Mendel's Experiment and his laws 2

Mendel performed three experiments-
1. Monohybrid cross
2. Dihybrid cross
3. Trihybrid cross
We will study two of them first in this article.

1. Monohybrid cross 

It is a cross between two pure individuals(Parents) which are homozygous in which inheritance pattern of only one pair of contrasting characters is studied.

Procedure of experiment-

Step 1-Selection of parents and obtaining pure lines

He selected pure tall pea plant as female plant(TT) and pure dwarf pea plant as a male parent(tt). He confirmed that the plants are true breeding by selfing them for three generations.

Step 2-Artificial cross of the selected plants to raise F1 generation

He then performed artificial cross, collected seeds, sowed them and waited for the results. He had expected that either the offspring's would be of intermediate height or 50% of them will be tall and 50% of them will be dwarf. But amazingly all were tall and dwarfness disappeared.

Step 3-Selfing of F1 hybrids to raise F2 generation

He then allowed self crossing of all the hybrids and collected seeds, sowed them and waited for the results. He observed that the dwarfness reappeared in some plants. But what was found is three were tall [One homozygous tall(TT) and two heterozygous tall(Tt)]and one was dwarf[Homozygous dwarf(tt)].The tallness was dominant over dwarfness.  

Step 4-Conclude by finding the genotypic and phenotypic ratios

So he obtained 

phenotypic ratio as 3:1 i.e. Tall : Dwarf = 3 :1 and

genotypic ratio as 1:2:1 i.e. TT : Tt : tt = 1:2:1 

This phenotypic ratio is called as the Mendel's Monohybrid Ratio. Diagram shows the same cross diagrammatically.

He did reciprocal cross (exchange of male and female plants) too and obtained the same ratio. Also He did experiments by considering all other six characters and formulated similar results.

So from his monohybrid cross he concluded and formulated two laws while the third is based on dihybrid cross.

Mendel's First Law of Inheritance-

• Also known as Law of Dominance
• Statement:In a cross between two organisms pure for any pair of contrasting characters, the character that appears in First filial generation is called dominant and the one which is suppressed is called recessive.
• So there is always uniform expression in F1 generation.
• It is not universally applicable because of co-dominance in some.We will study it in next article.

Mendel's Second Law of Inheritance-

• Also called law of segregation
• Statement:Members of allelic pair in a hybrid remain together without mixing with each other and separate or segregate during gamete formation.
• Hence also called Law of Purity of Gametes.
• It is Universally applicable.

2. Dihybrid cross

It is a cross between two pure individuals(Parents) which are homozygous in which inheritance pattern of two pair of contrasting characters is studied simultaneously.

Procedure of experiment-

Step 1-Selection of parents and obtaining pure lines

He selected pea plant with yellow and round seeds as female plant(YYRR) and pea plant with green and wrinkled seeds as a male parent(yyrr). He confirmed that the plants are producing respective types of seeds for three generations. 

Step 2-Artificial cross of the selected plants to raise F1 generation

He then performed artificial cross, collected seeds, sowed them and waited for the results.All seeds collected were round and yellow as expected as yellow seed colour is dominant over green seed colour as well as round seed coat is dominant over wrinkled seed coat. 

Step 3-Selfing of F1 hybrids to raise F2 generation

He then allowed self crossing of all the hybrids and collected seeds, sowed them and waited for the results.He was expecting 75% seeds to be yellow round and 25% to be green wrinkled. But he found four types of seeds in the pods as

1.Yellow and round

2.Yellow and wrinkled

3.Green and round

4.Green and wrinkled

Out of which first and last are parental combinations and rest two are newer ones.

Step 4-Conclude by finding the genotypic and phenotypic ratios

So he obtained 

phenotypic ratio as 9 : 3 : 3 : 1 

i.e. Yellow round : Yellow Wrinkled : Green Round : Green wrinkled = 9 : 3 : 3 : 1 and

genotypic ratio as 1: 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 

For details lets see the diagram

This phenotypic ratio is called as the Mendel's Dihybrid Ratio.

It is actually a product of two Monohybrid ratios i.e.(3:1)(3:1)=9:3:3:1

Mendel had applied the law of probability here i.e. "Probability of two events independent occurring simultaneously is the product of their individual probabilities "

He did reciprocal cross (exchange of male and female plants) too and obtained the same ratio. Also He did experiments by making other various combinations and formulated similar results.

Mendel's Third Law of Inheritance-

• Also Known as Law of Independent Assortment
• Statement:When the two homozygous parents differing in two pairs of contrasting traits are crossed , the inheritance of one pair is independent of the other.
• It is also not universally applicable.

Thank you , I hope you all understood what are experiments the which were performed by Mendel.

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