Mendel conducted his researches for over a period of eight years in Pisum sativum (pea plant).
Mendel used seven pairs of contrasting characters.
Mendel followed the inheritance of single, easily visible and distinguishable traits.
Mendels laws are in three steps. They are as follows,
1. Mendels Laws of dominance
2. Mendels Laws of segregation
3. Mendels Laws of independent assortment
1. Law of dominance
According to Mendel, A gene which occurs in pairs can control various traits.
Law of dominance states that “Out of two traits of the parents, only one trait appeared in F1 generation”.
The trait which expresses in F1 generation is the dominant trait while hidden trait is a recessive trait.
In the above cross, tallness is dominant trait and dwarfness is a recessive trait.
Similarly, in dihybrid cross tallness and red are dominant trait so in F1 generation all progeny are tall and red.
2. Law of segregation
In F1 hybrid, though there are both dominant and recessive alleles, plant shows dominant characters.
Law of segregation states that “two alleles of a trait remain together in heterozygote without contamination but they separate or segregate at the time of gamete formation”.
Due to segregation of alleles gametes bear only one of the contrasting alleles.
It means gametes are pure for a particular trait.
The paired condition of alleles is restored by a random fusion of gametes during fertilization.
In a monohybrid cross, F1 generation comprises both alleles T and t in hybrid.
The alleles do not blend in F1 generation but retain their individuality.
A recessive factor (t) is not altered by the presence of a dominant factor (T).
During gamete formation T and t separate, so each gamete bear either T or t.
During random fusion of the gametes, dwarfness reappears in the F2 generation.
The dwarfness which did not appear in F1 generation reappears in F2 generation is because of the segregation of alleles during gamete formation and random fertilization.
3. Law of Independent Assortment
Mendel formulated the law of segregation on the basis of the inheritance of single pairs of contrasting characters.
But to find how different characters would behave in relation to others.
Mendel performed a dihybrid cross and formulated the law of independent assortment.
It states that when two pairs of contrasting alleles present in parents, inherit to offspring, one pair of alleles is independent on another pair, they do not remain linked and assort randomly.
We can explain this law from a dihybrid cross.
In the above dihybrid cross, the height characters segregate and are inherited independently of the flower colour.
When two alleles of pair ( T, t and R, r ) segregate, both pairs assort randomly.
It means in a gamete T may assort with R or r, similarly, t may assort with R or r.
So 4 types of gametes ( TR, Tr, tR and tr ) are formed in F1 generation that on selfing results all combination of two characteristics in the F2 generation.
The ratio of 9 tall red : 3 tall white : 3 dwarf red: 1 dwarf white.
Reason for Mendel’s success
Concerning Mendels laws, The main reasons for Mendel’s success are as follows,
1. Mendel focused on the inheritance of a single character.
2. He used pure breeding plants which have two contrasting characters.
There was no intermediate.
3. Mendel kept accurate records in crosses and even maintained an exact number of seeds or plants produced.
4. He used a self-pollinated plant in which undesirable cross-pollination was totally safe.
5. Mendel made reciprocal and test cross to conform his result.
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