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

NCERT Solution Class 12th Biology Chapter – 5 Principles of Inheritance and Variation

NCERT Solution Class 12th Biology Chapter – 5 Principles of Inheritance and Variation Notes Heterozygous, Mutation discuss Principles of Inheritance and Variation Notes.

NCERT Solution Class 12th Biology Chapter – 5 Principles of Inheritance and Variation

Chapter – 5

Principles of Inheritance and Variation

Notes

Genetics is a branch of biology which deals with principles of inheritance arid is practices. Progeny resembling the parents in morphological and physiological features has attracted the attention of many biologists.

During the mid-nineteenth century, Gregor Mendel conducted hybridization experiments on garden peas and proposed the laws of inheritance in living organisms.

Mendel proposed the principles of inheritance, which today are referred to as Mendel’s Law of inheritance. He proposed that the factors (genes) regulating the characters are found in pairs known as alleles. The expression of the characters in the offspring follows a definite pattern in first-generation (F₁), second-generation (F₂), and so on.

The dominant characters are expressed when the factors are in heterozygous conditions (Law of Dominance). The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous conditions. A recessive character that was not expressed in heterozygous condition may be expressed again when becomes homozygous. Due to this reason, characters segregate while the formation of gametes (Law of Segregation).

Among all characters, some show incomplete dominance while some show co-dominance.

Mendel found that the factors independently assort and combine in all permutation and combination (Law of Independent Assortment.)

Different combinations of gametes are theoretically represented in a square tabular form known as ‘Punnett Square’.

The factors on chromosomes regulating the characters are called the genotype. The physical expression of characters is called the phenotype.

Later the Mendel’s Law was extended in the form of the ‘Chromosomal Theory of Inheritance’. However, later on, Mendel’s law of independent assortment does not hold true for the genes that were located on the same chromosomes. These genes were called ‘linked genes’.

Different genes were linked to sexes also. These are called sex-linked genes. Both males and females have a set of chromosomes. One set was common while the other set was different. The chromosomes that were different in the two sexes were named as sex chromosomes and the remaining set was named autosomes. In human beings, a normal male has 22 pairs of autosomes and a pair of sex chromosomes as XY. A female has 22 pairs of autosomes and a pair of sex chromosomes XX.

An analysis of traits in several generations of a family is called pedigree analysis. In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations.

Genetic disorders may be grouped into two categories: Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. The pattern of inheritance of Mendelian disorders can be traced in a family by the pedigree analysis. Some examples of Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anemia, Colour blindness, Phenylketonuria, Thalassemia, etc.

Chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Some common examples of chromosomal disorders are Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome, etc.

Hybridization – Mating between two (or more) individuals differing ¡n genotype.

Mutation – It is a sudden and heritable change in a character of an organism.

Heterozygous – Hybrids that contain alleles that express contrasting traits.

Punnett square – A graphical representation to calculate the probability of all possible genotypes öf offspring in a genetic cross.

Law of dominance – It explains the expression of only one of the parental characters in a monohybrid cross in the F₁ and the expression of both in the F₂.

Law of segregation – It ¡s based on the fact that the alleles do not show a blending and that both the characters are recovered as such in the F₂ generation though one of there is not seen at the F₁ stage.

Incomplete dominance – When the F₁ has a phenotype that does. not resemble either of the two parents and is in between the two it is incomplete dominance.

Aneuploidy – Failure of segregation of chromatin during cell division results in the gain or loss of a chromosome(s) is called aneupLoidy.

Pedigree traits – An analysis of traits in several generations of a family.

Mendelian disorders – Disorders mainly determined by alteration or mutation in a single gene.

Chromosomal disorders – Disorders caused due to the absence or excess or abnormal arrangement of one or more chromosomes.