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

These comprehensive RBSE Class 12 Biology Notes Chapter 5 Principles of Inheritance and Variation will give a brief overview of all the concepts.

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

→ The process of which characters are transferred from one generation to the next generation is called inheritance heredity.

→ The differences in trait of individuals of a progeny from each other and from their parents are called variations.

→ The branch of biology which deals with the study of inheritance and variation is called genetics.

→ Mendel was the first to study the phenomenon of inheritance. While studying the pattern of inheritance in Pisum sativum plants of contrasting character, he proposed the principles of inheritance, which are today reffered to as Mendel’s law of inheritance.    

→ Mendel observed that the expression of the characters in the offspring follow a definite pattern in different-fruit generation (F₁), second (F₂) and so on. 

→ Law of dominance states that when two alternative forms of a trait are present in an organism, only one factor expreses itself in F₁ progeny and is called dominant while the other that remains masked is called ricessive.

→ Law of segregation states that the factors or alleles of a pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show and blending.

→ According to law of independent assortment the two factors of each character assort or separate out independent of the factors of other characters of the time of gamete formation and get randomly rearranged in the offspring producing both parental and new combinations of characters.

→ Beside Mendel’s inheritance, other patterns of inheritance are also observed such as, incomplete dominance, co-dominance, pleiotropy, polygenic inheritances etc.

→ According to the chromosomal theory of inheritance, Mendelian factors are situated on chromosomes and chromosomes segregate and assort independently during meiosis or gamete formation.
 
→ Experimental verification of the chromosomal theory of inheritance in Drosophila was done by T.H. Morgan and his colleagues.

→ The physical association of two or more genes on a chromosome is called linkage. In other words, when two or more genes are closely located on a chromosome, then both the genes are passed on together in the next generation. This type of inheritance is called as linkage.

→ Recombination involves the generation of non-parental gene combinations.

→ The establishment of sex through differential development in an individual at the time of zygote formation is called sex determination.

→ In human, sex determination is of XX-XY type.

→ Mutation is a phenomenon which causes alteration of DNA sequences resulting in changes in the genotype and the phenotype of an organism.

→ Pedigree analysis is an analysis of traits in several generations of a family.

→ A number of disorders in human beings are associated with the inheritance of changed or altered gene or chromosomes. These are called genetic disorders.

→ Haemophilia, colourblindness, sickle-cell anaemia, thalasemia, phenyl-ketonuria etc., are mendelian disorders, Down’s syndrome, Turner’s syndrome, klinefelters syndrome etc., are chromosomal disorders.

→ Gene: An ordered sequence of nucleotide which acts as the functional subunit of hereditary information.

→ Gene locus: The specific location of a particular gene on a homologous chromosome.

→ Allele: Alternative form of gene, having the same locus in homologous chromosome.

→ Factor: The term used by Mendel for gene at his time.

→ Haploid: The situation where the number of chromosome is half as in gamete.

→ Diploid: The situation where two sets of chromosomes are present.

→ Genotype: Genetic makeup of an organism.

→ Phenotype: The visible trait of an organism.

→ Locus: The region of a chromosome that is associated with the expression of a particular gene.

→ Hybrid: An individual produced by *fhe mating of genetically unlike parents.    

→ Co-dominance: A pattern of inheritance in which both alleles of a gene are expressed.

→ Monohybrid Cross: A cross involving parents that differ with respect to alleles of one locus.

→ Dihybrid cross: A cross involving parents that differ with respect to two different pairs of alleles.

→ Gamete: A haploid sex cell.

→ Trait: A phenotypic characteristic associated witht the expression of a single gene.

→ Pure line: A strain of homozygous, true breeding organisms formed either through repeated self-fertilization or breeding between homozygous identical ancestors.

→ Genome: The total number of genes present in the haploid set at chromosomes.

→ Pleiotropic gene: When a gene produces more than one phenotypic effects totally unrelated, it is called pleotropic gene.

→ Linkage: Physical association of two or more genes on a chromosome.

→ Mutation: A phenomenon which causes alternation of DNA sequence resulting in changes in the genotype and phenotype of an organism.