Chapter 4: Principles of Inheritance and Variation – Class 12 Biology

Understanding how traits are transmitted from one generation to the next is a fundamental concept in biology. Chapter 4 of the NCERT Class 12 Biology textbook, titled “Principles of Inheritance and Variation,” lays a comprehensive foundation in classical genetics. It traces the revolutionary work of Gregor Mendel, expands into modern concepts of linkage, recombination, mutation, sex determination, and introduces genetic disorders with medical relevance.

This chapter plays a pivotal role in NEET preparation, often contributing several direct and application-based questions in the biology section. Mastery of this unit not only ensures academic success but also enhances conceptual clarity for biotechnology, medical genetics, and molecular biology in higher studies.

Key Topics Covered

1. Mendel’s Groundbreaking Experiments

Gregor Johann Mendel, an Austrian monk, laid the foundation of classical genetics through his methodical hybridization experiments on pea plants (Pisum sativum). He studied seven pairs of contrasting traits, such as flower color, seed shape, and pod color.

• Why peas? Short life cycle, self- and cross-pollination possible, and clearly distinguishable traits.
• Findings: Traits are controlled by discrete hereditary units called factors (genes). His observations led to the formulation of key inheritance laws that form the basis of classical genetics.

2. Mendel’s Laws of Inheritance

Mendel observed patterns of inheritance and proposed three fundamental laws:

• Law of Dominance: In a heterozygous condition, one trait masks the expression of the other.
• Law of Segregation: Allele pairs separate during gamete formation, and each gamete receives one allele.
• Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation, provided they are not linked.

These laws explain observed phenotypic and genotypic ratios in monohybrid (3:1, 1:2:1) and dihybrid (9:3:3:1) crosses.

3. Deviations from Mendelian Ratios

Modern genetics reveals patterns that deviate from Mendel’s predictions:

• Incomplete Dominance: Neither allele is completely dominant; heterozygote has intermediate phenotype (e.g., Mirabilis jalapa).
• Codominance: Both alleles express fully in heterozygotes (e.g., AB blood group).
• Multiple Alleles: More than two alleles exist for a gene in the population (e.g., ABO blood group).
• Pleiotropy: A single gene influences multiple phenotypic traits (e.g., sickle-cell anemia).

4. Chromosomal Theory of Inheritance

Proposed by Sutton and Boveri, this theory linked Mendel’s laws to chromosome behavior. Chromosomes, like genes, occur in pairs and segregate independently during meiosis.

• Experimental support came from Thomas Hunt Morgan’s work on Drosophila melanogaster, which also introduced the concept of sex-linked inheritance.

5. Linkage and Recombination

• Linkage: Genes that are located close to each other on the same chromosome tend to be inherited together. Strong linkage reduces chances of recombination. 
• Recombination: Crossing over during meiosis creates new combinations of alleles, enhancing genetic variability. 

Morgan’s experiments on Drosophila demonstrated that the frequency of recombination between linked genes depends on the distance between them.

6. Mechanisms of Sex Determination

Different organisms exhibit varied systems for determining the sex of an individual:

• XX-XY system: Found in humans and most mammals. Males determine the sex.
• XO system: Seen in some insects; absence of Y chromosome determines femaleness.
• ZZ-ZW system: Found in birds; female determines the sex (ZW), males are ZZ.
• Environmental sex determination: In some reptiles, sex is influenced by incubation temperature.

7. Mutation and Genetic Disorders

• Mutation: A sudden, heritable change in DNA sequence. It may occur naturally or be induced by mutagens like radiation.
• Chromosomal Disorders: Caused by abnormalities in chromosome number or structure.
  • Down Syndrome: Trisomy 21
  • Turner’s Syndrome: Monosomy of X chromosome (XO)
  • Klinefelter’s Syndrome: Extra X chromosome (XXY)
• Mendelian Disorders: Caused by mutation in a single gene. Inherited according to Mendelian laws.
  • Thalassemia, Sickle-cell anemia, Hemophilia, Phenylketonuria

NEET Practice MCQs

1. Who is considered the father of genetics? 
   • a) Watson 
   • b) Darwin 
   • c) Mendel ✅ 
   • d) Morgan 
2. What is the phenotypic ratio in Mendel’s dihybrid cross? 
   • a) 3:1 
   • b) 1:2:1 
   • c) 9:3:3:1 ✅ 
   • d) 2:1 
3. Which of the following exemplifies codominance? 
   • a) Sickle-cell anemia 
   • b) AB blood group ✅ 
   • c) Pink flowers in Mirabilis jalapa 
   • d) Down syndrome 
4. Which chromosomal disorder is caused by trisomy of chromosome 21? 
   • a) Klinefelter’s syndrome 
   • b) Turner’s syndrome 
   • c) Down syndrome ✅ 
   • d) Hemophilia 
5. The chromosomal basis of sex determination in humans is: 
   • a) XX-XY ✅ 
   • b) XO-XY 
   • c) ZZ-ZW 
   • d) XX-XO 

FAQs

Q1. Why were Mendel’s experiments significant?
Because they were systematic, quantitative, and the first to analyze inheritance patterns using statistical tools.

Q2. What is the significance of a test cross?
It helps determine whether an organism exhibiting a dominant trait is homozygous or heterozygous.

Q3. How do mutations impact genetic disorders?
Mutations in single genes can cause Mendelian disorders; chromosomal aberrations cause syndromic disorders.

Q4. Can recombination occur between linked genes?
Yes, but its frequency depends on the distance between the genes; closer genes recombine less frequently.

Q5. What is epistasis?
When one gene masks the effect of another gene at a different locus.

NEET Preparation Tips for Chapter 4

• Solve a variety of genetic cross problems including monohybrid, dihybrid, and test crosses.
• Understand and practice Punnett square-based problem-solving techniques.
• Memorize key genetic terms with examples.
• Focus on examples of inheritance patterns like codominance, incomplete dominance, and pleiotropy.
• Practice diagrams and revise chromosomal inheritance models.
• Attempt timed mock tests with previous year questions.

Conclusion

“Principles of Inheritance and Variation” is a cornerstone chapter for mastering the foundations of genetics. It connects classical Mendelian theories with modern-day genetic concepts and disorders, offering real-world relevance in medicine and biology. With consistent revision, application-based understanding, and regular practice, this chapter can become a high-scoring area in NEET 2025. Investing time in understanding this chapter is investing in your success in the biology section of the NEET exam.