CBSE Important Questions Class 12 Biology Chapter 4 – Principles of Inheritance and Variation

Class 12 Biology Chapter 4 Principles of Inheritance and Variation is the fourth chapter of the NCERT Class 12 Biology textbook. It explains how characters pass from parents to offspring: Mendel's laws of inheritance, dominance and incomplete dominance, the chromosomal theory of inheritance, sex determination, and genetic disorders such as sickle-cell anaemia that are caused by mutation.

These CBSE important questions for Class 12 Biology Chapter 4 cover incomplete dominance in snapdragon, what a gene is, the mechanism of heredity, point mutation and sickle-cell anaemia (with the normal and sickle haemoglobin gene sequences and red blood cell diagrams), the lethality of the HbS/HbS genotype, why one mutation is silent and another causes disease, and the rules of heredity. The set has multiple-choice, short-answer and long-answer questions with answers for board exam revision.

Q1. What is the probability of getting a red snapdragon flower if a pink snapdragon flower is crossed with a white snapdragon flower?
(a) 0
(b) 1
(c) 1/2
(d) 1/4

Answer: (a) 0
Flower colour in snapdragon is controlled by an incompletely dominant gene. In the heterozygous condition, the flower is pink. A cross between pink (heterozygous) and white (homozygous) produces heterozygous pink and homozygous white plants only. A red flower needs the homozygous red condition, so it is impossible to get a red flower from this cross.

Q2. A gene is a section of cellular DNA that provides information to
(a) DNA.
(b) RNA.
(c) nucleolus.
(d) centromere.

Answer: (b) RNA.
A gene is a specific sequence of nucleotides in the cellular DNA. According to the central dogma of molecular biology, genetic information flows from DNA to RNA to protein. During transcription, the information in the gene is copied into a complementary RNA molecule. So a gene provides information to RNA.

Q3. How does the mechanism of heredity work?

Answer: Cellular DNA (genes) is the information source which regulates metabolism and makes proteins in the cell. It also controls the characters of the progeny. During sexual reproduction, both the parents contribute equally to the DNA of the progeny. Thus all the cells have two copies of chromosomes, one from the male and the other from the female. The gametes (sperm and egg) are haploid (n); when the two germ cells combine, they maintain the total number of chromosomes (2n) and ensure the stability of the DNA of the species. Thus the progeny inherits the traits of the parents.

Q4. Give one example of a congenital disease caused due to a change in a single base pair.

Answer: Sickle cell anaemia. It is caused by a point mutation in the beta-globin gene in which the codon GAG is changed to GTG. As a result, glutamic acid is replaced by valine at the sixth position of the beta-globin chain of haemoglobin, and the red blood cells become sickle-shaped.

Q5. What is point mutation? Give one example.

Answer: A gene mutation involving the substitution, addition or deletion of a single nucleotide base is called a point mutation.
Example: Sickle-cell anaemia is caused by the substitution of glutamic acid by valine at the sixth position of the beta-globin chain of the haemoglobin molecule. This substitution is caused by a point mutation that changes the nucleotide A to T in the coding strand of the DNA.
Sequence for normal haemoglobin (HbA gene)

ATG GTG CAC CTG ACT CCT GAG GAG AAG TCT GCC GTT ACT
START Val His Leu Thr Pro Glu Glu Lys Ser Ala Val Thr

Normal red blood cell

Sequence for sickle cell haemoglobin (HbS gene)

ATG GTG CAC CTG ACT CCT GTG GAG AAG TCT GCC GTT ACT
START Val His Leu Thr Pro Val Glu Lys Ser Ala Val Thr

Sickle-shaped red blood cell

Explanation: Point mutations can have one of three outcomes:

Type of point mutation Outcome
Silent mutation Altered codon corresponds to the same amino acid
Missense mutation Altered codon corresponds to a different amino acid
Nonsense mutation Altered codon corresponds to a stop signal

Point mutations may arise from spontaneous mutations that occur during DNA replication. The rate of mutation may be increased by mutagens, which are physical, chemical or biological agents that increase the frequency of mutations above the normal level.
Sickle-cell anaemia is an autosome-linked recessive trait that is transmitted from parents to offspring when both parents are carriers of the mutated gene. The disease is controlled by a single pair of alleles, HbA and HbS. Double recessive individuals (HbSHbS) show the disease phenotype. Heterozygous individuals (HbAHbS) are only carriers of the disease and are unaffected.

