Biology 240 Chapter 4 notes
Biology 240 Chapter 4 notes Bio 240
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This 6 page Class Notes was uploaded by Izabella Nill Gomez on Saturday September 5, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 160 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Biology 240GeneticsChapter 4 Notes Gene Interaction situation in which a single phenotype is affected by more than one set of genes NeoMendelian genetics investigates observations of genetic data that did not conform precisely to expected Mendelian alleles Allele alternative form of a gene Allele that occurs most frequently or is normal is referred as the wild tvoe allelerepresented by a ex abc It is often but not always dominant The process of mutation is the source of new alleles For a new allele to be recognized by observation of an organism the allele must cause a change in phenotype A new phenotype is formed from a change in functional activity of cellular products speci ed by that gene Loss of function mutation causes a gene to create an enzyme that losesreduces the af nity for substrate A null allele results if loss is complete Gain of function mutation enhances the function of the wildtype product and increases the quantity of gene product Results in dominant alleles ExProto oncogens changed to oncogens which override regulation of gene product Function is always turned on Neutral mutations cause no phenotypic change or evolutionary tness change Traits are in uenced by many gene products Mutations can have a common effect in metabolic pathways with a failure to produce an end product IncompletePartial Dominance no one gene dominates the phenotype Creates an intermediate phenotype There can be alleles that are neither dominant nor recessive Haploinsuf ciency when a single allele is not suf cient to produce the wild type phenotype and mask the other allele The dominant allele does not produce enough gene productdose Ex red and white snapdragons RRred anthocyaminreceives two doses12 rrwhite receives no doses of pigment14 Rr pink receives one dose of anthocyaminnot 121 ratio Examination of gene product and activity often reveals intermediate gene expression not in the phenotype TaySachs disease is related to lipidstorage de ciency Those that are homozygous are affected but heterozygotes are only partially affected Threshold effect normal phenotypic expression occurs any time a certain level of gene product is attained ln Tay Sachs it is less than 50 of the time Codominance occurs when joint expression of both alleles occurs 2 alleles of a single gene are responsible Ex MN blood group Both alleles expressed Codominant inheritance is characterized by distinct expression of gene products of both alleles Different from incomplete dominance where products of both alleles Different from incomplete dominance where heterozygotes express intermediate blended phenotype When mutation modi es information on a gene each change produces a different allele Multiple Alleles when present in a population inheritance may be unique Multiple alleles can only be studied in populations one organism may have at most 2 homologous gen loci coupled by different alleles of the same gene Among members of a species numerous alternate forms of the same gene can exist Ex ABO Blood Groups characterized by the presence of antigens on the surface of red blood cells different from MN antigens Once combination exhibits codominant A B forms of inheritance I I for AB blood type are codominant to each other but dominant to i for 0 blood type All individuals possess H substance to which half of sugars are added carbohydrates are terminal sugars that determine A or B blood type Type O unlike A cannot add the H substance However there are mutants of FUTl gene that cannot express the sugar for the H substance even though they are type A or B functionally Bombay Phenotype Mutations resulting in nonfunctional genes can be tolerated in heterozygotes where the wild type might be enough to produce the essential product Recessive Lethal Allele mutations resulting in a nonfunctional gene lethal to homozygotes May respond as phenotype in heterozygotes Ex yellow pigment in mice Dominant in coat to wildtype so heterozygotes are yellow Homozygotes die before birth D U39 D I UH DH LE AYAY homozygous recessivedeath I Ll HLILI A A homozygous dominant agouti AY A heterozygotel 2 21 ratio signals lethal gene Dominant lethal allele the presence ofjust one copy of the allele results in death Ex Huntington s disease Delayed until adulthood in heterozygotes that results in motor and neurological degradation Dominant lethal alleles are rarely observed For dihybrid corsses such as albinism risk and blood type the 9331 ratio converts to 263121 establishing the probability of each phenotype Phenotype in many cases is affected by more than one gene Genetic in uence on phenotype is more complex than Mendel s encounters Gene Interaction expresses the idea that several genes in uence a particular characteristic DOES NOT MEAN THAT TWO OR MORE GENES INTERACT DIRECTLY TO INFLUENCE DOES MEAN THAT CELLULAR FUNCTION OF NUMEROUS GENE PRODUCTS CONTRIBUTES TO GENE DEVELOPMENT OF A COMMON PHENOTYPE Epigenesis a point in which each step of development increases the complexity of an organ or feature and is under the controlin uence of many genes Ex formation of inner ear in mammals Forms as a result of a cascade of intricate developmental events in uenced by many genes Mutations that interrupt these events leads to hereditary deafness In a sense many genes interact to produce a common phenotype Mutant phenotype then becomes the heterogeneous trait Epistasis the expression of one gene pair masksmodi es the effect of another Sometimes the genes involved in uence the same general phenotypic characteristic in an antagonistic manner which leads to masking But other times the genes involved exert in uence on one another in a complementary fashion Ex recessive homogeneous allele epistatic may override the expression of other alleles hypostatic at another locus 2 gene pairs may also complement one another such that at least one dominant allele in each pair is required to express a particular phenotype 97 ratio or 934 When a single character is being studied a ratio of 16 parts ex 3634 suggests that two gene pairs are interacting in the expression of the phenotype under consideration epistasis has an effect of combining