Describe steps or procedure of Mendel's experiment.

The first scientific explanation regarding inheritance was given by Gregor Johann Mendel in 1866.

He performed series of experiments on garden pea in a systematic and scientific manner, the steps or procedure of the experiments were as follows:

Procedure of the Experiments :

i) Mendel was very methodical.

ii) He first considered (studied) inheritance of just one trait at a time unlike others who had considered the organism as a whole.

iii) Then he studied two traits together and then three, i.e. he performed monohybrid, dihybrid and trihybrid crosses.

iv) He started with pure lines i.e. true breeding plants.

v) He meticulously maintained complete record of actual number of each type of offsprings.

vi) The quantitative as well as qualitative record was kept for three generations.

vii) He conducted ample crosses and reciprocal crosses to eliminate chance factor.

He conducted experiments in three steps as follows :

i) Step 1 - Selection of parents and obtaining pure lines :

Mendel not only started with pure lines.

The selected male and female parent plants are breeding true for the selected trait/traits by selfing them for three generations.

Breeding true or 'true breeding' means they produce offsprings with the same selected trait/traits only.

ii) Step 2 - Artificial cross of the selected parents to raise F1 generation :

Mendel first emasculated the flowers of the plant which he had selected as a female parent.

Then pollens from the flower of selected male parent were dusted on the stigma of the emasculated flower i.e. artificial cross.

Mendel crossed many flowers, collected seeds and raised the hybrids that represent first filial (filial means offsprings) generation or F1 generation.

iii) Step 3-Selfing of F1 hybrids to raise F2 generation.

Mendel allowed the natural self pollination in each F1 hybrid; collected seeds separately and raised F2 generation i.e. second filial generation.

F3 generation was obtained by selfing of F2 hybrids.

Write a note on emasculation.

i) Emasculation is removal of stamens well before anthesis i.e. formation of pollen grains.

ii) Therefore it is done in bud condition.

iii) Pea flowers are with papilionaceous -butterfly like- corolla with five petals.

iv) Stamens and carpels are enclosed in the innermost two petals that are appressed together forming somewhat boat shaped structure called carina.

v) Stamens and carpels mature simultaneously and naturally there is always self pollination.

vi) There are ten stamens but the filaments of nine are fused together and one is free.

vii) Therefore emasculation is easy, just open the carina and remove 9 + 1 by forceps.

Explain monohybrid cross experiment and monohybrid ratio.

Mendel conducted experiments in three steps as follows :

i) Step 1 - Selection of parents and obtaining pure lines :

ii) Step 2 - Artificial cross of the selected parents to raise F1 generation :

iii) Step 3-Selfing of F1 hybrids to raise F2 generation.

Monohybrid cross :

The cross between two pure parents differing in a single pair of contrasting character is called monohybrid cross.

The ratio for the cross is 3 : 1. e.g. Monohybrid cross between pure tall pea plant and pure dwarf pea plant.

i) Mendel started with pure lines and confirmed that the parents are true breeding.

Therefore parental tall plant is with factors (TT) and dwarf with (tt).

During gamete formation, factors separate and only one enters in each gamete.

Thus pure parents will produce only one type of gametes. Tall parent plant will produce all gametes with (T) and dwarf parent plant will produce all gametes with (t).

Therefore, all F1 generation offsprings (hybrids) are with (Tt). Due to dominance they all grow tall.

Thus all plants are tall in F1 generation.

ii) Mendel allowed selfing of F1 hybrids.

As factors separate during gamete formation F1 hybrids will produce two types of gametes in equal proportion i.e. 50% of all male gametes will be with capital (T) and 50% with small (t). Similarly 50% of female gametes will be with (T) and 50% with (t). (Pea flower are bisexual).

There will be random fusion and each type of male gamete has equal chance to fuse female gamete with (T) or (t) and vice versa.

iii) The fusion of gametes will produce (2 ×2 = 4) four combinations, which fall into three categories i.e. there are three different genotypes.

The 4 combination are (TT) pure tall, two are (Tt) hybrid tall and one is (tt) dwarf

They are shown in the above graphic representation by checker board or Punnett square.

Thus only one out of four i.e. 25% of the F2 generation offsprings will receive (t) from both the parents and therefore will be dwarf.

Thus, the recessive trait reappears in F2 generation and the ratio of dominants and recessives is 3 : 1.

However, all tall plants of F2 generation are not same, only 1/3rd are pure tall and 2/3nd are hybrid tall.

The dwarfs are always pure as recessive trait cannot be expressed in the presence of dominant allele.

