WATSON AND CRICKS MODEL OF DNA

Monohybrid cross Andmonohybrid ratio:-

Def: A cross between two pure (homozygous) parents having a single pair of contrasting characters is used or studied is called as monohybrid cross, and ratio obtained from monohybrid cross  is called as monohybrid ratio. (Phenotypic ratio = 3:1 & Genotypic ratio = 1;2;1)

This experiment involving 3 steps -

In first step (I)-

1. Mendel selected pure tall pea plant as a female parent and pure Dwarf pea plant as a male parent.
2. He confirmed whether they are true breeding or not by selfing them for three generations.

In step second (II)-

1.      Mendel performed artificial cross between female and male parent which were Tall & Dwarf.
2.      Mendel expecting progeny obtained from parent will be intermediate height or 50 % Tall &  
3.      50% dwarf but he surprised and found all were Tall in F1 generation (Tt).
4.    He started thinking, where the dwarfness has gone? Why and how it got eliminated ? He  thinked,     
5.      Dwarfness eliminated forever  or reappear in F2 generation!

In step (III)-

1. He performed F2- cross by selfing the F1 progeny.
2. He got F2 - phenotypic ratio 3:1 and F2 - genotypic ratio 1:2:1.
3. He found that dwarfness reappeared in some plant in F2 generation.
4. He collected seeds from many F1 hybrid plants when he self pollinated the F1 progeny.
5. He collected 1064 seed in F2- generation. Out of these seeds, 787 seeds grew tall and 277 remained dwarf in the ratio 3:1.
6. He thought that Tall plant produced more number than Dwarf plant due to Tall female parent would be dominant; so he performed reciprocal cross in which he selected female parent is dwarf and male parent is Tall but he got same result in F2- generation as the ratio 3:1               (3 Tall : 1 Dwarf).
7. Mendel again performed monohybrid cross by considering seven pairs of contrasting character one by one every cross in F2- generation he got same result i.e. 3:1 Contrasting pairs of characters considered by Mendel.

Diagram :-

Sr.No.	Cross	F1	F2	Ratio
1	Tall         x  dwarf	Tall	787 Tall,   277 dwarf	2.84:1
2	yellow   x  green seeds	Yellow seed	6022 yellow, 2001 green	3.01:1
3	Round    x wrinkled seeds	Round seed	5474 Round, 1850 wrinkled	2.96:1
4	Green     x yellow pods	Green pods	428 Green, 152 yellow	2.82:1
5	Inflated x constricted pods	Inflated pods	882 Inflated,  299 constricted	2.95:1
6	Axial       x terminal flower	Axial flower	651 Axial,  207 terminal	3.14:1
7	Violet     x white flower	Violet flower	705 Violet, 224 white	3.15:1
7a	Grey       x   white seed coat	Grey seed coat	705 Grey,  224 white	3.15:1

Conclusion:

1.        Mendel conclude that characters such as height of stem. colour of flower, colour of seeds &       so on inherited separately as a separate particles or units.
2.        He called them factors & recently called as genes.

·         Each gene present in alternative from now a day called as alleles.

·         Dominant gene expressed in F1 individual.

·         Inheritance of each character is controlled by pair of alleles (one from male parent and another from female parent ) eg. F1 hybrid  Tt.

·         Sexually reproducing individuals are diploid (2n). 

·         Reappearance of recessive trait in F2 progeny concluded that in F1 progeny factors do not mix with each other and they separate during gametogenesis of F1 hybrid.

·         In each gamete only one factor is received that is why gametes are haploid (n).

·         Recessive character expresses only when both the recessive factor contributed by male parent & female parent in their gamete & inherit in their progeny to form a pair (tt).

Diagram:-