one or more of 4 phenotypic categories in various ways Conventions 1 In each case a distinct phenotype classes produced disassemble from others Illustrate discontinuous variation phenotypic traits are discrete and different from each other 2 Genes considered in each cross are on different chromosomes and assort independently during gamete formation EX AaBb 3 When we assume complete dominance AAAaBBBb is equivalent to genetic effects of A or B means other aees present 4 P1 crosses are with homozygous individuals AABB aabbAAbb aaBB Each F1 has heterozygote AaBb 5 F2 is the main focus When 2 genes are involved genotypes fall into 916 A B 316 A bb and 116 aabb All genotypes are equivalent on the effect for the phenotype Ex mice A agouti aablack bbalbino bb masks A or a bb aI bi no 16 black aaB black 16 al bi no aabb aI bi no F2 Nove Phenotypes may be expressed in the generation in addition to the F1 modi ed dihybrids EX squash shape AaBb X AaBb both disc A bb and aaB yield sphere shape new phenotype qene interaction When 16th in ratios of dihybrid crosses where inheritance pattern is unknown think 2 gene pairs involved Heterogeneous trait go back to hereditary deafness Complementation analysis allows to determine whether 2 independently isolated mutations are in the same gene aees or represent mutations in separate genes May reveal if only a single gene is involved in a mutation or multiple genes EX F1 Drosophila bugs generation Case 1 All offspring have wings 2 recessive mutations that make bugs wingless in separate genes and not alleles being heterozygotes both genes have a normal copy of each gene and complement Case 2 No wings 2 mutations affect the same gene and are alleles of one another No complementation homozygous for 2 mutant alleles No normal product produced Complementation group is for all mutations present in a single gene Complement mutations are in all other groups Helps predict the number of genes involved in the determination of a trait Pleiotropy when a single gene has multiple phenotypic effects Ex Marfan Syndrome results from autosomal dominant mutations in the gene encoding brillin Also Poryphyria variegate does not allow for the metabolization of hemoglobin when the respiratory pigment is broken down Leads to toxic buildup A major portion of the Y chromosome is inert genetically There is only a small portion that is homologous to the X chromosome in order to separate during meiosis Xlinkage genes present on the X chromosome exhibit patterns of inheritance different from those with autosomal genes One of the rst examples was the white eye of the Drosophila female which was an Xlinked mutation that was expressed in male offspring only Since the Y chromosome lacks homology with almost all genes on the X alleles present on the X will be directly expressed in the phenotype Males cannot be homozygous or heterozygote for Xinked possession of only one copy of a gene in diploid is heterozygosity One result of Xlinkage is in a crisscross pattern where recessive Xinked genes passed from homozygous mom to all males Occurs because females express recessive mutant allele on both chromosomes Ex colorblindness f Xinked disorder debilitateslethal to individuals prior to reproduction disorder exclusively in males only source of lethal allele in heterozygote female quotcarriersquot Pass on to 12 of males Ex Duchenne muscular dystrophy n sexlinked inheritance and sexin uenced inheritance autosomal genes responsible for contrasting phenotypes but expression dependent on hormone constitution of the individual So heterozygotes may exhibit a phenotype in males and contrast in females Ex domestic fowl plumage distinctly different in males and females demonstrating sex limited inheritance controlled by a single pair of autosomal aees modi ed bv hormones Sex in uenced patterns of inheritance include pattern baldness in humans horn formation in sheep and coat patterns in cattle Autosomal genes are responsible while trait may show in both sexes expression dependent on hormone constitution Most gene products function within a cell cells interact with one another in various ways Organisms exist under diverse environmental in uences Gene expression and resultant phenotype modi ed through interaction between individuals genotype and external environment Degree of expression can be shown by determining penetrance and expressivity Penetrance Percentage of individuals that show at least some degree of expression of a mutant genotype Ex If 15 of ies have a mutant genotype with wildtype appearance penetrance of mutant gene is 85 Expressivity range of expression of mutant genotype Ex eyeess gene in ies can range from normal to partial to none experiments show genetics and environmental factors in uence expression Position effect effect of genetic background where physical location of a gene in relation to other genetic material may in uence expression Ex if a region of chromosome is relocated normal expression of genes in that region may be modi ed more True if gene is relocated to areas condensed and genetically inert heterochromatin Ex white eye of Xlinked Drosophila if a region of X chromosome with w allele is relocated caused red and white phenotype Temperature affectsin uences phenotypes through chemical activity dependent on kinetic energy Ex a primrose is red at 23 degrees Celsius and white at 18 degrees Siamese cat and Himalayan rabbitfur black in cold areas and white in warmer ones temperature sensitive mutations These are rexamples of conditional mutations Permissive conditions allow an organism to grow restrictive conditions require and organism to use essential genes and arrests Nutritional mutations are crucial in bacteria mutations that prevent synthesis of nutrient molecules may kill microorganisms They can also prevent organisms from metabolizing a substance Ex lactose intolerance Some genes are expressed at different phases of life prenatal childhood preadult and adult Genetic Anticipation a form of heritable disorders that exhibit progressively earlier ages of onset and increased severity of the disorder in each successive generation Es Myotonic dystrophy GenomicParental Imprinting process of selective gene silencing that occurs during early development impacting subsequent phenotypic expression Impact depends on parental origin of genesregions involved Leads to direct phenotypic expression of alleles on homologs that are not silenced