CHAPTER 4 MODIFICATIONS OF MENDELIAN GENETICS 4 1 Alleles Alter Phenotypes in Different Ways Alleles are alternative forms of the same genes o Wild type allele is the one that occurs more frequently in a population designated as normal Mutation is the source of alleles o A mutant allele contains modified genetic information and often specifies an altered gene product New Phenotypes due to mutations result from changes in functional activity of the cellular product specified by that gene o Mutations may Change overall enzyme shape and thus reduce eliminate affinity for the substrate Loss of function mutation Cause complete loss of function Null allele May enhance allelic function Gain of function mutations Usually result in dominant alleles since one copy in a diploid organism is sufficient to alter the normal phenotype Not show any change in function Neutral mutation 4 2 Geneticists Use a Variety of symbols for Alleles The initial letter of the name of a recessive trait lowercased and italicized denotes the recessive allele and the same letter in uppercase refers to the dominant allele In Drosophila Melanogaster an initial letter or combination of two three letters of the name of the mutant is used o Ex Body color Ebony mutant is indicated by e Normal gray Wild type is indicated by e e e or e e wild type e e mutant If no dominance exists between alleles italic uppercase italic letters and superscripts are used to denote alternative alleles R 1 R 2 C W C R The indicates the two allele designations represent the same locus on two homologous chromosomes Many diverse systems of genetic nomenclature are used to identify genes in The symbol used should reflect the function of the gene or even a disorder various organisms caused by the gene o Yeast cdk represents the cyclin dependent kinase gene whose product is involved in cell cycle regulation o In bacteria leu refers to a mutation that interrupts the biosynthesis of the amino acid leucine and the wild type gene is designated leu o The symbol dnaA represents a bacterial gene involved in DNA synthesis o In humans capital letters are used to name genes BRCA1 the gene associated with breast cancer 4 3 Neither Allele is Dominant in Incomplete or Partial Dominance A cross between parents with contrasting traits may generate offspring with an intermediate phenotype in the heterozygote Incomplete or Partial dominance o Red snap dragons X white snapdragons F1 is all pink o Pink X Pink 1 Red 2 Pink 1 White o The phenotypic ratio is identical to the genotypic ratio o Neither trait is dominant in incomplete dominance no uppercase or lower case use subscript C R C R red C W White have a intermediate phenotype Tay Sachs disease in humans o Offspring from a cross between parents with contrasting traits may o Homozygous recessive die from fatal lipid storage disorder when hexosaminidase activity is absent in individual o Heterozygotes have only a single copy of the mutant gene and are phenotypically normal but have only 50 percent of the enzyme activity found in homozygous normal individuals 4 4 In Codominance the Influence of Both Alleles in a Heterozygote is Clearly Evident Codominance joint expression of two alleles of a gene in a heterozygote results in phenotypic detection of both gene products o Inheritance in which two alleles of a gene pair in a heterozygote both have full phenotypic expression Codominant inheritance is characterized by distinct expression of the gene products of both alleles The MN blood group in humans illustrates Codominance Phenotype o Genotype o LMLM M o LMLN MN o LNLN N types 1 2 1 A mating with two heterozygotes may produce children with all three blood 4 5 Multiple Alleles of a Gene May Exist in a Population Mutations modify the information stored in any gene by producing different alleles o For any specific gene there can be more than two alleles within o Multiple alleles refer to three or more alleles of the gene create members of a population unique mode of inheritance Can only be studied in populations not individuals Human ABO blood groups provide an example of multiple alleles in a population o The ABO system exhibits the codominant mode of inheritance o 4 phenotypes are present A B AB O o I A and I B are responsible form making their respective A and B antigens and I is an allele that does not produce either antigens I A alleles are in the nucleus I A gene products are in the golgi A antigens are on the cell surface The Bombay Phenotype o A woman was typed as type O but One parent was AB blood and she passed the I B allele to her children o A and B antigens are derived from a precursor molecule H Substance o In the Bombay Phenotype The H substance incompletely formed and thus No antigens were formed Type O was expressed Bombay Phenotype is Even with I A I B alleles no A or B antigens were added to cell surface FUT1 is the gene mutation that causes this Wild type allele for Drosophila is w r Eye color of wild type fruit fly is red 4 6 Lethal Alleles Represent Essential Genes Ex Mouse A Y allele Ex Human H allele causing Huntington s disease Essential genes are those that are required for survival One wild type allele may be sufficient to produce enough of the essential product for survival o Such a mutation behaves as a recessive lethal allele Mutations resulting in the synthesis of a nonfunctional protein can often be tolerated in the heterozygote o Recessive lethal alleles result in homozygous recessive individuals that don t survive Approx 1 3 of all genes are essential for survival 8700 In some cases the allele responsible for a lethal effect when homozygous may also result in a distinctive mutant phenotype when present heterozygously behaves as a recessive lethal allele but is dominant with respect to the phenotype A mutation in mice causes a yellow coat color that varies in the normal agouti Wild type coat phenotype o Homozygous yellow coats are lethal in mice Die before birth o A Y yellow fur dominant to Agouti A o A Wild type allele recessive to A Y o Heterozygous mice have a yellow coat The allele responsible for lethal effect when it is homozygous can also result in a distinctive mutant phenotype when it is heterozygous The mouse A Y an allele exhibits a o Recessive mode of inheritance with respect to the lethality phenotype o Dominant mode of inheritance with respect to the coat color phenotype Some mutations may behave as a dominant lethal allele where one copy results in death Huntington s
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