Assertion (A):
In a monohybrid cross, all the F₁ progeny are tall, resembling only one of the parents.
Reason (R):
The Law of Dominance explains that in a dissimilar pair of factors, one factor (T) dominates the other (t), resulting in the expression of only the dominant trait in F₁.

[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 observed that all F₁ progeny plants were tall, like one parent. This is explained by the Law of Dominance, where T is dominant over t in the dissimilar pair Tt.

Assertion (A):
In the F₂ generation of a monohybrid cross, the tall and dwarf traits appear in a 3:1 phenotypic proportion.
Reason (R):
This ratio is explained by the Law of Dominance and the Law of 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: The F₂ stage results in a 3:1 ratio of tall to dwarf. This proportion is explained by the Law of Dominance (F₁ expression and F₂ ratio) and the Law of Segregation (separation of alleles).

Assertion (A):
Human ABO blood grouping is an example of multiple alleles.
Reason (R):
Multiple alleles for a trait can only be found when population studies are made, as any single individual can possess only two alleles for the 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: The ABO system involves three alleles ($I^A, I^B, i$), demonstrating multiple alleles. Since an individual is diploid and carries only two alleles, R explains why population studies are required to identify multiple alleles.

Assertion (A):
In the ABO blood group, allele $I^A$ is completely dominant over allele $i$.
Reason (R):
Allele $i$ does not produce any sugar on the plasma membrane of the red blood cells.

[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: Alleles $I^A$ and $I^B$ are completely dominant over $i$. This is because allele $i$ does not produce any sugar, allowing the $I^A$ allele to express fully in the $I^A i$ heterozygote.

Assertion (A):
The F₂ phenotypic ratio 9:3:3:1 in a dihybrid cross confirms the Law of Independent Assortment.
Reason (R):
This ratio is derived as a combination series of the 3:1 ratio for each independent trait, confirming that the segregation of one pair of characters is independent of the other.

[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 ratio 9:3:3:1 confirms the Law of Independent Assortment. R explains this because the ratio is derived from multiplying the independent 3:1 ratios, meaning the traits segregate independently.

Assertion (A):
T.H. Morgan coined the term linkage.
Reason (R):
Linkage describes the physical association of genes on a chromosome, which prevents the independent segregation seen in Mendel’s Law of Independent Assortment.

[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: Morgan coined the term linkage to describe the physical association of genes on a chromosome. This association caused a deviation from the 9:3:3:1 ratio, meaning they did not segregate independently.

Assertion (A):
The genes white and miniature wing in Drosophila are considered loosely linked.
Reason (R):
These genes showed a relatively high recombination frequency of 37.2 per cent, indicating a significant distance between them on the 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 white and miniature wing genes are loosely linked (Cross B). R provides the evidence: they showed 37.2 per cent recombination, indicating looser linkage.

Assertion (A):
Chromosomal disorders are caused due to aneuploidy or polyploidy.
Reason (R):
These conditions result from the absence, excess, or abnormal arrangement of one or more whole 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: B  [ B ]

Remark: A is True: Chromosomal disorders include aneuploidy and polyploidy.
R is True: Chromosomal disorders are defined by the absence, excess, or abnormal arrangement of one or more chromosomes. However, R is a definition of chromosomal disorders, not a direct causal explanation limited to just aneuploidy/polyploidy (though they are subtypes). They are both true, related concepts.

Assertion (A):
Haemophilia, an X-linked recessive disease, shows transmission from unaffected carrier female to some of the male progeny.
Reason (R):
Males are hemizygous for X-linked traits, meaning they express the trait if they inherit a single recessive allele on their 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: Haemophilia is an X-linked recessive trait that transmits from carrier females to male progeny. This occurs because males have only one X chromosome and will express the recessive trait if inherited.

Assertion (A):
Sickle-cell anaemia is an autosome linked recessive trait.
Reason (R):
The defect is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta globin chain due to a single base substitution (GAG to GUG).

[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 an autosome linked recessive trait. The defect is molecularly characterized by a point mutation (GAG to GUG) leading to Glu substitution by Val. R explains the specific single-gene molecular basis of this Mendelian disorder (A).