Assertion (A): Male drones in honey bees cannot have sons. Reason (R): Males are haploid and produce sperms by mitosis.
The drone characteristic (A) is that they cannot have sons. R explains the genetic basis: being haploid and producing sperm by mitosis means they lack the paternal component required to produce diploid sons (fertilised eggs result in females, unfertilised results in males).
Question 512 of 519
📘 CLASS XII
Assertion (A): In humans (XY type), the sex determination mechanism is designated as male heterogamety. Reason (R): Males produce two different types of gametes: 50 per cent X-carrying sperm and 50 per cent Y-carrying sperm.
Male heterogamety (A) is defined by the production of two types of sex-determining gametes (R).
Question 513 of 519
📘 CLASS XII
Assertion (A): The possibility of a human female becoming haemophilic is extremely rare. Reason (R): This requires the female's father to be haemophilic (often unviable in the later stage of life) and the mother to be at least a carrier.
A is True: Female haemophilia is extremely rare. R provides the specific, rare parental genotypes required for a female to be $X^h X^h$, explaining the rarity of A.
Question 514 of 519
📘 CLASS XII
Assertion (A): Phenylketonuria results in mental retardation. Reason (R): The affected individual lacks the enzyme that converts phenylalanine into tyrosine, leading to accumulation of phenylpyruvic acid in the brain.
Mental retardation (A) is a manifestation of Phenylketonuria. R explains the biochemical pathway defect (lack of enzyme leading to accumulation of phenylpyruvic acid in the brain) that causes A.
Question 515 of 519
📘 CLASS XII
Assertion (A): Thalassemia is a quantitative problem regarding globin synthesis. Reason (R): Sickle-cell anaemia is a qualitative problem of synthesising an incorrectly functioning globin.
A is True: Thalassemia is a quantitative problem (too few molecules). R is True: Sickle-cell anaemia is a qualitative problem (incorrectly functioning globin). Both are true but R does not explain A; they are contrasting definitions.
Question 516 of 519
📘 CLASS XII
Assertion (A): Down’s syndrome results in the presence of an additional copy of chromosome 21. Reason (R): This disorder is caused by aneuploidy, which is the gain or loss of a chromosome(s) due to the failure of chromatid segregation.
Down's syndrome is Trisomy 21 (A). R explains that Trisomy is a form of aneuploidy resulting from the failure of segregation of chromatids, providing the causal mechanism for A.
Question 517 of 519
📘 CLASS XII
Assertion (A): Polyploidy is commonly observed in plants. Reason (R): Polyploidy is the phenomenon of an increase in a whole set of chromosomes, caused by the failure of cytokinesis after the telophase stage of cell division.
A is True: Polyploidy is often seen in plants. R is True: R defines polyploidy and its mechanism (failure of cytokinesis). R is the cause/definition of the phenomenon A, but does not explain why this condition is common specifically in plants.
Question 518 of 519
📘 CLASS XII
Assertion (A): Mendel’s work remained unrecognised until 1900 partly because communication was difficult in those days. Reason (R): Due to difficult communication, his work could not be widely publicised.
A states the initial problem (difficult communication). R explains the direct consequence (lack of wide publicity) which contributed to the decades of non-recognition.
Question 519 of 519
📘 CLASS XII
Assertion (A): Mendelian disorders may be dominant or recessive, and their pattern of inheritance can be traced by pedigree analysis. Reason (R): Mendelian disorders are mainly determined by alteration or mutation in a single gene.
A describes the classification and tracing method for Mendelian disorders. R provides the genetic basis (single gene mutation), which allows for the predictable dominant/recessive patterns that are traceable via pedigree analysis.
