created by Titas Mallick
Biology Teacher • M.Sc. Botany • B.Ed. • CTET (CBSE) • CISCE Examiner
Created by Titas Mallick
Biology Teacher • M.Sc. Botany • B.Ed. • CTET (CBSE) • CISCE Examiner
Competency Based Questions on Principles of Inheritance
1. Analyze the Pedigree: A male has a rare genetic disorder. He marries a healthy female. All their daughters are affected, but none of their sons are affected. What is the most likely mode of inheritance? a) Autosomal Dominant b) X-linked Dominant c) X-linked Recessive d) Y-linked Answer: b) X-linked Dominant Explanation: An affected father passes his X chromosome to all his daughters, making them affected. Sons receive the Y chromosome, so they are spared.
2. Predict the Outcome: In a cross between AABB and aabb, the F1 progeny is AaBb. If the genes are completely linked (0% recombination), what will be the phenotypic ratio of the F2 generation? a) 9:3:3:1 b) 1:1:1:1 c) 3:1 d) 1:2:1 Answer: c) 3:1 Explanation: In complete linkage, the alleles A and B stay together, and a and b stay together. The F1 (AB/ab) behaves like a single gene cross. F2 phenotypes will be the same as the parents (AABB and aabb) in a 3:1 ratio.
3. Assertion (A): Sickle-cell anaemia is an example of a point mutation. Reason (R): It involves the substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain. 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. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: A single nucleotide change (GAG to GUG) results in a single amino acid change, which is the definition of a point mutation.
4. Evaluate: A couple, both carriers for Thalassemia (minor), want to know the risk of having a child with Thalassemia major. What is the probability? a) 100% b) 50% c) 25% d) 0% Answer: c) 25% Explanation: Thalassemia is an autosomal recessive disorder. The cross is Tt x Tt (Heterozygous x Heterozygous). Offspring: 25% TT (Normal), 50% Tt (Carrier), 25% tt (Major).
5. Calculate: In a population of 100 individuals, 16 are homozygous recessive (aa). According to Hardy-Weinberg equilibrium, what is the frequency of the heterozygous (Aa) individuals? a) 0.48 b) 0.36 c) 0.16 d) 0.60 Answer: a) 0.48 Explanation: . Then . Frequency of Aa () = .
6. Analyze the blood group: A child has blood group 'O'. The father has blood group 'A' and the mother has blood group 'B'. What are the genotypes of the parents? a) , b) , c) , d) , Answer: b) , Explanation: A child with group O has genotype . This means both parents must contribute one '' allele.
7. Identify the Phenomenon: In Mirabilis jalapa, crossing a red-flowered plant with a white-flowered plant yields pink-flowered offspring. This is due to: a) Codominance b) Incomplete Dominance c) Epistasis d) Pleiotropy Answer: b) Incomplete Dominance Explanation: The dominant allele (Red) does not completely mask the recessive allele (White), resulting in a blended, intermediate phenotype (Pink).
8. Assertion (A): Frequency of crossing over between two genes is directly proportional to the physical distance between them. Reason (R): Genes located farther apart on a chromosome are more likely to undergo recombination. 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. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: As distance increases, the chance of a chiasma forming between those points increases, leading to higher recombination frequency.
9. Clinical correlation: A sterile female with a webbed neck, short stature, and karyotype 45,XO is diagnosed with: a) Down's Syndrome b) Klinefelter's Syndrome c) Turner's Syndrome d) Edwards Syndrome Answer: c) Turner's Syndrome Explanation: This is a chromosomal disorder caused by the loss of one X chromosome in females (Monosomy).
10. Analyze: Why does color blindness occur more frequently in males than in females? a) The gene is on the Y chromosome. b) Males have only one X chromosome, so a single recessive allele is expressed. c) Females have a stronger immune system against the defect. d) Males have higher testosterone levels affecting vision. Answer: b) Males have only one X chromosome... Explanation: Males are hemizygous. Since they have only one X, any recessive gene on it is expressed. Females (XX) need two recessive alleles to show the trait.
