Assertion (A):
Alleles that produce a non-functional enzyme usually result in a dominant trait.
Reason (R):
The modified allele, which is responsible for producing a non-functional enzyme or no enzyme, is generally the recessive 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: D  [ D ]

Remark: A is False: If an allele produces a non-functional enzyme, the phenotype may be effected, and the resulting trait (if seen only in homozygous condition) is generally recessive.
R is True: The modified allele producing a non-functional enzyme or no enzyme is generally the recessive allele.

Assertion (A):
In humans, I^A and I^B alleles are completely dominant over allele i in ABO blood grouping.
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: I^A and I^B are completely dominant over i. This dominance is explained because allele i does not produce any sugar, so when combined with I^A or I^B, only the sugar produced by the dominant allele is expressed.

Assertion (A):
If starch grain size is considered the phenotype in pea seeds, the alleles B and b show incomplete dominance.
Reason (R):
Heterozygotes (Bb) produce round seeds, but the starch grains produced are of intermediate size.

[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: If starch grain size is the phenotype examined, the alleles show incomplete dominance because Bb seeds produce intermediate size starch grains. The reason (R) correctly states that while the seed shape is round (dominance), the grain size is intermediate (incomplete dominance), supporting the assertion that the dominance relationship depends on the phenotype chosen.

Assertion (A):
In a typical test cross, the organism showing a dominant phenotype is crossed with the recessive parent.
Reason (R):
The progenies of the test cross can be easily analyzed to predict the genotype of the test organism, distinguishing between homozygous dominant and heterozygous dominant genotypes.

[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 test cross involves crossing an organism with a dominant phenotype (unknown genotype) with the recessive parent. This is done because the resultant progenies can easily be analyzed to determine if the test organism was homozygous or heterozygous.

Assertion (A):
Mutation is a phenomenon that leads to variation in DNA.
Reason (R):
Loss (deletions) or gain (insertion/duplication) of a segment of DNA results in alteration in chromosomes, leading to abnormalities or aberrations.

[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: Mutation results in the alteration of DNA sequences, leading to variation. The loss or gain of DNA segments results in chromosomal alteration or aberrations because genes are located on chromosomes.

Assertion (A):
The F₁ hybrid plant Tt is considered a monohybrid.
Reason (R):
The Tt plant is heterozygous for genes controlling only one character (height).

[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 Tt plant is a monohybrid. This is because it is heterozygous for only one character (height).

Assertion (A):
Klinefelter’s Syndrome individuals are tall statured with overall masculine development.
Reason (R):
The affected individual has an additional copy of the X-chromosome (47, XXY), which leads to the expression of feminine development (gynaecomastia) despite having masculine characteristics.

[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: Klinefelter’s Syndrome involves an XXY karyotype. The individual shows overall masculine development but also feminine traits like gynaecomastia. The presence of the extra X chromosome (R) leads to these ambiguous characteristics (A).

Assertion (A):
Mendel’s approach of using mathematics to explain biological phenomena was totally new and was well-received by his contemporaries.
Reason (R):
Mendel’s concept of genes as stable and discrete units was rejected because biologists favored the idea of 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: D  [ D ]

Remark: A is False: Mendel’s approach using mathematics was totally new and unacceptable to many biologists of his time.
R is True: His concept of discrete factors was rejected by contemporaries in favor of the apparently continuous variation seen in nature.

Assertion (A):
The total number of chromosomes in a normal human cell is 46, consisting of 22 pairs of autosomes and one pair of sex chromosomes.
Reason (R):
Failure of cytokinesis after the telophase stage of cell division leads to aneuploidy in humans, causing conditions like Down’s Syndrome.

[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: C  [ C ]

Remark: A is True: A normal human cell has 46 chromosomes (22 pairs of autosomes + one pair of sex chromosomes).
R is False: Failure of cytokinesis leads to polyploidy (an increase in a whole set of chromosomes), which is common in plants. Down’s syndrome (aneuploidy) is caused by the failure of segregation of chromatids.

Assertion (A):
Genetics is the branch of biology that deals with inheritance, as well as the variation of characters from parents to offspring.
Reason (R):
Inheritance is the basis of heredity, and variation is the degree by which progeny differ from their parents.

[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: Genetics deals with inheritance and variation.
R is True: Inheritance is the process characters are passed on (basis of heredity), and variation is the degree progeny differ from parents. R defines the components but does not explain why the subject (Genetics) deals with both.