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COURSE Principles of inheritance and variations Lesson 11 A Critical evaluation of Mendel's Laws Incomplete Dominance Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET (JRF). SRF, GATE 21Years of Pre-medical teaching experience
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1. INCOMPLETE DOMINANCE According to Mendel's first principle, in F1 generation or in heterozygous state, only one allele is able to express itself phenotypically, while other remains suppressed. Further, the two alleles do not show any interaction. However, in Mirabilis jalapa (4 'O' clock plant), or in Antirrhinium majus (Snapdragon), when a cross is made between pure white and red flowered plants, all F1 individuals have pink flower. (According to Mendel there should have been either white or red flowers) (phenotypic ratio). (However, according to Mendel, the F2 phenotypic ratio should be 3: 1). dominance of one allele over the other allele is not complete or perfect. On selfing, these pink flowered F1 individuals produced red, pink and white flowered plants in 1: 2: 1 The appearance of pink flower in heterozygous condition suggested that in some cases, the Therefore, both alleles interact with each other and produce a mixed or blended character. the blending of the character is called incomplete dominance (or blending inheritance) dominance. This type of inheritance where the two alleles of a character interact in such a way, so as to cause The incomplete dominance clearly and directly opposes the universality of the Mendel's principle of
Red White R r R r Pink Pink Pink Pink F1 generation
Pink Pink Red Pink Pink White F 2 generation
Another Example of Incomplete Dominance Starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b). Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are produced. On the other hand, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains. After maturation of the seeds, B seeds are round and the b seeds are wrinkled. Heterozygotes (Bb) produce round seeds, and so B seems to be the dominant allele. But, the starch grains produced are of intermediate size in Bb seeds. So, if starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance. This case is also an example of pleiotropy because same gene B is determining more than one trait, i.e., seed shape and seed size.
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