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supplementary2 - CHAPTER 4 EXTENSIONS OF MENDELIAN...

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1 CHAPTER 4: EXTENSIONS OF MENDELIAN INHERITANCE. Many crosses do not yield simple Mendelian ratios. Instead, the ratios are modified. These modifications reflect complexities in gene expression not complexities in inheritance of the genes. In other words, the genes (and genotypes) are still inherited according to Mendelian rules, but the phenotypes do not conform to simple Mendelian rules. I. Variations of Dominance A. Incomplete dominance 1. Phenotype of the heterozygote is intermediate between the phenotypes of the homozygotes snapdragon plant with red petals (C 1 C 1 ) x plant with white petals (C 2 C 2 ) 1/4 red C 1 C 1 1/2 pink C 1 C 1 1/4 white C 2 C 2 B. Co-dominance 1. Phenotype of the heterozygote shows the phenotype of both homozygotes. For example, the gene required for production of a glycoprotein on the surface of red blood cells (basis for the M and N blood groups). L M L M has only M antigens on the surface L N L N has only N antigens on the surface L M L N has both antigens on the surface. C. The distinction between incomplete dominance and codominance is dependent upon the phenotypic “level” to which you are referring (i.e. sickle cell anemia example pg. 92-93 of your textbook).
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