Q6. Sickle cell anaemia is caused due to the substitution of glutamic acid by
(a) Isoleucine.
(b) Valine.
(c) Leucine.
(d) Alanine.

Answer: (b) Valine.
In sickle cell anaemia, the codon GAG for glutamic acid at the sixth position of the beta-globin chain changes to GTG, which codes for valine. This replaces a hydrophilic amino acid with a hydrophobic one, so haemoglobin molecules polymerise under low oxygen and the red blood cells become sickle-shaped.

Q7. In the homozygous state (HbS/HbS), sickle cell disorder shows
(a) sub lethality.
(b) no lethality.
(c) absolute lethality.
(d) delayed lethality.

Answer: (c) absolute lethality.
Individuals with the HbS/HbS genotype produce only sickle haemoglobin. The sickled red blood cells break down early and block small blood vessels, causing severe anaemia and organ damage. Without proper treatment the disease is fatal, so the homozygous condition shows absolute lethality.

Q8. Study the following carefully and explain why mutation (A) did not cause any sickle cell anaemia in spite of a change in the molecular structure of the gene which codes for haemoglobin, when a similar mutation (B) did. (The question is based on the properties of the genetic code. C = codon, a = amino acid, Hb = haemoglobin)
Codons for Hb: C1-C2-C3-C4-C5-GAA-GAA-C8 .....
Amino acids in Hb: a1-a2-a3-a4-a5-Glutamic acid-Glutamic acid-a8 ..... (Normal haemoglobin)
Mutation (A): C1-C2-C3-C4-C5-GAG-GAA-C8 .....
a1-a2-a3-a4-a5-Glutamic acid-Glutamic acid-a8 ..... (Normal haemoglobin)
Mutation (B): C1-C2-C3-C4-C5-GUG-GAA-C8 .....
a1-a2-a3-a4-a5-Valine-Glutamic acid-a8 ..... (Sickle cell haemoglobin)

Answer: 1. The genetic code is degenerate, i.e. more than one codon can code for the same amino acid. Both GAG and GAA code for glutamic acid.
2. In mutation (A), only the third base of the sixth codon changes (GAA to GAG). Because of degeneracy and the wobble hypothesis, the amino acid does not change, so there is no change in the phenotype. This is a silent mutation and haemoglobin stays normal.
3. In mutation (B), the sixth codon changes to GUG, which codes for a different amino acid, valine, in place of glutamic acid. This is a missense mutation.
4. The structure of the protein changes: normal Hb (HbA) becomes sickle cell Hb (HbS), and the red blood cells become sickle-shaped, causing sickle cell anaemia.

Q9. The rules of heredity determine the process by
(a) behavioural trends
(b) traits and characteristics
(c) size of population
(d) environmental conditions

Answer: (b) traits and characteristics
Heredity is the process by which genetic information passes from parents to offspring. Mendel's laws of segregation and independent assortment describe how alleles separate and combine to form genotypes, which are expressed as observable traits and characteristics in the offspring.

Q10. The rules of heredity determine the process by which traits and characteristics are reliably
(a) inferred.
(b) inherited.
(c) chosen.
(d) picked.

Answer: (b) inherited.
Genes, the units of heredity, are segments of DNA on chromosomes. Mendel's laws explain how alleles separate during gamete formation and combine at fertilisation. Because of these laws and the stable transmission of DNA, traits and characteristics are reliably inherited from one generation to the next.

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