On the basis of his experiments of hybridization Mendel proposed following rules of heredity –
1. Law of Paired Factors
Traits of each individual are unit characters. These characters remain independent from each other in terms of heredity. The unit trait of any organism is expressed by two factors at least. These two factors are found on one segment of the chromosome. They represent only one or alternate characters. The factors representing alternate, contrasting or similar traits are called as allele.
2. Law of Dominance
When pollination is done between plants of mutually contrasting characters, after fertilization, only one character appears in the next generation. The other character remains recessive. The character which appears is called as dominant character.
The process of combining two different characters through sexual reproduction to produce a new offspring having both the characters is called as Hybridization. Such an individual is called as a Hybrid.
When Mendel performed artificial pollination between pea plants of contrasting characters say Tall (T) and Dwarf(t) and allowed for fertilization, only one character i.e. Tall appeared in the first generation. The character for dwarfness remained hidden as recessive character in the first generation. This process can be expressed through following –
T x t
Hybrid Tall F1 Generation
3. Law of Segregation
When self-pollination is allowed between two plants of the first generation and fertilization is carried out safely, the character which remained recessive in the first generation appears in the second generation. This process can be expressed through following diagram.
During the experiment of Mendel out of 1064 pea plants in the F2 generation 387 remained long and 277 remained dwarf. This he got the ratio of 787:277=2.84:1 or 3:1. This ratio is called as Monohybrid Ratio.
4. Law of independent Assortment
When a cross is made between plants having two pairs of contrasting characters all the characters appear independently at the time of the gamete formation.
Through this law of Mendel it is clear that parental traits or genes remain independent from each other. One trait or gene has no impact on the other.
Since Mendel selected two pairs of characters for artificial pollination in his experiment this type of cross is called as Dihybrid cross. In the cross when single pair of contrasting characters are taken for pollination the cross is called as Monohybrid cross.
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