CBSE Class 12 Biology Principles Of Inheritance And Variation Notes

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Principles Of Inheritance And Variation Notes Class 12 Biology Revision Notes

Class 12 Biology students should refer to the following concepts and notes for Principles Of Inheritance And Variation Notes in standard 12. These exam notes for Grade 12 Biology will be very useful for upcoming class tests and examinations and help you to score good marks

Principles Of Inheritance And Variation Notes Notes Class 12 Biology


 Principles of Inheritance and Variation

Genetics: Genetics is a branch of biology which deals with principles of inheritance and its practices.

Heredity: It is transmission of traits from one generation to another through the molecular mechanism.

Variation: Variation is the degree by which progeny differ from their parents.

Allele: Various or slightly different forms of a gene, having same position on chromosomes.

Phenotype: The observable or external characteristics of an organism.

Genotype: The genetic constitution of an organism.

Monohybrid cross : A cross between two individuals of species, considering the inheritance of single pair of contrasting character e.g., a cross between pure tall (TT) and Dwarf (tt).

Dihybrid cross: A cross between two individuals of a species, considering the inheritance of two pairs of contrasting traits/characters e.g., a cross between Round & Yellow seeds (RRYY) and wrinkled & green seeds (rryy).

Incomplete dominance: When one of the two alleles of a gene is incompletely dominant over the other allele.

Co-dominance: When two alleles of a gene are equally dominant and express themselves even when they are together.

Multiple allelism : When a gene exists in more than two allelic forms e.g., gene for human blood group exist in three allelic forms, IA, IB and i.

Aneuploidy : The phenomenon of gain or loss of one or more chromosome(s), that results due to failure of separation of homologous pair of chromosomes during meiosis.

Trisomy : The condition in which a particular chromosome is present in three copies in a diploid cell/nucleus.

Male heterogamety : When male produces two different types of gametes/sperms e.g., In human beings X and Y.

Female Heterogamety : When female produces two different types of gametes/ova e.g., female bird produces Z and W gametes.

Mutation : The sudden heritable change in the base sequence of DNA, or structure of chromosome or a change in the number of chromosomes.

Pedigree Analysis : The analysis of the distribution and movement of trait in a series of generations of a family.

Law of Dominance: i) Characters are controlled by discrete units called factors. Ii) Factors occur in pairs. iii) In a dissimilar pair of other (recessive).

Law of Segregation: The members of allelic pair that remained together in the parent Segregate/separate during gamete formation and only one of the factors enters a gamete.

Law of Independent Assortment: In the inheritance of two pairs of contrasting characters, the factors of each pair of characters segregate independently of the factors of the other pair of characters.

Test Cross: When offspring or individual with dominant phenotype, whose genotype is not known, is crossed with an individual who is homozygous recessive for the trait.

Aneuploidy: Failure of segregation of chromatids during cell division results in the gain or loss of a chromosome(s) called aneuploidy.

Polyploidy: Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is known as polyploidy.

Pleiotropism: Pleiotropism is defined as a phenomenon when single gene may produce more than one effect (the multiple effect of a gene) or control several phenotypes depending on its position.

Carrier: A person with a ‘defective recessive gene’ and a ‘dominant normal gene’ on homologous pair of chromosomes and therefore, not affected by the disorder but transmit the defective gene to the next progeny through gametes.

Genome: All the genetic material in the chromosomes of a particular organism; size generally given as its total number of base pairs.

Germ Cell- An egg or sperm cell. A gamete. In humans, a germ cell contains 23 chromosomes. Haploid= A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells.

Hemophilia = sex-linked recessive. Males get it most often.

Homologous chromosomes: A pair of chromosomes containing the same linear gene sequences, each derived from one parent

Karyotype: Photomicrograph of an individual’s chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type.

Linkage: Proximity of two or more genes on a chromosome. The closer together the genes, the lower the probability that they will be separated during meiosis and hence the greater the probability that they will be inherited together.

Linkage map: relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together.

Locus (pl. loci): The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean regions of DNA that are expressed.

Non-Disjunction: When homologous chromosomes fail to segregate properly during meiosis. Down syndrome, Turner syndrome and Klinefelter syndrome result from non-disjunction.

Relationship between genes and chromosome of diploid organism and the terms used to describe them

CBSE Class 12 Biology - Principles of Inheritance and Variation

Exceptions to Mendel’s laws of inheritance


I. Incomplete Dominance
When the dominant allele does not completely the phenotypic expression of the recessive allele in a heterozygote, then a blending of both dominant and recessive traits takes place in F1 and F2 hybrids. This phenomenon is called incomplete dominance.

e.g.i) In snapdragon, broad leaf is incomplete dominant over narrow leaf.
ii) In snapdragon,(or Antirrhinum sp./dog flower) red flower is incompletely dominant over white flower
iii) In Mirabilis, red flower is incompletely dominant, over white flower.

II. Co-dominance ( Multiple allelism)
When both dominant and recessive allele lack dominant recessive relationship, then both are expressed side by side. This phenomenon is known as co-dominance.

e.g. ABO blood groups in human beings are controlled by the gene I. The gene (I) has three alleles IA, IB and i. Since there are three different alleles, there are six different combinations of these three alleles that are possible a total of six different genotypes of the human ABO blood types.

III. Polygenic Inheritance
At least in some instances the same character can be determined by more than one gene, each with the same but cumulative phenotypic effect Quantitative characters like plant height, yield of crops (size, shape and number of seeds and fruits per plant), intelligence in human beings and milk yield in animals have been found to be determined by many genes and their effects have been found to be cumulative. This phenomenon is known as polygenic inheritance. This is also considered as “ Quantitative inheritance” or “multiple factor inheritance”.
Other examples that can be studied are the kernel colour in wheat and inheritance of cob length in maize.

IV. Pleiotropism
Pleiotropism is defined as a phenomenon when single gene may produce more than one effect (the multiple effect of a gene) or control several phenotypes depending on its position.

Eg.In drosophila white eye mutation leads to depigmentation in many other parts of the body, giving a pleitropic effect.

In transgenic organisms, the introduced gene can produce different effects depending on where the gene has introgressed.

 CBSE Class 12 Biology - Principles of Inheritance and Variation

Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the “chromosomal theory of inheritance”.

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