HERIDITY

Transformation of characters from parents into offspring is called heredity.

Variation
The dissimilarity between the individuals of the same species is called variation. The main causes of
variation are:

o Different organisms have different sets of chromosomes.
o Formation of gametes by meiosis cell division.
o Transformation of characters from father and mother both.
o Environmental influence or environmental cause.

Genetics
The branch of biology concerned with the study of the mechanism of heredity and variation is called
Genetics.Gregor Johann Mendel is called Father of Genetics.

Monohybrid cross
In a cross, the character which can express itself is called dominant character and that which is hidden by dominant character is called recessive character. For example, if a cross is made between pure tall and pure short pea plants (monohybrid cross), in F 1 generation all the pea plants are found tall. Here tall is dominant character and short is recessive character.

Genotype and Phenotype
External appearance of an organism is called phenotype and genetic make up of an organism is called
genotype of that organism. For example, a tall pea plant having trait. Tt has phenotype tall but genotype tall and short both.

Hybrid
Hybrid is that individual which has different phenotype and genotype. For example by pea plant having traits Tt is not pure tall but hybrid tall because its phenotype is tall but genotype is both tall and short.

Allele
A pair of genes controlling a pair of contrasting characters is known as an allelic pair and the members of an allelic pair are called allele of each other. For example, in Tt, T is allele of t and vice-versa.

Mendel selected a pea plant for his experiment because the following characters are found in a pea plant
(Pisum sativum)

o It is a naturally self pollinated and cross pollination also can be made artificially in it.
o It has a number of distinguishable contrasting characters.
o The hybrid plants obtained as a result of cross pollination are fertile.
o A large number of progeny can be obtained from a single plant.
o The life cycle is of short duration.
o The plants are convenient to handle.

Mendel’s Laws
Law of Dominance: This law states that “When a cross is made between a pure contrasting characters, only one of them is able to express itself phenotypically where others remains hidden in F 1 generation. The one, which is expressed, is dominant and the other is recessive.” For example, when a cross is made in between pure tall (TT) and pure short (tt) pea plants, in F 1 generation, all the pea plants are found tall (Tt).

Law of segregation / Law of purity of gametes:
This law stated that – “The two members of a pair of factors separate during the formation of gametes. They do not blend with each other but segregate out into different gametes.” For example, in Mendel’s experiment when selfing occurred in two monohybrid pea plants (Tt) if F 1 generation, in F 2 generation, the tall and short pea plants were found in the ration of 3 :1 (Phenotypic ration), while the genotypic ratio was 1:2:1.

Law of independent assortment:
This law states that “When two pairs of allele are brought together in the hybrid (F 1 ), they, at the time of gamete formation, segregate or assort independently at random and freely.”