Supplementary2 - CHAPTER 4 EXTENSIONS OF MENDELIAN INHERITANCE Many crosses do not yield simple Mendelian ratios Instead the ratios are modified

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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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2 II. Multiple Alleles A. Although an individual organism (say you) can have only two alleles of a gene (because there are only two chromosomes), among a population (say the entire class), there may be multiple alleles which are said to be in an allelic series B. For example, the genes that are responsible for the synthesis of sugars bound to a lipid on the surface of the red blood cells (basis for another type of human blood groups) 3 alleles (i or I A or I B ) but any individual can only have two of them. Blood Group Genotype Phenotype of sugar on surface O ii no sugar on surface A I A I A or I A i N-acetyl galactosamine B I B I B or I B i galactose AB I A I B N-acetyl galactosamine and galactose I A and I B are dominant to i but codominant to each other. C. Other allelic series C - = full color - dark gray c ch = chinchilla - pale gray c h = himalayan - albino with dark extremities c = albino Dominance = C >c ch >c h >c D. Test for Allelism How does one know that a set of contrasting phenotypes is determined by alleles of only one gene (vs. 2 sets of genes that influence each other which is described in Section IV) Test for allelism and simple dominance: Observation of F2 ratios (3:1) from crosses of all pairwise combinations of pure breeding strains.
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3 III. Lethal
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This note was uploaded on 07/10/2011 for the course BIO 390 taught by Professor Dr during the Spring '11 term at McMaster University.

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Supplementary2 - CHAPTER 4 EXTENSIONS OF MENDELIAN INHERITANCE Many crosses do not yield simple Mendelian ratios Instead the ratios are modified

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