NCERT Solutions Class 12 Biology Chapter 4 Principles of Inheritance and Variation

Chapter 4, Principles of Inheritance and Variation from Class 12 Biology teaches students how traits/characteristics are passed from parents to offspring and why certain differences appear among individuals of the same species. It also explains Mendel’s experiments, basic genetic terms, and the patterns of inheritance seen in different crosses. Concepts like linkage, recombination, and chromosomal basis of inheritance, etc., are introduced in this chapter.

ALLEN provides a set of NCERT Solutions that turn complex genetic cross-diagrams into easy-to-understand lessons. Topics like Punnett squares and pedigree analysis are simplified to ensure you never get stuck. These solutions are crafted to supplement your classroom preparation, giving you the edge with expert-verified answers and clear explanations. The solutions will also be beneficial for those preparing for competitive exams like NEET.

1.0Download NCERT Solutions Class 12 Biology Chapter 4 Principles of Inheritance and Variation : Free PDF

This chapter explains how genes determine the characteristics of individuals and how variations can arise across generations. The NCERT Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation help students in the revision of concepts including genetic crosses, key laws, and important terms easily. Download the free PDF from the link below to study anytime and prepare confidently for exams.

2.0Class 12 Biology Chapter 4: Key Concepts

Some of the key lessons covered in this chapter are:

• Mendel’s Laws of Inheritance: Helps in understanding the laws of dominance, segregation, and independent assortment.
• Monohybrid and Dihybrid Crosses: Studying genetic crosses and interpreting phenotypic and genotypic ratios.
• Chromosomal Theory of Inheritance: helps in learning how genes are located on the chromosomes and are inherited during cell division.
• Linkage and Recombination: Understanding how genes on the same chromosome are inherited together or reshuffled.
• Sex Determination: Learning how sex is genetically determined in humans and other organisms.
• Genetic Disorders: Studying examples like haemophilia, sickle-cell anaemia, and colour blindness.
• Variation: Understanding the causes and importance of genetic differences in populations.

3.0NCERT Class 12 Biology Chapter 4 Principles of Inheritance and Variation : Detailed Solutions

1. Mention the advantages of selecting pea plants for experiment by Mendel. 

Ans: Below are the advantages of selecting a pea plant by Mendel:

• Easy to grow and maintain: Pea plants can be grown easily under controlled conditions and do not require any special care. This makes them very useful for condiucting large scale experiments.
• Short life cycle: They complete their life cycle in a short period, allowing Mendel to observe several generations within a limited time.
• Presence of contrasting characters: Pea plants show many clearly distinguishable traits such as tall/dwarf, round/wrinkled, and yellow/green seeds. This makes the study of inheritance simple and accurate.
• Naturally self-pollinating: Self-pollination helps in maintaining pure (true-breeding) lines over generations.
• Easy cross-pollination: Artificial cross-pollination can be performed easily by removing the anthers, allowing controlled hybridization.
• Produces large number of seeds: A large sample size increases the reliability of results and statistical analysis.

2. Differentiate between the following – 

(a) Dominance and Recessive 

(b) Homozygous and Heterozygous 

(c) Monohybrid and Dihybrid. 

Ans: (a) Dominance and Recessive 

(b) Homozygous and Heterozygous 

(c) Monohybrid and Dihybrid. 

3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced? 

Ans: According to the law of independent assortment, a diploid organism that is heterozygous can produce different gametes. 

The number of gametes produced = 2ⁿ

Here, n=4

Therefore, the number of gametes produced = 2⁴ = 16

4. Explain the Law of Dominance using a monohybrid cross. 

Ans: According to the Law of Dominance, when two contrasting alleles for a character are present in a heterozygous condition, only one allele (the dominant one) expresses itself, while the other allele (the recessive one) will remain masked.

Example - Monohybrid cross in a pea plant
Mendel crossed a pure tall pea plant (TT) with a pure dwarf pea plant (tt).

• Parental Generation (P): TT × tt
• Gametes: T and t
• F₁ Generation: All offspring are Tt (Tall)

This shows that the tallness trait is dominant, as it appears in all F₁ plants, while the dwarf trait does not express itself in the presence of the dominant allele.

5. Define and design a test-cross. 

Ans: A test cross is a genetic cross between an individual showing a dominant phenotype but with an unknown genotype and a homozygous recessive individual. It is performed to determine whether the dominant individual is homozygous or heterozygous.

Design of a Test Cross:

• The individual with the dominant trait is crossed with a pure recessive parent.
• For example, in pea plants, a tall plant of unknown genotype (T?) is crossed with a dwarf plant (tt).

Possible Outcomes:

• If all offspring are tall, the test plant is homozygous dominant (TT).
• If the offspring show a 1 : 1 ratio of tall and dwarf, the test plant is heterozygous (Tt).

Thus, a test cross helps in identifying the genotype of an individual expressing a dominant trait by analysing the phenotypic ratio in the offspring.

6. Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus. 

Ans: Homozygous Female (tt) × Heterozygous Male (Tt)

• Female Gametes: t, t
• Male Gametes: T, t

Result: 50% Tall, 50% Dwarf. Phenotypic ratio is 1:1.

7. When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be 

(a) tall and green. 

(b) dwarf and green. 

Ans: Given: Tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy) = TtYy x Ttyy

Where, T=tall, t=dwarf, Y=yellow seed, y=green seeds

Step 1: Gametes

Parent 1 (TtYy), Possible gametes = TY, Ty, tY, ty

Parent 2 (Ttyy), Possible gametes = Ty, ty

Step 2: Punnet Square: Total outcomes - 8

Step 3: Required Phenotypes

(a) Tall and Green (T…yy):

These are: TTyy, Ttyy, Ttyy

Number = 3

Therefore, proportion = 3/8

(b) Dwarf and Green (tt yy):

This is: ttyy

Number = 1

Therefore, proportion = 1/8

8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross? 

