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

Question

Victoria Angeli · Accepted Answer

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 experimented on pea plants, noting that as many as seven distinct traits in pea have two distinct alternative expressions; that some pea plants growing in nature were tall and others were dwarf, with a distinct difference in height; that some pea plants were bearing purple flowers and others were bearing white flowers. Mendel also noted that some tall plants produced only tall plants on sellfing (allowing only self pollination), while other tall plants produced both tall and dwarf plants; dwarf plants, on the other hand, always produced only dwarf plants on selfing. To put it simply, Mendel described tall plants producing only tall plants as pure tall and others as impure tall, meaning that tall plants can be either pure or impure and dwarf plants are 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. Since at that time there was no knowledge of genes, chromosomes, or cell division, Mendel attempted to explain his observations by speculating and used these assumptions to calculate the results of his monohybrid cross experiments. He made the assumption that each trait is represented by two factors. For instance, there are two factors for height: one for tallness and the other for dwarfness. One of these factors is dominant (T), and the other is recessive (t), meaning that it cannot express itself when present in conjunction with the dominant factor. As a result, a plant will be dwarf if both of the factors are for dwarfness (tt).** Additionally, he made the assumption that these two characteristics of a trait split during gamete development. He thus proposed two laws on the basis of his experiments on monohybrid crosses: 1. Law of Dominance 2. Law of Segregation. The results computed on the basis of these two assumptions were the same as observed in his experiments. 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. Mendel proposed the Law of Independent assortment based on assumptions made in the dihybrid cross and comparing those with actual observations. I won't go into detail about these assumptions because the answer is getting too long.

Cooper Alexander · Answer

undefined Mendel derived the Law of Segregation from observing that alleles separate during gamete formation, ensuring each gamete carries only one allele for a trait. The Law of Independent Assortment came from his observation that different traits segregate independently during gamete formation.