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UConn MCB 2410 - MCB Notes

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Chapter 3 Genetic Terminology ● Gene- an inherited factor that determines a characteristic ● Alleles- different variations of a gene ● Locus (loci)- the place on a chromosome an allele is found ● Genotype- the set of alleles an organism has ● Phenotype- manifestation or appearance of a characteristic ● Homozygous (homozygote)- a diploid organism with 2 identical alleles at a given locus ● Heterozygous (heterozygote)- a diploid organism with 2 different alleles at a given locus Gregor Mendel ● Discovered the principles of inheritance using peas Monohybrid Crosses ● Used to track single characteristic ● Mendel cut off anthers to prevent self pollinating ● Principle of segregation ● Concept of dominance ● Used pure-breeding plants (homozygous for a trait) ● Began with plants that differed in only one character (= monohybrid cross) ● Homo and hetero cross = 3:1 pheno and 1:2:1 geno ratios Mendel’s Conclusions (monohybrid cross) ● Each F1 plant must posses 2 genetic factors (alleles) encoding a character ● The 2 alleles in each plant separate and each gamete only gets one (principal of segregation) ● 2 alleles of a plant separate with equal probability ● The concept of dominance ○ Trait that remained unchanged in F1= dominant ○ The trait that disappears in F1= recessive Principle of Segregation ● Chromosome segregation is the basis of this ● In anaphase 1= 2 chromosome pairs (n=4) and anaphase 2= 1 chromatid (n=1) ● Each allele has 50% chance of being found in any of the four gametes produced Punnet Square ● Can be used to predict genetic crosses● 4 x Rr heterozygotes round seed (monohybrid cross) ● In each parent gametes can have one of two alleles ● Geno 1:2:1 and pheno 3:1 ratios Probability in Genetics ● Multiplication rule- the probability of two or more independent events occurring together is calculated by multiplying their independent probabilities ● Probability of rolling a four and then another four on a dice is ⅙ x ⅙ = 2/6 = ⅓ ● “And” = multiply ● Addition rule- the probability of any one of two or more mutually exclusive events is calculated by adding the probabilities of these events ● Probability of rolling a three or a four on dice is ⅙ + ⅙ = 2/6 = ⅓ ● “Or” = add The Principle of Independent Assortment ● When two alleles separate, their separation is independent of the separation of other alleles at other loci ● Dihybrid crosses reveal this Dihybrid Crosses ● Pure breeding plants for two characters ● Cross F1 RY ry Ry rY= ● The allele encoding color separated independently of the allele encoding seed shape, producing 9:3:3:1 ratio in the F2 progeny Branch Diagram Method ● For solving genetic cross probabilities ● Testcross- one individual of unknown genotype is crossed with another individual with a homozygous recessive genotype (reveals the genotype of first individual) ● RrYy x rryy ○ Testcross between a round yellow plant and a wrinkled green ○ Break cross down into two monohybrid crosses ○ Then combine the monohybrid ratios using the multiplication rule ● Example ○ Probability- independently assorting loci ○ AABbCcDD x AabbCcdd ○ P(AaBbCcDd)? ○ P(Aa) = ½ ○ P(Bb) = ½ ○ P(Cc) = ½○ P(Dd) = 1 ○ ½ x ½ x ½ x 1 = ⅛ Chi-Square ● Goodness-of-fit test ● Statistical test used to evaluate the role chance plays in getting results ● Determines the probability that the difference between the observed and expected results are due to chance ● ● Degrees of freedom = n-1 ● DF= # of ways expected classes are free to vary ● N = number of different expected phenotypes ● Chi-square can indicate the probability that the difference between observed and expected is due to chance ● Cannot tell if ○ The cross has been correctly carried out ○ The results are correct ○ We have chosen the correct explanationChapter 5 Extensions to Mendelian Genetics ● Lethal alleles ● Concept of dominance: incomplete, codominant ● Multiple alleles for a single locus (gene) ● Gene interactions: including epistasis ● CFTR- a case study- multiple alleles/pleiotropy ● Penetrance and expressivity ● sex-influenced/linked/limited/genomic imprinting ● Environmental contributions Lethal Alleles ● Cause death early in development ● Can alter phenotypic ratios ● Can be dominant or recessive Concept of Dominance ● Interaction between alleles at the same locus ● Dominant- heterozygote is same as one parent ● Incomplete dominance- heterozygote is intermediate ○ Phenotypic ratio will match genotypic ratio ○ Ex. red + white = pink ● Codominance- heterozygote exhibits both phenotypes ○ Create greater variety in genotypes and phenotypes ○ Ex. blood types AB Gene Interaction ● Genes at multiple loci determine a single phenotype ● Products of different loci combine to produce a phenotype ● Phenotypes are not predictable from single-locus effects alone ● Interaction among genes at 2, 3, 4, or more loci is common Epistasis ● One gene hides/masks the effect of another gene at a different locus ● Recessive epistasis- the presence of two recessive alleles inhibits the expression of an allele at a different locus ● Dominant epistasis- the presence of a single copy of an allele can inhibit the expression of an allele at a different locus ● Epistasis gene- the gene that does the masking ● Hypostatic gene- the gene whose effect is maskedCystic Fibrosis ● Persistent coughing wheezing or pneumonia ● Excessive appetite but poor weight gain ● Infertility with 95% of males with CF ● Infertility with 20% of females with CF ● 35-40 year life expectancy Classical Genetics of CF ● Most common recessive genetic disease in caucasians of european descent ● Affects about 30,000 children and young adults in the U.S. ● Occurs in approx 1 of every 3,500 live births in U.S. ● More than 10 million Americans are unknowing, symptomless carriers ● Median age of survival is 35-40 years CF Mode of Inheritance ● Single gene (CFTR) ● Can be unaffected carrier ○ Paternal Rr, Maternal Rr ○ Kids: 1:2:1 (unaffected, unaffected carrier, affected) Function of CFTR ● Maintains the fluidity of mucus and other secretions ● Key to discovery of functions was similarity to bacterial and fruit fly genes involved in transport ● Forms channels that transport chloride ions through cell membrane ●


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