Assertion (A): To determine the genotype of a tall F2 plant, Mendel crossed it with the dwarf parent, calling this a test cross. Reason (R): The progenies of the test cross can be easily analysed to predict the genotype of the test organism, distinguishing between TT and Tt compositions.
A test cross (crossing the dominant phenotype with the recessive parent) is necessary because, by simply looking at the phenotype, the genotypic composition (TT or Tt) cannot be known. R explains the functional purpose of performing A.
Question 422 of 519
📘 CLASS XII
Assertion (A): Mendel’s work gained greater credibility due to his use of statistical analysis and large sampling sizes. Reason (R): Confirmation of his inferences from experiments on successive generations proved that his results pointed to general rules of inheritance.
Mendel's use of statistical analysis and large sampling size gave greater credibility to his data. Confirmation of these findings over successive generations proved the universality of his general rules of inheritance.
Question 423 of 519
📘 CLASS XII
Assertion (A): A true-breeding pea line, established through continuous self-pollination, shows stable trait inheritance for several generations. Reason (R): Mendel selected true-breeding lines to ensure the parental traits were homozygous and consistent before starting his hybridisation experiments.
A is True: A true-breeding line is stable due to continuous self-pollination. R is True: Mendel used these lines because they were stable and homozygous (e.g., TT or tt). R is a broader context/purpose, not the direct explanation of the stability mechanism described in A.
Question 424 of 519
📘 CLASS XII
Assertion (A): The phenotype of the F1 heterozygote (Tt) in a monohybrid cross is exactly like the TT parent in appearance. Reason (R): In a pair of dissimilar factors, one (T, the dominant factor) dominates the other (t, the recessive factor), so only the dominant trait is expressed in F1.
Mendel found the F1 heterozygote Tt phenotype to be exactly like the TT parent (Tall). This is explained by the Law of Dominance, where the dominant factor masks the recessive one.
Question 425 of 519
📘 CLASS XII
Assertion (A): The starch grain size in heterozygous pea seeds (Bb) is intermediate, showing incomplete dominance for this phenotype. Reason (R): If seed shape is chosen as the phenotype, the same genotype (Bb) results in a round shape, demonstrating complete dominance.
A is True: Starch grain size in Bb is intermediate, reflecting incomplete dominance. R is True: Seed shape in Bb is round, reflecting complete dominance. R provides a contrast of phenotypes influenced by the same gene, supporting the complexity but not explaining why A (grain size) is intermediate.
Question 426 of 519
📘 CLASS XII
Assertion (A): The modified allele of a gene is generally the recessive allele. Reason (R): This is because the modified allele often produces a non-functional enzyme or no enzyme at all, making the phenotype dependent solely on the expression of the unmodified (dominant) allele.
The modified allele is typically recessive because it often results in non-functional or no enzyme production. The unmodified allele must then produce enough enzyme to carry out the function, making the modified trait only visible when homozygous recessive.
Question 427 of 519
📘 CLASS XII
Assertion (A): Thalassemia is a qualitative problem, while sickle-cell anaemia is a quantitative problem regarding haemoglobin synthesis. Reason (R): The former is due to the reduced rate of synthesis of globin chains, and the latter is due to the synthesis of an incorrectly functioning globin.
A is False: Thalassemia is a *quantitative* problem (too few globin molecules), and sickle-cell anaemia is a *qualitative* problem (incorrectly functioning globin). R is True: Thalassemia involves reduced synthesis rate (quantitative), and sickle-cell involves incorrect functioning (qualitative).
Question 428 of 519
📘 CLASS XII
Assertion (A): The severity of $\alpha$ Thalassemia is related to the number of genes that are mutated or deleted. Reason (R): $\alpha$ Thalassemia is controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16, resulting in a total of four genes per individual.
A is True: The more genes affected (deleted/mutated), the less alpha globin is produced, hence greater severity. R is True: $\alpha$ Thalassemia is controlled by four total genes (HBA1 and HBA2 on each chromosome 16). This structure (R) directly explains why the number of affected genes determines severity (A).
Question 429 of 519
📘 CLASS XII
Assertion (A): Polyploidy, resulting in an increase in a whole set of chromosomes, is often seen in plants but is rare in animals/humans. Reason (R): Polyploidy is caused by the failure of cytokinesis after the telophase stage of cell division.
A is True: Polyploidy is commonly observed in plants. R is True: Polyploidy results from the failure of cytokinesis after telophase. R explains the mechanism of polyploidy, but not why this condition is commonly seen only in plants (A).
Question 430 of 519
📘 CLASS XII
Assertion (A): Aneuploidy refers to the gain or loss of a chromosome or chromosomes. Reason (R): This phenomenon occurs due to the failure of segregation of chromatids during the cell division cycle.
Aneuploidy is the gain or loss of a chromosome(s). This gain or loss results from the failure of chromatid segregation (non-disjunction).
