1CHAPTER 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 ininheritance of the genes. In other words, the genes (and genotypes) are stillinherited according to Mendelian rules, but the phenotypes do not conform to simpleMendelian rules.I.Variations of DominanceA.Incomplete dominance1.Phenotype of the heterozygote is intermediate between thephenotypes of the homozygotessnapdragon plant with red petals (C1C1) x plant with white petals (C2C2)1/4 redC1C11/2 pinkC1C11/4 whiteC2C2B.Co-dominance1.Phenotype of the heterozygote shows the phenotype of bothhomozygotes.For example, the gene required for production of a glycoprotein onthe surface of red blood cells (basis for the M and N blood groups).LM LM has only M antigens on the surfaceLNLN has only N antigens on the surfaceLM LNhas both antigens on the surface.C.The distinction between incomplete dominance and codominance isdependent upon the phenotypic “level” to which you are referring (i.e. sicklecell anemia example pg. 92-93 of your textbook).
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