Important Questions Class 12 Biology Chapter 4, Download PDF

Important Questions Class 12 Biology Chapter 4: Class 12 Biology Chapter 4 focuses on the Principles of Inheritance and Variation. It explains how traits are transmitted from parents to offspring and why variations occur among organisms. 

The chapter covers Mendel’s laws of inheritance, dominance, co-dominance, incomplete dominance, linkage, mutation, sex determination, and pedigree analysis.

These topics form the core foundation of genetics and develop a clear scientific understanding of heredity.

From the Class 12 board exam perspective, this chapter carries significant importance. Questions are commonly asked in MCQ, short answer, long answer, case-based, assertion-reason, and diagram-based formats. 

Important Questions Class 12 Biology Chapter 4

 Class 12 Biology Chapter 4 covers key areas such as Mendel’s monohybrid and dihybrid crosses, chromosomal theory of inheritance, inheritance of genetic disorders like sickle cell anaemia, and human sex determination, which are frequently included in examinations. Practising Important Questions Class 12 Biology Chapter 4 is therefore essential for structured revision and effective preparation.

Below are some carefully selected Important Questions Class 12 Biology Chapter 4 based on key topics from the chapter “Principles of Inheritance and Variation.” These questions are important for board exams and help in strong conceptual revision.

1. All genes located on the same chromosome:

(1) Form different groups depending upon their relative distance
(2) Form one linkage group
(3) Will not from any linkage groups
(4) Form interactive groups that affect the phenotype

Ans. (2) Form one linkage group

2. In sickle cell anaemia glutamic acid is replaced by valine. Which one of the following triplets codes for valine?

(1) GGG
(2) AAG
(3) GAA
(4) GUG

Ans. (4) GUG

3. Occasionally, a single gene may express more than one effect. The phenomenon is called:

(1) multiple allelism
(2) mosaicism
(3) pleiotropy
(4) polygeny

Ans. (3) pleiotropy

4. Mention the advantages of selecting a pea plant for the experiment by Mendel.

Ans. Gregor Mendel demonstrated characters of inheritance acquired by offspring from parents. He selected a pea plant for this experiment for the following reasons:

• Peas exhibit several visible contrasting features, such as dwarf/tall plants, wrinkled/round seeds, yellow/green pods, white/purple flowers and so on.

• As they possess bisexual flowers, they can easily undergo self-pollination. This is why pea plants are able to produce offspring with the same traits over generations. 

• Cross-pollination can easily be achieved through emasculation, wherein the stamen of the flower is plucked without any disturbance to the pistil.

• These plants have a short life span wherein they produce plenty of seeds in one generation alone.

5. How is sex determined in human beings?

Ans. The chromosomal mechanism of determination of sex in human beings is of the genotype XX-XY. The nucleus of each cell consists of 23 pairs of chromosomes or 46 chromosomes, out of which 22 pairs are autosomes and the last pair, the 23rd pair, is the sex chromosome. Females are homomorphic, i.e., they possess two same-sex chromosomes, XX, whereas males are heteromorphic; they possess two different sex chromosomes, XY. Females are homogametic, producing only one type of egg (22+X), whereas males are heterogametic, producing two types of sperms – (22+X) & (22+Y).

In the process of fertilisation, if the sperm containing an X chromosome fertilises the egg, then the developing offspring would be a female (XX) and if the sperm containing a Y chromosome fertilises the egg, then the developing offspring would be a male (XY). Therefore, the sex ratio that is produced in the progeny is 1 : 1. Heterogamesis is the chromosomal mechanism of sex determination which can be male heterogamety or female heterogamety.

6. Explain the following terms with an example.

(a) Co-dominance
(b) Incomplete dominance

Ans.

(a) Co-dominance: These are the alleles that are able to express themselves independently when found together in a heterozygote. They are termed co-dominant alleles.

(b) Incomplete dominance: It is the phenomenon of inheritance where none of the contrasting characters in a pair is dominant. In the F1 hybrid, the expression of the trait is a fine mixture of the expression of the two factors. For instance, the Mirabilis jalapa produces pink flowers when a hybrid cross between a red and white flower is carried out.

7. What is point mutation? Give one example.

Ans. The abrupt change in the gene structure due to a change in the single-base pair of DNA due to inversion and substitution without any alteration in the reading of succeeding bases. For example, sickle cell anaemia.

A change in the shape of the RBCs from biconcave discs to elongated sickle-shaped structures is caused due to substitution of a single nitrogen base guanine to adenine at the sixth codon of the β-globin chain of the haemoglobin molecule. A typical RBC is biconcave, resembling a flexible disc-like that facilitates the cells to pass through large and small blood vessels to convey oxygen. Whereas sickle-shaped RBCs are elongated, non-flexible and rigid in structure which causes the cells to stick to the walls of the blood vessels leading to a blockage slowing or stopping the blood flow, hence sickle cell anaemia.

8. What is pedigree analysis? Suggest how such an analysis can be useful.

Ans. Pedigree analysis is the study of family history over generations to trace the inheritance of specific traits. It is useful in:

• Identifying genetic disorders and understanding inheritance patterns.

• Helping genetic counselors assess the risk of genetic diseases.

• Understanding how certain traits are passed through families.

9. Define and design a test cross.

Ans. A test cross is used to determine the genotype of an individual displaying a dominant phenotype. This is done by crossing the individual with a homozygous recessive individual.

Example:
Cross a tall (Tt) plant with a dwarf (tt) plant. If the offspring include both tall and dwarf plants, the tall parent is heterozygous (Tt). If all offspring are tall, the tall parent is homozygous (TT).

10. Differentiate between the following.

(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid

Ans.

(a) Dominance and Recessive:
• Dominance: A dominant allele expresses itself in the presence of a recessive allele.
• Recessive: A recessive allele only expresses itself in the absence of a dominant allele.

(b) Homozygous and Heterozygous:
• Homozygous: An individual has two identical alleles for a trait (e.g., TT or tt).
• Heterozygous: An individual has two different alleles for a trait (e.g., Tt).

(c) Monohybrid and Dihybrid:
• Monohybrid: A cross between two organisms differing in a single trait (e.g., seed color).
• Dihybrid: A cross between two organisms differing in two traits (e.g., seed color and seed shape).

Class 12 Principle of Inheritance and Variation Important Questions PDF

Students often look for a structured revision format before exams. A well-organized PDF containing Important Questions Class 12 Biology Chapter 4 can help in quick revision. Practicing these regularly improves accuracy and speed. It also helps students understand patterns of questions asked in previous years.

When solving the principle of inheritance and variation class 12 mcq, focus on keywords like dominance, linkage, mutation, and pedigree. MCQs test conceptual clarity rather than memorization.

Principle of Inheritance and Variation Class 12 PDF