11. Differentiate: Which of the following differentiates Polygenic Inheritance from Pleiotropy? a) Polygenic: One gene -> Many traits; Pleiotropy: Many genes -> One trait. b) Polygenic: Many genes -> One trait; Pleiotropy: One gene -> Many traits. c) Polygenic involves blending; Pleiotropy involves dominance. d) Polygenic is qualitative; Pleiotropy is quantitative. Answer: b) Polygenic: Many genes -> One trait; Pleiotropy: One gene -> Many traits. Explanation: Height (Polygenic) is controlled by many genes. PKU (Pleiotropic) is one gene mutation affecting mental status, hair color, and skin color.
12. Assertion (A): In Honeybees, females are diploid (32 chromosomes) and males are haploid (16 chromosomes). Reason (R): Males develop parthenogenetically from unfertilized eggs. 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. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: This is the Haplodiploid sex determination system.
13. Map the genes: If recombination between A-B is 20%, B-C is 15%, and A-C is 35%, what is the order? a) A-C-B b) A-B-C c) B-A-C d) C-A-B Answer: b) A-B-C Explanation: The distance A-C is the largest (35 units). A-B (20) + B-C (15) = 35. Therefore, B must be in the middle of A and C.
14. Identify the chromosomal disorder: A karyotype shows 47, XX, +21. The individual has a broad palm with a simian crease and mental retardation. a) Patau Syndrome b) Down's Syndrome c) Turner's Syndrome d) Cri-du-chat Syndrome Answer: b) Down's Syndrome Explanation: It is caused by Trisomy of the 21st chromosome.
15. Reasoning: In T.H. Morgan's experiment on Drosophila, why did the 'white eye' and 'yellow body' genes show very low recombination (1.3%)? a) The genes were tightly linked. b) The genes were on different chromosomes. c) Mutation rates were high. d) Environment. Answer: a) The genes were tightly linked. Explanation: Tightly linked genes (physically close on the chromosome) rarely separate during crossing over.
Haemophilia is a sex-linked recessive disorder where blood fails to clot.
16. Analyze: If a carrier female marries a normal male, what is the probability of their sons being haemophilic? a) 0% b) 25% c) 50% d) 100% Answer: c) 50% Explanation: The mother is . Each son has a 50% chance of receiving the (affected) allele from her. (Note: Only looking at sons).
17. Explain: Why is it rare to find a female with haemophilia? Answer: To be haemophilic, a female must be . This requires a carrier/haemophilic mother AND a haemophilic father (). Since haemophilic males often died young in the past, this combination was extremely rare.
18. Contrast: Difference between Haemophilia and Thalassemia? Answer: Haemophilia is a sex-linked recessive disorder (on X chromosome). Thalassemia is an autosomal recessive disorder (on autosomes).
A gardener crosses pure red and pure white snapdragons. All F1 are pink.
19. Identify: The phenotypic ratio in F2? Answer: 1 Red : 2 Pink : 1 White (1:2:1 ratio).
20. Interpret: Does this disprove Mendel's Law of Segregation? Answer: No. Even though F1 was pink (blended), the original Red and White traits reappeared in F2. This proves the alleles themselves did not blend; they remained discrete and segregated.
Morgan crossed yellow-bodied, white-eyed females with wild-type males.
21. Deduce: Why did the ratio deviate from 9:3:3:1? Answer: Because the genes for body color and eye color were Linked on the same chromosome (X). They did not assort independently as Mendel's laws assumed.
22. Define: Physical association of genes on a chromosome is called Linkage.
23. Designing a Cross: Determine if a tall plant is TT or Tt. Answer:
24. Formulating a Hypothesis: Industrial Melanism. Answer:
25. Creating a Pedigree: Autosomal Dominant disorder. Answer:
26. Visualizing Development: Sickle Cell hemoglobin. Answer:
27. Analyzing Complexity: Skin color is polygenic. Answer:
28. Debating Ethics: Embryo selection (PGD). Answer:
29. Solving a Genetic Puzzle: Group A man + Group B woman = Group O daughter. Answer:
30. Concept Map: Answer: Chromosome -> contains -> DNA -> segments are -> Genes -> at specific -> Loci -> variations are -> Alleles -> pair is -> Genotype -> expresses -> Phenotype.
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