CBSE Class 12 Biology Notes Chapter 5 Principles of Inheritance and Variation

The principles of inheritance and variation are the fundamental concepts that explain how traits are passed down from parents to offspring, and how those traits can differ within a species.

• The process of sharing traits from parents to offspring is known as heredity.
• Genetics is the science that explains how these traits are passed down through generations.
• Gregor Mendel, the "Father of Genetics", discovered key principles of inheritance through experiments with pea plants.
• These principles, known as Mendel's Laws explain how characteristics are transmitted.
• Variation, the diversity of life, is the raw material for evolution.

CBSE Class 12 Biology Notes for Chapter 5 Principles of Inheritance and Variation are given in the article below for easy preparation and understanding of the concepts involved.

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Class 12 Biology Chapter 5 Notes – Principles of Inheritance and Variation

Important Terms

• Heredity: It is the process of transfer of characteristics from parents to offspring.
• Genetics: The study of the inheritance of character from parents to offspring is called genetics.
• Inheritance: It is the basis of heredity and the process through which characteristics are passed on from parents to children.
• Variation: Variations refer to differences between individuals within a species or even between populations of the same species.

Mendel’s Experiment

• Gregor Johann Mendel is known as the Father of Genetics.
• He conducted experiments on garden peas.
• Mendel selected 14 true-breeding pea plants. 
• These plants exhibited seven distinguishable characteristics, each with two contrasting traits.
• He introduced "factors," now known as genes, which are units of heredity passed from parents to offspring.
• Genes controlling a pair of opposing traits are called "alleles."

Mendel’s Experiments

Mendel’s Laws of Inheritance

• Law of Dominance: It states that in hybrids (heterozygotes), one allele masks the other. The expressed allele is dominant, while the masked one is recessive.
• Law of Segregation of Genes: Alleles separate during gamete formation, with one from each parent uniting at fertilization.
• Law of Independent Assortment: Trait alleles segregate independently during gamete formation, allowing any combination to appear in offspring.

Incomplete Dominance

• Incomplete dominance occurs when neither allele for a trait is fully dominant. 
• This results in the heterozygote expressing an intermediate phenotype, often a blend of the traits seen in the homozygous parents. 
• An example is the snapdragon flower color. 
• Crossing a red-flowered plant (RR) with a white-flowered plant (rr) produces all pink-flowered offspring (Rr) in the F1 generation. 
• Self-pollination of the pink F1 plants (Rr) leads to a 1:2:1 ratio of red:pink: white flowers in the F2 generation.

Incomplete dominance

Co-dominance

• Codominance describes a situation where both alleles of a gene are expressed simultaneously in an individual.
• A classic example is the ABO blood group system in humans.
• This system is controlled by a gene with three alleles: IA, IB, and i. 
• IA and IB each code for a specific sugar molecule on red blood cells, while i is recessive and doesn't produce any sugar. 
• IA and IB are co-dominant with each other, meaning if both are present in an individual (e.g., IA and IB), both alleles are expressed, leading to blood type AB.

Codominance

Chromosomal Theory of Inheritance

• Building on Mendel's work, Sutton and Boveri proposed that chromosomes carry genes. 
• These genes reside on homologous pairs of chromosomes, with each pair containing alleles for a trait at the same location. 
• During meiosis, homologous chromosomes separate, ensuring that gametes receive only one chromosome from each pair. 
• Importantly, chromosome segregation and assortment occur independently. 
• Fertilization then combines gametes, restoring the diploid chromosome number in offspring, similar to the parents.
• Further solidifying this theory, Morgan's experiments with fruit flies (Drosophila melanogaster) provided strong experimental evidence for the chromosomal theory of inheritance.

Linkage and Recombination

• Linkage: It is the physical association of genes located on a chromosome.

Types: Complete linkage and Incomplete linkage

• Recombination: It is a process of producing new combinations of alleles by the recombination of DNA molecules.

Types: Homologous, Non-homologous, Site-specific, and Mitotic Recombination

Sex Determination

• Different organisms utilize various systems for sex determination.
• Sex chromosomes, distinct from the remaining autosomes, determine the development of sexual characteristics. 
• The first X chromosome was observed by Henking and named the "X body.
• Two types of sex chromosome systems exist i.e. male heterogamety and female heterogamety.
• In male heterogamety, males produce two distinct gamete types (e.g., humans, grasshoppers, Drosophila).
• In female heterogamety, females produce two different gametes (e.g., birds).

Sex determination

Mutation

• A mutation is any change in the DNA sequence. 
• Viable mutations, those not lethal to the organism, can be passed on to offspring, altering both the organism's genotype and phenotype. 
• While mutations are linked to various diseases, not all are harmful. 
• Deletions, insertions, duplications, substitutions, and similar changes in the DNA sequence can all cause mutations.
• Ultraviolet (UV) rays are one example of a mutation-inducing agent (mutagen).

Mutation

Genetic Disorders

• Genetic disorders are a variety of conditions caused by changes in genes or chromosomes. 
• These changes can be inherited from parents or occur spontaneously. 
• Types:

• Mendelian Disorders: These are disorders due to alteration in the single gene
• Chromosomal Disorders: These are disorders due to excess, absence, or abnormal arrangement of chromosomes

There are Some important List Of Top Biology Questions On Principles of Inheritance and Variation Asked In CBSE CLASS XII