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Mendel’s Principle of segregation

 generation

From his controlled self-pollinations, Mendel germinated and grew the “pure line” seeds of plants with several different phenotypes: seed shape, seed color, pod shape, pod color, flower color and stem length. He collectively called pure line plants the P generation, the parent generation. The P generation served as the starting point for his inheritance experiments

F1 generation

Mendel mated peas representing two extreme “pure line” phenotypes from the P generation. The resulting offspring are the first filial (or F1) generation. We will focus on his experiment with different flower colors: purple and white. 

Results of the F1 generation

Mendel’s results for all of his physical traits did not support the blended inheritance hypothesis. Rather, he found that one of the extreme traits appeared in a cross of different pure lines. He called these expressed phenotypes dominant, meaning that if there is a mix of two pure lines this phenotype will be expressed. For flower color, purple dominated over white, meaning if a pure-line, purple-flowered plant is mated with a pure-line, white-flowered plant, all of the resulting offspring have purple flowers. In contrast, the phenotype that is masked is known as therecessive phenotype. White flowers are recessive to purple flowers in pea plants. 

F2 generation

Mendel pondered, “If one phenotype dominates over another, how can the recessive phenotype even exist in a population?.” This led him to conduct another controlled cross, this time between plants of the F1 generation. While the P generation was composed of pure line plants, he knew that the F1 generation was composed of half the genetic information from each plant in the P generation. What happens if the hybrids are crossed? The resulting generation is the F2 generation (hybrids of hybrids), and the results awaiting him were another surprise to Mendel.

Results of the F2 generation from Mendel's monohybrid cross. 

Results of the F2 generation from Mendel’s monohybrid cross. 

For all the different phenotypes Mendel analyzed, the recessive characteristics reemerged in the F2generation! And they did so with a predictable regularity. The ratio of dominant to recessive phenotypic ratio of all of the characteristics Mendel analyzed were all very close to 3 dominant: 1 recessive. In other words, in the F2generation ¾ of the pea plants expressed the dominant phenotype, while ¼ expressed the recessive phenotype.