Assertion–Reason Questions

🧬 Genetic Disorders | Class 12 Biology

Directions (CBSE Pattern):

For each question, choose the correct option:

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

Question 1

Assertion (A): Sickle cell anaemia is a Mendelian disorder.

Reason (R): It is caused by a point mutation in the β-globin gene.

✅ Correct Option: A

Explanation: A single-gene mutation follows Mendelian inheritance.

Question 2

Assertion (A): Sickle-shaped RBCs appear only under low oxygen conditions.

Reason (R): Valine replaces glutamic acid in the haemoglobin chain.

✅ Correct Option: A

Question 3

Assertion (A): Phenylketonuria leads to mental retardation if untreated.

Reason (R): Phenylalanine accumulates due to lack of phenylalanine hydroxylase.

✅ Correct Option: A

Question 4

Assertion (A): Haemophilia affects more males than females.

Reason (R): It is an X-linked recessive disorder.

✅ Correct Option: A

Question 5

Assertion (A): A female carrier of haemophilia is usually normal.

Reason (R): One normal allele masks the effect of the defective allele.

✅ Correct Option: A

Question 6

Assertion (A): Colour blindness is inherited only through females.

Reason (R): The gene for colour blindness is located on the X chromosome.

✅ Correct Option: B

Question 7

Assertion (A): Huntington’s disease appears at an early age.

Reason (R): It is an autosomal dominant disorder.

✅ Correct Option: C

Question 8

Assertion (A): Down syndrome is a chromosomal disorder.

Reason (R): It is caused by trisomy of chromosome 21.

✅ Correct Option: A

Question 9

Assertion (A): Down syndrome results due to nondisjunction.

Reason (R): Homologous chromosomes fail to separate during meiosis.

✅ Correct Option: A

Question 10

Assertion (A): Turner syndrome individuals are sterile females.

Reason (R): They have only one X chromosome (XO).

✅ Correct Option: A

Question 11

Assertion (A): Klinefelter syndrome affects males.

Reason (R): It is caused due to the presence of an extra X chromosome.

✅ Correct Option: A

Question 12

Assertion (A): Cri-du-chat syndrome is a structural chromosomal disorder.

Reason (R): It occurs due to deletion of the short arm of chromosome 5.

✅ Correct Option: A

Question 13

Assertion (A): All chromosomal disorders follow Mendelian inheritance.

Reason (R): Chromosomal disorders are due to gene mutations.

✅ Correct Option: D

Question 14

Assertion (A): Aneuploidy results in abnormal chromosome number.

Reason (R): It occurs due to nondisjunction during cell division.

✅ Correct Option: A

Question 15

Assertion (A): Mendelian disorders are caused by gene mutations.

Reason (R): They involve changes in the structure or number of chromosomes.

✅ Correct Option: C

Question 16

Assertion (A): Phenylketonuria is an autosomal recessive disorder.

Reason (R): The disorder appears only when both alleles are defective.

✅ Correct Option: A

Question 17

Assertion (A): Chromosomal disorders are less common than Mendelian disorders.

Reason (R): Chromosomal disorders are always lethal.

✅ Correct Option: C

Question 18

Assertion (A): Males cannot be carriers of X-linked recessive disorders.

Reason (R): Males have only one X chromosome.

✅ Correct Option: A

Question 19

Assertion (A): Genetic counseling helps prevent genetic disorders.

Reason (R): It provides information about inheritance and risks.

✅ Correct Option: A

Question 20

Assertion (A): Polyploidy is a common genetic disorder in humans.

Reason (R): Humans can survive with multiple sets of chromosomes.

✅ Correct Option: D