Ans: When two heterozygous parents are crossed, and the two loci are completely linked, the law of independent assortment does not apply. As a result, the genes are inherited together and recombinant types are not formed.

The F₂ generation shows mainly the parental phenotypes, and the typical 9 : 3 : 3 : 1 ratio is not obtained. Instead, the phenotypic ratio resembles a monohybrid cross, that is 3 : 1, with only parental combinations appearing.

9. Briefly mention the contribution of T.H. Morgan in genetics. 

Ans:

• T.H. Morgan used the fruit fly Drosophila melanogaster as a model organism to study inheritance because of its short life cycle and easily observable traits.
• He supported the chromosomal theory of inheritance by providing strong experimental evidence that genes are arranged linearly on chromosomes.
• Morgan showed that some genes are inherited together because they are located on the same chromosome, a phenomenon known as linkage.
• He also explained that crossing over during meiosis can produce recombinant types, which helped in understanding genetic variation.
• His work laid the foundation for genetic maps, where the distance between genes can be estimated based on recombination frequency.

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

Ans: Pedigree analysis is the study of the inheritance pattern of a particular trait or genetic disorder in a family across several generations. It is represented using standard symbols to show relationships and the appearance of the trait in males and females.

Below are some uses of the pedigre analysis:

• It helps to determine the mode of inheritance, such as autosomal dominant, autosomal recessive, or sex-linked traits.
• Assists in identifying carriers of genetic disorders within a family.
• Useful in predicting the probability of a trait or disease appearing in future generations.
• Aids in genetic counselling and making informed medical or reproductive decisions.

11. How is sex determined in human beings? 

Ans: In humans, sex is determined by the XY type mechanism asmentioned below:

• Females have two identical sex chromosomes (XX). They produce only one type of gamete (X).
• Males have two different sex chromosomes (XY). They produce two types of gametes (X and Y) in equal proportions.
• The sex of the child is determined by whether the X-bearing egg is fertilized by an X-bearing sperm or a Y-bearing sperm.
• If the final zygote has XX chromosome, it is a girl and if it has a XY chromosome, it is a boy.

12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings. 

Ans: Given,

Child: Group = O; Genotyoe = ii

Father: Group = A; Genotype = I^Ai

Mother: Group = B; Genotype = I^Bi

Therefore, other possible genotypes of offsprings are:

I^AI^B (Group AB)

I^Ai (Group A)

I^Bi (Group B)

13. Explain the following terms with example 

(a) Co-dominance 

(b) Incomplete dominance 

Ans: (a) Co-dominance: It is a pattern of inheritance in which, in a heterozygous individual, both alleles of a gene express themselves equally and independently. Neither allele is dominant nor recessive, so the phenotype shows the simultaneous expression of both traits.

Example: In the AB blood group system in humans, the alleles Iᴬ and Iᴮ are co-dominant. 

An individual with the genotype IᴬIᴮ expresses both A and B antigens on the red blood cells, resulting in AB blood group

(b) Incomplete dominance: It is a pattern of inheritance in which neither allele is completely dominant, and the heterozygous individual shows an intermediate phenotype that is a blend of the two parental traits.

Example: In the snapdragon (Antirrhinum) plant, a cross between a red-flowered plant (RR) and a white-flowered plant (rr) produces pink-flowered plants (Rr) in the F₁ generation. In the F₂ generation, the phenotypic ratio is 1 red : 2 pink : 1 white.

14. What is point mutation? Give one example. 

Ans: A genetic mutation in which there is a change in a single nucleotide or base pair in the DNA sequence of a gene is called a point mutation. This small change can alter the codon and may lead to the formation of an abnormal or altered protein.

Example: Sickle cell anaemia is a classic example of point mutation. It is caused by a single base substitution in the gene for the beta chain of haemoglobin, where the amino acid glutamic acid is replaced by valine, resulting in the formation of abnormal, sickle-shaped red blood cells.

15. Who had proposed the chromosomal theory of the inheritance? 

Ans: Walter Sutton and Theodor Boveri independently proposed the chromosomal theory of inheritance in 1902. They suggested that genes are located on chromosomes and that the behaviour of chromosomes during meiosis explains the patterns of inheritance described by Mendel.

16. Mention any two autosomal genetic disorders with their symptoms.

Ans: Sickle Cell Anaemia: Sickle cell anaemia is an autosomal recessive disorder caused by a mutation in the gene coding for the beta chain of haemoglobin.

Symptoms include: Formation of abnormally shaped, sickle-like red blood cells results in the reduced oxygen-carrying capacity of blood, causing symptoms such as fatigue, breathlessness, anaemia, and episodes of pain due to blockage of blood vessels.

Thalassemia: Thalassemia is an autosomal recessive disorder in which the synthesis of either the alpha or beta globin chain of haemoglobin is reduced or absent. This results in severe anaemia, weakness, pale skin, delayed growth, and enlargement of the spleen due to increased destruction of red blood cells.

4.0Key Features of NCERT Solutions for Class 12 Biology Chapter 4

• Genetic crosses are clearly explained in the chapter, helping students to follow each step and calculate the correct ratios.
• Practicing these questions strengthen the understanding of inheritance patterns and chromosome behaviour.
• The solutions follows the latest NCERT syllabus and are solved for the NCERT Class 12 exercise section.
• To improve recall of certain genetic terms and laws of inheritance, regular revision of these solutions are recommended.
• The chapter provides strong fundamentals to support in the preparation for competitive exams like NEET.