How did Mendel derive the Law of Segregation and Independent Assortment from his experiments?

1 Answer
Apr 15, 2018

Mendel derived law of segregation and law of independent assortment on the basis of observations from his experiments on mono-hybrid and di-hybrid cross.


Mendel carried out experiments on pea plants. He observed that certain pea plants growing in nature were tall and others were dwarf, with distinct difference in height . Some pea plants were bearing purple flowers and others were bearing white flowers. He observed that as many seven different traits in pea have two distinct alternative expressions.

He also observed that on sellfing (allowing self pollination only), certain tall plants produced only tall plants and others produced both tall and dwarf plants. However, dwarf plants always produced only dwarf plants on selfing. Mendel termed tall plant producing only tall plants as pure tall and others as impure tall in a layman's language. Thus tall plants can be pure or impure and dwarf plant plants always pure.

He crossed a pure tall plant with pure dwarf plant. This is mono-hybrid cross as only one trait is observed. All the plants in #F1# generation were tall. The plants of #F1# generation were allowed to breed freely among themselves to get #F2 # generation. The plants in # **F2#generation were tall and dwarf in approximate ratio of 3:1.** He carried out monohybrid cross , considering other six traits like colour of flower, colour of cotyledons , etc. All the seven traits behaved in uniform manner.

Mendel tried to explain his observations by making certain assumptions as there was no knowledge of cell division, genes or chromosomes at that time. He calculated the results of his experiments on monohybrid cross on the basis of these assumptions.

He assumed that every trait is represented by two factors. For example height has two factors , one for tallness and other for dwarfness. One of these factors is dominant (T) and other recessive (t) (not able to express when present along with dominant factor. Thus a pure tall plant will be TT and impure tall as Tt. Plant will be dwarf if both the factor are for dwarfness ( tt).**

He also assumed that these two factors of a trait separate at the time of gamete formation.

The results calculated on the basis of these two assumptions were the same as observed in his experiments. He thus proposed two laws on the basis of his experiments on monohybrid cross;
1. Law of Dominance
2. Law of segregation.

He also performed experiments on dihybrid cross, i.e observing the behaviour of two traits simultaneously. For example he crossed a pure tall plant bearing purple flowers with pure dwarf plant bearing white flowers. All the plants in #F1# were tall bearing purple flowers. These plants were allowed to interbreed freely among themselves to get #F2# generation. The plants in #F2# generation were Tall purple, tall white , dwarf purple and dwarf white in the ration of 9:3:3:1.

On the basis of assumptions made in dihybrid cross and comparing those with actual observations, Mendel proposed Law of Independent assortment. As the answer is getting too long, I can't explain in detail these assumptions.