(i) The term mutation was introduced by Hugo De Vries in 1901 when he observed phenotypic changes in the evening Primrose plant, Oenothera lamarckiana.
(ii) Mutation is an inheritable sudden change in the genetic material (DNA) of an organism.
(iii) Mutations are classified into two main types, namely chromosomal mutation and gene mutation. Chromosomal mutation: The sudden change in the structure or number of chromosomes is called chromosomal mutation. This may result in
Changes in the structure of chromosomes:
Structural changes in the chromosomes usually occurs due to errors in cell division. Changes in the number and arrangement of genes takes place as a result of deletion, duplication, inversion and translocation in chromosomes. Changes in the number of chromosomes: They involve addition or deletion in the number of chromosomes present in a cell. This is called ploidy. There are 2 types of ploidy:
Euploidy in plants and animals:
(a) Euploidy: It is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). If it has four haploid sets of chromosomes. It is called Tetraploidy. Tetraploid plants are advantageous as they often result in increased fruit and flower size.
(b) Aneuploidy: It is the loss or gain of one or more chromosomes in a set. It is of 3 types:
(i) Monosomy (2n-1),
(ii) Trisomy (2n+1)
(iii) Nullisomy (2n-2). In man, Down's syndrome is one of the commonly known aneuploid condition.
Gene or point mutation:
(i) Gene mutation is the changes occurring in nucleotide sequence of a gene.
(ii) It involves substitution, deletion, insertion or inversion of a single or more than one nitrogenous base.
(iii) Gene alteration results in abnormal protein formation in an organism.

Structure of a Chromosome:
(i) The chromosomes are thin, long and thread like structures consisting of two identical strands called sister chromatids.
(ii) They are held together by the centromere.
(iii) Each chromatid is made up of spirally coiled thin structure called chromonema.
(iv) The chromonema has number of bead-like structures along its length which are called chromomeres.
(v) The chromosomes are made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions.
(vi) These proteins provide structural support to the chromosome.
A chromosome consists of the following regions:
Primary constriction:
(i) The two arms of a chromosome meet at a point called primary constriction or centromere.
(ii) The centromere is the region where spindle fibres attach to the chromosomes during cell division.
Secondary constriction:
(i) Some chromosomes possess secondary constriction at any point of the chromosome.
(ii) They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).
Telomere:
(i) The end of the chromosome is called telomere. Each extremity of the chromosome has a polarity and prevents it from joining the adjacent chromosome.
(ii) It maintains and provides stability to the chromosomes.
Satellite:
(i) Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite.
(ii) The chromosomes with satellites are called as the sat-chromosomes

(i) Allosomes are chromosomes which are responsible for determining the sex of an individual.
(ii) They are also called as sex chromosomes or hetero-chromosomes.
 

(i) Pea plant is naturally self-pollinating and so is very easy to raise pure breeding individuals.
(ii) It has a short life span as it is an annual and so it was possible to follow several generations.
(iii) It is easy to cross-pollinate.
(iv) It has deeply defined contrasting characters.
(v) The flowers are bisexual.

DNA is a large molecule consisting of millions of nucleotides. Hence, it is also called a polynucleotide. Each nucleotide consists of three components.
(i) A sugar molecules - Deoxyribose sugar.
(ii) A nitrogenous base.
There are two types of nitrogenous bases in DNA.
They are
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine and Thymine)
(iii) A phosphate group
Nucleoside and Nucleotide:
Nucleoside = Nitrogen base + Sugar
Nucleotide = Nucleoside + Phosphate
The nucleotides are formed according to the purines and pyrimidines present in them.
Watson and Crick model of DNA:
(i) DNA molecule consists of two polynucleotide chains.
(ii) These chains form a double helix structure with two strands which run anti-parallel to one another.
(iii) Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.
(iv) Pairing between the nitrogenous bases is very specific and is always between purine and pyrimidine linked by hydrogen bonds.
a) Adenine (A) links Thymine (T) with two hydrogen bonds (A = T)
b) Cytosine (C) links Guanine (G) with three hydrogen bonds( C ≡ G) This is called complementary base pairing.
(v) Hydrogen bonds between the nitrogenous bases make the DNA molecule stable.
(vi) Each turn of the double helix is 34 A^o (3.4 nm). There are ten base pairs in a complete turn.
(vii) The nucleotides in a helix are joined together by phosphodiester bonds.
Significance of DNA:
(i) DNA is responsible for the transmission of hereditary information from one generation to next generation.
(ii) It contains information required for the formation of proteins.
(iii) It controls the developmental process and life activities of an organism.

(i) Dihybrid cross involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time.
(ii) The two pairs of contrasting characteristics chosen by Mendel were shape and colour of seeds: round-yellow seeds and wrinkled-green seeds.
(iii) Mendel crossed pea plants having round - yellow seeds with pea plants having wrinkled green seeds.
Mendel made the following observations:
(i) Mendel first crossed pure breeding pea plants having round-yellow seeds with pure breeding pea plants having wrinkled green seeds and found that only round yellow seeds were produced in the first generation (F₁).
(ii) No wrinkled-green seeds were obtained in the F₁ generation.
(iii) From this it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green color of the seeds.
(iv) When the hybrids of F₁ generation pea plants having round-yellow seeds were cross-breed by self pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation or F₂ generation.
(v) They were round-yellow, round-green, wrinkled yellow and wrinkled-green seeds.
(vi) The ratio of each phenotype (or appearance) of seeds in the F₂ generation is 9:3:3:1. This is known as the Dihybrid ratio.
(vii) From the above results it can be concluded that the factors for each character or trait remain independent and maintain their identity in the gametes.
(viii) The factors are independent to each other and pass to the offsprings (through gametes).

Results of a Dihybrid Cross:
(i) Mendel got the following results from his dihybrid cross.
Four Types of Plants:
(i) A dihybrid cross produced four types of F₂ offsprings in the ratio of 9 with two dominant traits, 3 with one dominant trait and one recessive trait, 3 with another dominant trait and
another recessive trait and 1 with two recessive traits.
New Combination:
(i) Two new combinations of traits with round green and wrinkled yellow had appeared in the dihybrid cross (F₂ generation).

Monohybrid Cross	Dihybrid Cross
Cross involving inheritance of only one pair of contrasting character.	Cross involves the inheritance of two pair of contrasting character.
E.g. Stem length	E.g. seed shape and seed colour

(i) In the body cells of sexually reproducing organisms, the chromosomes generally occur in pairs. This condition is called diploid (2n).
(ii) The gametes produced by the organisms contain a single set of chromosomes. Hence, the gametes are said to be haploid (n).

Law of independent assortment states: "In case of inheritance of two or more pairs of characters simultaneously, the factors or genes of one pair assort out independently of the other pair

(i) Addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy.
(ii) Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy. Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in increased fruit and flower size reased fruit and flower size.

(i) Some chromosomes possess secondary constriction at any point of the chromosome.
(ii) They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).

Substitution, Deletion, Insertion and Inversion

Genes are segments of DNA, which are responsible for the inheritance of a particular phenotypic character.