Assertion (A):
In pea seeds, the heterozygous genotype (Bb) results in a Round seed shape.
Reason (R):
If starch grain size is the phenotype examined, the same genotype (Bb) results in intermediate size starch grains, showing incomplete dominance.

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

• (A) Both Assertion and Reason are true, and Reason is not the correct explanation of Assertion
• (B) Both Assertion and Reason are true, and 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: Both A and R are true observations from the pea starch synthesis gene study. A describes dominance for shape, and R describes incomplete dominance for grain size. R does not explain why the seed shape (A) is round.

Assertion (A):
The phenotypic ratio 9:3:3:1 observed in the F₂ generation of a dihybrid cross confirms the Law of Independent Assortment.
Reason (R):
The Law of Independent Assortment states that segregation of one pair of characters is independent of the other pair of characters.

[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 9:3:3:1 ratio (A) is the observation that led Mendel to propose the Law (R), which states the independent segregation of two pairs of traits.

Assertion (A):
Mendel's work remained unrecognised until 1900.
Reason (R):
His concept of genes (factors) as stable and discrete units was not accepted by contemporaries due to the 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 work was unrecognized (A) for several reasons, including the fact that his discrete, non-blending factor concept (R) contradicted the prevailing view of continuous variation.

Assertion (A):
Chromosomes and genes (factors) both occur in pairs.
Reason (R):
Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of the factors they carried.

[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 occur in pairs (A). R describes the conclusion of the Chromosomal Theory of Inheritance, where the physical behavior of chromosomes (pairing/separation) explains the behaviour of genes (segregation).

Assertion (A):
Mendelu2019s Law of Independent Assortment does not hold true for genes that are closely located on the same chromosome.
Reason (R):
These genes are called 'linked genes' and exhibit high proportions of parental combinations because of their physical association.

[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 Law of Independent Assortment fails for linked genes (A). R explains this failure by noting that linked genes show physical association, leading to higher parental combinations rather than independent assortment.

Assertion (A):
The genes for yellow body and white eyes in Drosophila are very tightly linked.
Reason (R):
These genes showed a very low recombination frequency of 1.3 per cent.

[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: Tightly linked genes (A) are defined by very low recombination rates (R). R provides the specific experimental evidence (1.3%) demonstrating tight linkage.

Assertion (A):
Human skin colour is a classic example of a polygenic trait.
Reason (R):
In polygenic inheritance, the phenotype reflects the additive contribution of each allele.

[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: Human skin colour is polygenic (A). This means it is controlled by multiple genes where the effect of each allele is additive (R), leading to a range of phenotypes.

Assertion (A):
Phenylketonuria exhibits pleiotropy, manifesting as mental retardation and reduced hair/skin pigmentation.
Reason (R):
The underlying mechanism of pleiotropy is the effect of a single gene (mutation in phenyl alanine hydroxylase gene) on multiple metabolic pathways.

[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: Phenylketonuria is pleiotropic (A). R explains this phenomenon by stating that a single gene affects metabolic pathways, leading to multiple phenotypic outcomes.

Assertion (A):
In human sex determination (XY type), the female is homogametic.
Reason (R):
Females produce only one type of ovum carrying an X-chromosome.

[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 female is homogametic (A). R explains this by confirming that females produce only one type of gamete (ova carrying X).

Assertion (A):
Pedigree analysis is a crucial tool in human genetics to trace the inheritance of specific traits or diseases.
Reason (R):
Since control crosses that can be performed in plants are not possible in case of human beings, studying the family history provides an alternative approach to understand inheritance patterns.

[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: Pedigree analysis is used to trace human inheritance (A). R explains that this method is necessary because control crosses are not possible in humans.