01Introduction to Genetics and Inheritance
Mastering the fundamental concepts of this chapter is essential for any medical aspirant. This comprehensive principles of inheritance and variation pdf guide provides a high-yield analysis of how traits are passed from parents to offspring and the mechanisms that drive biological diversity. Genetics is the branch of biology that explores inheritance (the process of passing characters) and variation (the degree by which progeny differ from parents). For NEET, this chapter consistently yields 4-5 high-weightage questions, making it a “must-perfect” topic for securing a top rank.
Basic Terminology for Genetics
- Gene: The functional unit of inheritance (Mendel’s “factors”).
- Allele: Alternative forms of a gene occupying the same locus on homologous chromosomes.
- Genotype: The genetic constitution of an organism (e.g., TT, Tt).
- Phenotype: The observable physical expression of traits (e.g., Tall, Dwarf).
02Mendel’s Experiments with Pea Plants
Gregor Mendel, known as the Father of Genetics, conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863). In your principles of inheritance and variation pdf study, you must memorize the seven pairs of contrasting traits he selected, such as stem height, seed shape, and flower color.
| Trait | Dominant Expression | Recessive Expression |
|---|---|---|
| Stem Height | Tall | Dwarf |
| Seed Shape | Round | Wrinkled |
| Seed Color | Yellow | Green |
| Flower Position | Axial | Terminal |
03Monohybrid Cross and Mendelian Laws
A cross involving only one pair of contrasting traits is a monohybrid cross. When Mendel crossed tall and dwarf plants, the F1 generation was entirely tall, while the F2 showed both traits. This led to the formulation of two fundamental laws.
Genotypic Ratio: 1 : 2 : 1
1. Law of Dominance
Characters are controlled by discrete units called factors (genes) which occur in pairs. In a dissimilar pair, one member dominates (dominant) the other (recessive).
2. Law of Segregation
Alleles do not show any blending and both characters are recovered as such in the F2 generation. During gamete formation, the factors or alleles of a pair segregate such that a gamete receives only one of the two factors. This is also known as the Law of Purity of Gametes.
04Dihybrid Cross: Principles of Inheritance and Variation PDF Guide
A dihybrid cross involves two pairs of contrasting traits simultaneously (e.g., Seed Shape and Seed Color). This led to Mendel’s third law. Understanding the 16-square Punnett grid is a vital skill for any principles of inheritance and variation pdf worksheet.
(9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green)
Law of Independent Assortment
When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. This law holds true as long as the genes are located on different chromosomes or are far apart on the same chromosome.
05Non-Mendelian Inheritance: Deviations
Post-Mendelian research revealed that inheritance is often more complex than simple dominance. These deviations are frequent NEET targets in principles of inheritance and variation pdf assessments.
- Multiple Alleles: Presence of more than two alleles for a single gene in a population (e.g., Gene ‘I’ has 3 alleles).
- Pleiotropy: A single gene influencing multiple phenotypic traits (e.g., Phenylketonuria).
- Polygenic Inheritance: Multiple genes controlling a single trait (e.g., Human skin color, height).
06Chromosomal Theory, Linkage, and Recombination
Proposed by Sutton and Boveri, the Chromosomal Theory of Inheritance identifies chromosomes as the carriers of genetic material. T.H. Morgan later provided experimental evidence using Drosophila melanogaster.
07Sex Determination: Principles of Inheritance and Variation PDF Summary
Sex determination mechanisms vary across the animal kingdom. In humans, the presence of the Y-chromosome dictates the male sex. In birds, however, the female is the heterogametic sex.
| Mechanism Type | Heterogametic Sex | Example Organism |
|---|---|---|
| XX − XY | Male (XY) | Humans, Drosophila |
| XX − XO | Male (XO) | Grasshopper |
| ZZ − ZW | Female (ZW) | Birds, Butterflies |
| Haplodiploidy | Male (Haploid) | Honeybees |
08Genetic Disorders: Mendelian and Chromosomal
Disorders are broadly categorized based on their inheritance patterns. This section of the principles of inheritance and variation pdf is highly tested for pedigree analysis.
1. Mendelian Disorders
Determined by alteration or mutation in a single gene. They follow Mendelian inheritance patterns.
- Haemophilia: Sex-linked recessive; blood fails to clot.
- Sickle-cell Anaemia: Autosomal recessive; GAG changes to GUG at 6th position of β-globin chain.
- Phenylketonuria (PKU): Autosomal recessive; metabolic error in enzyme phenylalanine hydroxylase.
2. Chromosomal Disorders
Caused by absence, excess, or abnormal arrangement of one or more chromosomes. Often due to non-disjunction during meiosis.
Quick Revision Summary
- Mendel: Selected 7 pairs of traits in garden peas.
- Segregation: Alleles separate during gamete formation (Universal Law).
- Test Cross: Crossing F1 with recessive parent to find genotype.
- ABO Blood Group: Shows Multiple Allelism and Co-dominance.
- Morgan: Discovered Linkage in Drosophila.
- Henking: Discovered the X-body (X-chromosome).
- Sickle-cell: Point mutation; Glutamic acid replaced by Valine.
- Aneuploidy: Gain or loss of a chromosome (e.g., 2n + 1).
- Polyploidy: Increase in whole set of chromosomes (Common in plants).
- Pedigree Chart: Square = Male, Circle = Female, Colored = Affected.
09Frequently Asked Questions
What is a Test Cross and what is its significance?
Explain the difference between Dominance and Co-dominance.
Why did Mendel choose the Pea plant for his experiments?
What is the molecular basis of Sickle-cell Anaemia?
Explain the concept of Linkage.
What is Trisomy and provide an example?
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Table of Contents
Biology — Class 12
| 01 | Sexual Reproduction in Flowering Plants | Go to page |
| 02 | Human Reproduction | Go to page |
| 03 | Reproductive Health | Go to page |
| 04 | Principles of Inheritance and Variation | Go to page |
| 05 | Molecular Basis of Inheritance | Go to page |
| 06 | Evolution | Go to page |
| 07 | Human Health and Disease | Go to page |
| 08 | Microbes in Human Welfare | Go to page |
| 09 | Biotechnology: Principles and Processes | Go to page |
| 10 | Biotechnology and its Applications | Go to page |
| 11 | Organisms and Populations | Go to page |
| 12 | Ecosystem | Go to page |
| 13 | Biodiversity and Conservation | Go to page |
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