The ratio of 'pure tall : hybrid tall : dwarf' is 1 : 2 : 1 this is called genotypic ratio but the phenotypic ratio, 3 : 1 is called monohybrid ratio.

iv) The monohybrid ratio is defined as the phenotypic ratio of different types of offsprings (dominants and recessives) obtained in F2 generation of a monohybrid cross. It is 3 : 1 in all Mendelian crosses.

State and explain Mendel's first law or law of dominance.

Law of dominance states that "in a cross between two homozygous organisms differering in a single pair of contrasting character,

- the character which is expressed in the F1 generation is called dominant character.

- the character which is not expressed or suppressed is recessive character.

e.g. Tallness in pea plant is a dominant character while dwarfness is a recessive character.

Phenotype of parents → Pure tall × Pure dwarf

Genotype → TT tt

Gametes → T t

Tt

F1 generation → Hybrid tall

i) In a cross between pure tall and pure dwarf pea plant, only tall character is expressed in all the individuals of F1 generation.

ii) Hence it can be inferred that in pea plants, tallness is the dominant character while dwarfness is recessive character.

iii) Tallness in F1 hybrid is determined by genotype Tt in which the dominant allele 'T' suppresses the recessive allele 't' thereby suppressing its expression in the phenotype.

Give examples of 1st law or Law of dominance.

i) Guinea pigs, black coat colour is dominant over white.

ii) In human beings, curly hair is dominant trait over straight hair and brown eye colour over green or blue.

iii) In jowar, pearly grain is dominant over chalky grain and so on.

iv) Law of dominance can be explained with the help of a monohybrid cross experiment.

Give significance of 1^st Law or Law of dominance..

The harmful recessive traits are masked, i.e. The Recessive allele is not expressed in presence of dominant allele.

e.g. Recessive characters in humans are : idiocy, diabetes and hemophilia.

State and explain 2^nd law or Law of segregation.

Mendel's second law of inheritance (law of segregation) :

i) Mendel's law of segregation is second law of inheritance.

It states that when a pair of con-trasting characters occur together in a hybrid, they remain together without mixing with each other and segregate during the formation of gametes.

ii) Thus, for example, when Mendel crossed a homozygous tall plant with a homozygous dwarf plant, the offspring resulted from this cross was found to be a hybrid tall.

iii) The hybrid tall thus produced had two genes, viz., T (tallness) and t (dwarfness). When this hybrid tall formed the gametes, the two genes, viz., T and t were segregated as shown under :

iv) Thus, each gamete receives only one allele for each character and not both. Owing to the segregation, the gametes that are formed are pure for the characters which they carry. Hence, this law is also called the law of purity of gametes.

Explain why law of segregation is also called law of purity of gametes ?

The Law of segregation states that "when unidentical alleles are brought together in a hybrid they remain together without mixing and separate from each other during formation of gametes".

Thus, gametes receive only one of the two factors so organisms can be hybrid or pure but gametes are always pure. Therefore it is also called Law of purity of gametes.

Eg. Tt (F1 hybrid) →(T) + (t)

What is dihybrid cross ? Explain with suitable example and checker board method.

A cross between two pure (homozygous) parents in which the inheritance pattern of two pairs of contrasting characters is considered simultaneously is called Dihybrid cross.

The phenotypic ratio of different types of offsprings (with different combinations) obtained in F2 generation of dihybrid cross is called dihybrid ratio.

Dihybrid Ratio is 9 : 3 : 3 : 1.

For example, when we cross a yellow and round seed pea plant with a green and wrinkled seed pea plant, we get 9 yellow round, 3 yellow wrinkled, 3 green round and 1 green wrinkled plants in the F2 generation.

Distinguish between Monohybrid cross and Dihybrid cross

Why law of independent assortment is not universally applicable ?

i) When a dihybrid forms gametes, assortment (distribution) of alleles of different traits is independent of their original combination in the parents.

ii) Recombination during meiosis is responsible for assortment of allele.

iii) Many genes are located on one chromosome i.e. they are linked. Therefore, they pass through gametes in the form of a linkage group.

iv) Therefore, the law of independent assortment is applicable only for the traits which are located on different chromosomes. Thus law of independent assortment is not universally applicable.

Why law of independent assortment is not universally applicable ?

i) When a dihybrid forms gametes, assortment (distribution) of alleles of different traits is independent of their original combination in the parents. It is caused by recombination.

ii) Recombinations are due to the crossing over that takes place during meiosis

iii) Many genes are located on one chromosome i.e. they are linked. Therefore, they pass through gametes in the form of a linkage group.

iv) Therefore, the law of independent assortment is applicable only for the traits which are located on different chromosomes.

Thus law of independent assortment is not universally applicable.