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Chapter: 11 th 12th std standard Bio Botany plant tree Biology Higher secondary school College Notes

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Inheritance Laws of Mendel

Based on his experiments of monohybrid and dihybrid cross, Mendel proposed three important laws which are now called Mendel's Laws of Heredity. 1.Law of dominance and recessiveness 2.Law of segregation or Law of purity of gametes, 3.Law of independent assortment.

Laws of Mendel

 

Based on his experiments of monohybrid and dihybrid cross, Mendel proposed three important laws which are now called Mendel's Laws of Heredity.

 

      Law of dominance and recessiveness

       

        Law of segregation or Law of purity of gametes

 

          Law of independent assortment

 

i. Law of Dominance

 

The law of dominance and recessiveness states : "When two homozygous individuals with one or more sets of contrasting characters are crossed, the

characters that appear in the F1 hybrid are dominant and those that do not appear in F1 are recessive characters".


ii. Law of Segregation or Law of Purity of Gametes

 

The Law of segregation states that "When a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote or hybrid, the two members of the allelic pair remain together without mixing and when gametes are formed the two separate out, so that only one enters each gamete".

 

This law though it was a conception originally and propagated by Mendel, now it has been confirmed by cytological studies. Dominance or no dominance segregation holds good for all cases.

 

iii. Law of independent Assortment

 

Law of independent assortment states : "In case of inheritance of two or more pairs of characters simultaneously, the factors or genes of one pair assort out independently of the other pairs".

 

Mendel gave this law based on his dihybrid cross experiment. Here the total number of individuals in F2 will be sixteen which occur in a ratio of 9:3:3:1 where two parental classes and two new combinations will be produced.

 

Back cross and Test Cross

 

In Mendelian inheritance F2 offsprings are obtained by selfing the hybrid, but if the F1 hybrid is crossed to any of the pure breeding parents it is called a back cross. If the hybrid is crossed to the dominant parent, all the F2 offsprings will show dominant character.

 

If the hybrid is crossed to recessive parent, dominant and recessive phenotypes with appear in equal proportions as shown, which is called a test cross.

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