Explain Mendel's pea plant experiment showing the dihybrid and monohybrid cross( also write the genotypic and phenotypic ratio).

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​Gregor Johann Mendel (1822 – 1884) was the first to carryout the study on the transmission of characteristics from the parents to the offsprings. He proposed that heredity is controlled by factors, which are now believed to be segments of chromosomes or genes.

Mendel’s Experiments

Mendel performed experiments on a garden pea (Pisum sativum) with different visible contrasting characters. He worked on the following seven traits of garden pea:

S. No.	Character	Dominant	Recessive
1	Stem height	Tall	Dwarf
2	Flower colour	Violet	White
3	Flower position	Axial	Terminal
4	Pod shape	Inflated	Constricted
5	Pod colour	Green	Yellow
6	Seed shape	Round	Wrinkled
7	Seed colour	Yellow	Green

• True breeding pea lines were obtained by continuous self pollination for several generations.
• Fourteen true breeding pea lines were selected as pairs, which were similar except for one character with contrasting traits.
• Artificial cross pollination (hybridisation) was performed on such varieties to obtain first hybrid generation known as the first filial progeny or F₁.
• After hybridisation, the F₁ generation so obtained resembled only one of its parents (say, all tall; no dwarf).
• When 2 plants from F₁ generation were self pollinated, the second filial progeny or F₂ generation was obtained.
• Revival of unexpressed trait (dwarf) was observed in some F₂ progeny. Both traits, tall and dwarf, were expressed in F2 in ratio 3:1.
• Mendel proposed that something is being passed unchanged from generation to generation. He called these things as ‘factors’ (presently called genes).
• Factors contain and carry hereditary information.
• Alleles − Slightly different form of same factor

Two alleles code for a pair of two contrasting traits. (e.g., tall and dwarf)

Monohybrid cross is the cross that involves only a single character.
Let us take the example of cross between the two homozygous plants considering a single character height with genotype TT and tt. The cross can be illustrated as:

 
Phenotype ratio in F₂ generation = 3 : 1 i.e 3 tall and 1 dwarf
Genotype ratio in F₂ generation = 1 : 2 : 1 i.e 1 TT, 2 Tt and 1 tt.
 

Inheritance of Two Genes (Dihybrid Cross)

• In dihybrid cross, we consider two characters. (e.g., seed colour and seed shape)

• Yellow colour and round shape is dominant over green colour and wrinkled shape.

Phenotypic ratio 9:3:3:1

Round yellow 9

Round green 3

Wrinkled yellow 3

Wrinkled green 1

Mendel performed his experiments on Garden Pea and proposed three laws

1. Law of dominance- According to this law, characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other in a dissimilar pair.

2. Law of segregation- This law states that the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors. In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions.

3. Law of independent assortment- When two pairs of traits are combined in a hybrid, one pair of character segregates independent of the other pair of character. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes (RY, Ry, rY, ry) are produced. Each of these segregate independent of each other, each having a frequency of 25% of the total gametes produced.

Hope this information will clear your doubts about the Mendel's experiment.
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