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Principles of Genetics

by: Kirstin Kuhic

Principles of Genetics SOCR 330

Kirstin Kuhic
GPA 4.0

Sarah Ward

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Sarah Ward
Class Notes
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This 11 page Class Notes was uploaded by Kirstin Kuhic on Monday September 21, 2015. The Class Notes belongs to SOCR 330 at Colorado State University taught by Sarah Ward in Fall. Since its upload, it has received 16 views. For similar materials see /class/210115/socr-330-colorado-state-university in Soil Science at Colorado State University.


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Date Created: 09/21/15
12 September Incomplete Dominance Creme gene in horses has 2 alleles C and Ccr CC sorrel chestnut CC39Ccr cremello Heterozygote CCcr Palomino Variable Penetrance Trait is not expressed at all in some individuals while others with the same genotype do express trait Example Rb gene and retinoblastoma Rb Gene and Retinoblastoma Rb gene in normal unmutated state helps regulate cell cycle Most individuals have normal RbRb genotype Children born with one mutated rb version of gene have 90 chance of developing retinoblastoma by age 5 10 of Rb individuals do not develop retinoblastoma The mutated rb allele is 90 penetrant Variable Expressivity Trait is expressed in different amounts or intensities in different individuals with the same genotype Results in a variable phenotype A variably expressive allele is always expressed compare with variable penetrance Variable Expressivity Polydactyly The polydactyly allele P is a dominant mutation causing the growth of extra digits fingers and toes Pleiotropic Lethal Allele Manx Cat All Manx cats are Mm Normal tailed cats are mm M allele causes Lack of tail Multiple toes Skeletal deformities Pleiotropy multiple phenotypic effects from one allele Homozygous MM is lethal Offspring from mating two Manx cats Mm x Mm will segregate 2 Manx Mm 1 normal mm SexInfluenced Traits Autosomal genes express differently in male and female Dependent on hormonal background Pattern baldness males BB or Bb bald females only BB bald 31 August Assumptions Underlying Mendel s Principles Organism is diploid Only two alleles at a locus Complete dominance Independent assortment of alleles no linkage No interaction between loci Uncertainty in Transmission Genetics Many genes and traits violate at least one Mendelian assumption How do we known when we are NOT seeing Mendelian segregation Simple Probability Probability is defined as the number of times a particular event occurs divided by the total number of opportunities for the event to occur wa s in whichA can ocmr Probability of event A occurrIng possible outcomes Example of Simple Probability Peven score when throwing dice g V2 05 Podd score 05 These are MUTUALLY EXCLUSIVE EVENTS if an even score is thrown no odd score is possible PA 0 event A is impossible PA 1 event A is certain Ptwo sixes when throwing two dice X 36 Probability that if a woman has four children they will all be boys 1 V2XV2XV2XV2E Rules of Simple Probability Sum Rule combined probability of two or more mutually exclusive events is the SUM of their individual probabilities Pl 2 34 5or6beingthrown on dicel Product Rule joint probability of both of two or more independent events occurring is the PRODUCT of their individual probabilities 1 1 i Pthrowmg two Sixes on dIce 6 x 6 36 Rules of Simple Probability If A and B are mutually exclusive independent events PAg B PA PB PA m B PA x PB 7 November Molecular Basis for Mutation Mutations result from Rearrangement of sections of genome Change in nucleotide sequence Change in single nucleotide pair is a point mutation Change on one base always results in change in complementary base on opposite DNA strand Effect of point mutation varies from none to major depending on location of change in DNA sequence Point Mutations Substitution changing one base for another Transitions change purine 9 purine A H G or pyramidine9pyramidine C H T Transversions change purine H pyramidine T H A C H G etc Transitions less likely to be