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chapter 4 notes - Chapter 4"Modifications of Mendelian...

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Chapter 4 “Modifications of Mendelian Ratios” Specific phenotypes are often controlled by one or more gene pairs whose alleles exhibit modes of expression other than dominance and recessiveness. In all such cases, however, the Mendelian principles of segregation and independent assortment are operative during the distribution of the alleles into gametes. 4.1, 4.2 Function and symbols for alleles 62-63 4.3 Incomplete dominance 63-64 4.4 Codominance 64 4.5 Multiple alleles 64-65 4.6 Lethal alleles 65-67 4.7, 4.8 Combinations of two gene pairs and gene interaction 67-72 4.9 Complementation 72-73 4.10 Genes on the X-chromosome 73-76 4.11 Sex limited and sex influenced inheritance 76-77 4.12 Phenotype not related to genotype 77-80 4.13 Extranuclear and Maternal Inheritance 80-86 P. 85-86 Mitochondrial DNA and the Mystery (optional) 4.1 Alleles Alter Phenotypes What is an allele? Recall that an allele is an alternate form of a gene Types of Alleles 1. “Wild - type” alleles (the term wild type means what occurs in nature) - most commonly referred to the allele that occurs most frequently in a population (what is normal). -Usually dominant to other alleles (Tall vs short in pea) -Responsible for the wild type phenotype and the standards against which all mutations are compared 2. Mutant alleles -modified alleles that have an altered gene product (protein) -The mutant allele can become the most common allele if it has a selective advantage in the environment (is it still the mutant allele then?) -Usually recessive Note: Some genes have several versions of a gene (ie several alleles, in which none are classified as wild-type or mutant, but are thought of as having multiple alleles).
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4.2. Symbols for Alleles One system used to represent alleles assuming complete dominance (Mendel’s work) is to use upper case for the dominant form and lower case for the recessive form. YY = Yellow seed coat color yy = green seed coat color Another system was developed in genetic studies of the fruit fly ( Drosophila melongaster ) that was developed to imply wild type alleles vs mutant alleles. e +/ e + = gray homozygote (wild type phenotype) e + /e = gray heterozygote (wild type phenotype) e/e = ebony homozygote (mutant phenotype) In this example, the e + allele is the wild type allele found in Drosophila that conditions gray body color. A recessive mutation (e) occurred that conditions ebony body color in the homozygous recessive state. Allele e + is dominant to e. To abbreviate this notation further, we can use the following notation: +/+= gray homozygote (wild type phenotype) +/e= gray heterozygote (wild type phenotype) e/e = ebony homozygote (mutant phenotype) When the mutant is dominant to the wild type. Wr + /Wr + = normal wing (wild type phenotype) Wr + /Wr = wrinkled wing (mutant phenotype) Wr/Wr = wrinkled wing (mutant phenotype) 4.3 Incomplete (Partial) Dominance Incomplete dominance occurs when: T he phenotype of the heterozygote is intermediate to either homozygous phenotype .
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