Assertion (A):
Male drones in honey bees cannot have sons.
Reason (R):
Males are haploid and produce sperms by mitosis.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: 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).

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: Male heterogamety (A) is defined by the production of two types of sex-determining gametes (R).

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: 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.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: 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.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: B  [ B ]

Remark: 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.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: 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.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: B  [ B ]

Remark: 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.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: A states the initial problem (difficult communication). R explains the direct consequence (lack of wide publicity) which contributed to the decades of non-recognition.

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.

[Principles-of-Inheritance-and-Variation] [class-xii ]

• (A) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion
• (B) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion
• (C) Assertion is true but Reason is false
• (D) Assertion is false but Reason is true
Correct Option: A  [ A ]

Remark: 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.