detected and repaired Frameshift Mutations Deletion loss of base AATAGCA 9 AATGCA Addition insertion of extra base AATAGCA 9 AATI39AGCA Frameshifts have major effects change all codons downstream Silent MisSense and NonSense Point Mutations Silent mutation has no effect on gene function AGA ser 9 A66 ser Missense mutation changes amino acid GAA leu 9 GGA pro Nonsense mutation causes premature termination of translation ATG tyr 9 ATC stop All downstream amino acids in polypeptide lost Effect of a Missense Point Mutation Normal betaglobin polypeptide Glutamic acid is 6 h amino acid GAG Sicklecell betaglobin polypeptide Valine is 6 h amino acid GTG Common Causes of Spontaneous Point Mutations Deamination Loss of NH2 group from a base Converts cytosine to uracil CG pair becomes AT after replication Also converts adenine to hypoxanthine which pairs with cytosine AT pair becomes GC after replication Tautomeric shifts Proton Hi ion moves within thymine keto9enol form or cytosine amino 9imino form Changes pairing Wrong base inserted during DNA replication Loss of nitrogenous base from intact double helix 9apurinic site if G or A 9apyrimidinic site if C or T Excision Repair of DNA Lesions Mismatched base or distortion in helix 0 39 39 by Section of faulty DNA strand removed typically 13 nucleotides in prokaryotes 2829 in eukaryotes Methyl groups on old DNA strand direct repair to newlysynthesized faulty strand DNA pol and ligase repair gap Triple Repeat Mutations in Humans Fragile X Syndrome Affects 1 in 1250 males 1 in 5000 females Normal X chromosome has 6 54 C66 repeats in untranslated FMR1 gene region Fragile X chromosome has 100 9 1000 CGG repeats Causes moderate to severe mental retardation Impact varies with individual Triple Repeat Mutations in Humans Huntington s Disease Inherited as autosomal dominant lethal mutation Trinucleotide repeat CAG on chromosome 4 Normal gene has up to 26 CAG repeats Huntington s gene has 40 9 100 CAG repeats Greater number of repeats earlier onset More severe if inherited on paternal chromosome quotgenomic imprintingquot 4 November Translational Gene Regulation Masked mRNA Unfertilized sea urchin eggs store mRNA complexed with protein inhibits translation After fertilization proteins removed translation increases 50x Categories of Mutation Mutation is a heritable change in the genome Mutations can occur in any DNA sequence but are more likely at llhot spots within the genome eg CMEG Germinal mutations occur in gametes Affect next generation Somatic mutations occur in nongamete cells Affect individual in which they occur Spontaneous mutations occur naturally Frequently from errors in DNA replication Induced mutations occur from exposure to mutagens Chemicals Ionizing radiation Effect of Germinal Mutation Albino individuals homozygous for mutant hypostatic allele c Albino allele prevents expression at pigment loci Inherited as germinal mutations via gametes from parents Effect of Somatic Mutation Somatic mutation in single cells prevents pigmentation Cell continues to divide 9 gray patch Mutation Rates Mutation rates vary with organism Typically higher in rapidly dividing cells E coli makes errors in DNA replication approximately 1 in 100000 bases Size of E coli genome is 4639221 bp 24 August Basic Cell Structure Every living organism contains genetic material usually DNA but can be RNA DNA is organized into structures called chromosomes n eukaryotes most DNA within nucleus with some in mitochondria and chloroplasts extra nuclear DNA cpNDA DNA in chloroplasts mtDNA DNA in mitochondria Prokaryotes lack nuclear membrane Humans have 22 pairs of autosomes 1 pair of sex chromosomes Chromosomes Chromosomes are quotpackagesquot of condensed DNA Each chromosome has a distinctive shape The full chromosome complement of a eukaryote is the karyotype Trisomy 21 Down syndrome Karyotypes can show abnormalities Types of Chromosomes Cytogenetics study of chromosomes Chromatid one strand of a chromosome Centromere attaches strands Short arm is p for petite Long arm is q Metacentric chromosomes long arm short arm roughly


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