Assertion (A): Trisomy and Monosomy conditions lead to very serious consequences in the affected individual. Reason (R): Trisomy is the presence of an additional copy of a chromosome, while Monosomy means an individual lacks one of any one pair of chromosomes.
Both Trisomy and Monosomy involve the gain or loss of a chromosome (aneuploidy). R accurately defines these two specific situations that result in the serious consequences noted in A.
Question 482 of 519
📘 CLASS XII
Assertion (A): Individuals with Klinefelter’s Syndrome often express feminine development like gynaecomastia. Reason (R): Klinefelter’s Syndrome is caused due to the presence of an additional copy of the X-chromosome, resulting into a karyotype of 47, XXY.
The XXY karyotype (R) leads to overall masculine development but also feminine traits like gynaecomastia (A) because of the extra X chromosome.
Question 483 of 519
📘 CLASS XII
Assertion (A): $\alpha$ Thalassemia is controlled by two closely linked genes (HBA1 and HBA2) on chromosome 16 of each parent. Reason (R): $\beta$ Thalassemia is controlled by a single gene (HBB) on chromosome 11 of each parent.
A and R are both true statements concerning the genetic control and location of $\alpha$ and $\beta$ Thalassemia, respectively. However, R is a separate fact about $\beta$ Thalassemia and does not explain the gene linkage of $\alpha$ Thalassemia (A).
Question 484 of 519
📘 CLASS XII
Assertion (A): Sickle-cell anaemia is a classical example of a point mutation. Reason (R): A point mutation is defined as a change in a single base pair of DNA.
Sickle-cell anaemia is cited as a classical example of a point mutation. R provides the definition of a point mutation, explaining the basis for A.
Question 485 of 519
📘 CLASS XII
Assertion (A): Haemophilia, an X-linked recessive disease, shows transmission from an unaffected carrier female to some of the male progeny. Reason (R): The possibility of a female becoming a haemophilic is extremely rare.
A is True: Haemophilia transmission is classically carrier female to male progeny. R is True: Female haemophilia is extremely rare. Both are true facts about Haemophilia but R does not explain the carrier transmission pattern described in A.
Question 486 of 519
📘 CLASS XII
Assertion (A): Mendel’s approach of using statistical analysis and mathematical logic was unacceptable to many biologists of his time. Reason (R): At that time, biologists had little idea about the scientific basis of inheritance and preferred the observation of apparently continuous variation seen in nature.
Mendel's mathematical approach was new and unacceptable to contemporaries. This resistance was partly due to their difficulty in accepting discrete factors (genes) in light of continuous variation (R).
Question 487 of 519
📘 CLASS XII
Assertion (A): Chromosomes and genes (factors) both occur in pairs. Reason (R): The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
A states the parallel occurrence of chromosomes and genes. R explains this parallelism by specifying that the two alleles are physically located on the homologous chromosomes.
Question 488 of 519
📘 CLASS XII
Assertion (A): Male drones in honey bees produce sperm by mitosis. Reason (R): Since males are haploid (16 chromosomes), they cannot undergo meiosis for gamete production.
Males produce sperm by mitosis. R explains this necessity because males are haploid (16 chromosomes), meaning they lack the diploid set required for reduction division (meiosis).
Question 489 of 519
📘 CLASS XII
Assertion (A): A gene that exhibits multiple phenotypic expressions is called a pleiotropic gene. Reason (R): The underlying mechanism of pleiotropy is the effect of a single gene on multiple metabolic pathways which contribute towards different phenotypes.
A defines pleiotropy. R explains the cellular/molecular reason for the multiple effects described in A: the gene affects a core metabolic pathway that influences various end products/traits.
Question 490 of 519
📘 CLASS XII
Assertion (A): In a dihybrid cross, the segregation of the R/r gene pair is independent of the segregation of the Y/y gene pair. Reason (R): This independent segregation results in four genotypes of gametes (RY, Ry, rY, ry), each with a frequency of 25 per cent (1/4th) in the F1 hybrid.
A is the statement of the Law of Independent Assortment in terms of gamete production. R provides the quantifiable result of this independent segregation (four types, each 1/4th), thus explaining the consequence of the principle A.
