WATSON AND CRICKS MODEL OF DNA

Monohybrid cross And monohybrid 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 & 50% dwarf but he surprised and found all were Tall in generation (Tt).

3.       He started thinking, where the dwarfness has gone? Why and how it got eliminated ? He thinked, Dwarfness eliminated forever  or reappear in  generation!

In step (III)-

1.       He performed - cross by selfing the progeny.

2.       He got - phenotypic ratio 3:1 and - genotypic ratio 1:2:1.

3.       He found that dwarfness reappeared in some plant in  generation.

4.       He collected seeds from many hybrid plants when he self pollinated the progeny.

5.       He collected 1064 seed in - 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 - 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 - 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 individual.

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

·         Sexually reproducing individuals are diploid (2n). 

·         Reappearance of recessive trait in  progeny concluded that in progeny factors do not mix with each other and they separate during gametogenesis of 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:-