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University of Waterloo
BIOL 239
Christine Dupont

Biology 139 Genetics Final Exam Review Biology Undergraduate SocietyMendelian Genetics Why garden peas Easy to cross fertilizeLarge numbers of offspringShort growing season TermsPhenotypeobservable characteristic largely determined by genotype Commonly referred to as a traitGenotypegenetic makeupMonohybrid Crossmatings between individuals that only differ in one traitGeneunit of inheritanceAllelealternative form of a single genePolymorphicMore then one wildtype allele allele frequency greater than 1MonomorphicOnly one wildtype allele only one allele above 1 frequency Mendels law of segregation The two alleles for each trait separate segregate during gamete formation then unite at random one from each parent at fertilizationAt the DNA level alleles vary in nucleotide sequence This can result in o New amino acid sequence o Change in the amount of protein o EgGarden peasR gives the pea a round shape because this version of the Starch Branching Enzyme is normalThe r allele as a different amino acid sequence and the enzyme is nonfunctional ie no branched starch Genetic CrossesTest CrossBreeding an unknown genotype with the homozygous recessive will reveal its genotypeIf it was homozygous dominant all progeny are heterozygous and have the same phenotypeIf it was heterozygous we would see a 11 phenotypic ratioThe law of product The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itselfThe law of sum The probability of either of two mutually exclusive events occurring is the sum of their individual probabilitiesDihybrid CrossMatings of individuals that differ in 2 traitsMore phenotypic classes are seen because there are more allele combinations for independent assortmentMating between dihybrids produces a 9331 phenotypic ratioMultihybrid CrossMatings that differ in 3 or more traits Biology 139 Genetics Final Exam Review Biology Undergraduate Societyo Huge punnet square Use a simpler method Eg Aa Bb Cc Dd x Aa Bb Cc Dd o Probability of an AA bb Cc Dd offspring xxx164 o Modifications of Mendellian RatiosCrossingWhen a gene has more than 2 alleles reciprocal crosses help determine dominance relationships by crossing pure breeding linesA dominance series shows most dominant to least dominant Types of DominanceIncomplete DominanceThe F Hybrid doesnt resemble either parent intermediate 1phenotype o Eg Flower pigments A red flower and a white flower produce all pink FsThe Fs will 12have a 121 phenotypic ratioCodominanceBoth traits are expressed in the F Hybrid 1o Eg A dotted parent and a spotted parent mate to make an F that is all dotted and 1spotted The F generation also has a 121 ratio 2 Blood type is also an example of codominance Ao Imakes enzyme to add A sugar Bo Imakes enzyme to add B sugar Oo I or ino enzymesno sugars added ABo IIboth enzymes to add both AB sugars Alleles and InheritanceAllele Frequencyfrequency of a given allele within a population o Mutant AlleleLess than 1 o Wildtype AlleleGreater than 1PleiotropyMultiple phenotypic effects from one allele Eg sickle cell anemiaRecessive LethalHomozygotes for this allele die F is 21 when heterozygotes are crossed 2o EgThe yellow allele in mice is the dominant for colour but recessive for lethality Multifactorial Inheritance Arising from the action of two or more genes polygenic or from interactions between genes and the environment F is 9331 same as dihybrid cross 2 Complementary Gene ActionTwo or more genes can work in tandem in the same biochemicalpathway to produce a particular trait F is 97 2 Heterogeneous TraitA mutation at any one of a number of genes can give rise to the same phenotypeComplementation Testingdetermines if 1 gene or 2 or more are involved in producing a trait o If the mutation is on a different gene complementation can happenoNo complementation on the same gene Biology 139 Genetics Final Exam Review Biology Undergraduate Society EpistasisA gene interaction in which the effects of an allele at one gene hide the effects of alleles at another geneRecessivewhen both alleles are recessive it hides expression Dominantdominant allele hides expressionHypothesis TestingCan determine mode of inheritance fromF ratios but not in humans2o Must use pedigrees in humans Penetrance vs ExpressivityPenetrancepercentage of the population with a particular genotype that demonstrate the expected traitExpressivitythe degree or intensity with which a particular genotype is expressed in a phenotype within a population o Eg Retinoblastoma 75 penetrance 25 do not develop the diseaseof those who do get the disease some get in it only one eye 30 expressivity Sexlinked traits Due to genes ON the X or Y chromosomeSexlimited traitsaffect a structure or process that is found in one sex but not the otherSexinfluenced traitsshow up in both sexes but their expression may differ between the two sexesNote the environment can also influence the phenotype of a given genotype Himalayan coatConditional lethalityoccurs with a particular type of allele which is lethal under only certain conditions o Conditions are said to be permissive or restrictive Pedigree Analysis Many traits do not show the Mendelian pattern of inheritance because most traits in humans are influenced by more than one gene Most single gene traits in humans are lifethreatening or disablingExamples of singlegene traits Thalassemia Sicklecell disease and PKU Thalassemia there is a mutation in front of the coding region for the Bglobin chain of haemoglobinSicklecell anemia there is a change in the amino acid sequence that changes the conformation of the Bglobin protein Albinism and Huntingtons are also singlegene traits seen in humans
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