33 2.1 Autosomal inheritance phenotypic ratio in the F 2 generation was a more fun-damental 1:2:1 ratio: Further studies showed that such 1:2:1 ratios un-derlie all the 3:1 F 2 phenotypic ratios that Mendel had observed. Thus, his challenge was to explain not the 3:1 ratio, but the 1:2:1 ratio. Mendel’s explanation was a classic example of a creative model or hypothesis derived from observation and suitable for testing by fur-ther experimentation. His model contained the follow-ing concepts: 1. The existence of genes. Contrasting phenotypic differences and their inheritance patterns were attributed to certain hereditary particles. We now call these particles genes. 2. Genes are in pairs. The gene may have different forms, each corresponding to an alternative phenotype of a character. The different forms of one type of gene are called alleles. In adult pea plants, each type of gene is present twice in each cell, constituting a gene pair. In different plants, the gene pair can be of the same alleles or of different alleles of that gene. Mendel’s reasoning here was probably
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