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.

[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: 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.

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.

[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 XXY karyotype (R) leads to overall masculine development but also feminine traits like gynaecomastia (A) because of the extra X chromosome.

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.

[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 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).

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.

[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: 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.

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.

[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: 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.

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.

[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: 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).

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.

[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 parallel occurrence of chromosomes and genes. R explains this parallelism by specifying that the two alleles are physically located on the homologous chromosomes.

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.

[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: 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).

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.

[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 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.

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.

[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 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.