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Transcript
Lecture 3 Mendelian Genetics Phenotype and Genotype Genotype and Phenotype • Genotype – genetic constitution of an • - organism: Total set of alleles of an individual Describe the following : AA: Homozygous Dominant (Wild Type) Aa: Heterozygous aa: Homozygous Recessive (double mutant) • Phenotype – observable characteristic • Genotype and environment • Contribution of environment varies between genes • Can be controlled by many genes • Random developmental events Mendel’s Experimental Design Mendelian Genetics • Modern genetics began with Gregor Mendel’s quantitative genetic experiments • Austrian monk • Mathematician • Numerical and observational data • Several generations Stamen Carpel Mendelian Genetics • Heritable, obvious traits • Simple crosses at first • Used peas because: • Easy to grow and available • Many distinguishable characteristics • Self-fertilization • True breeding peas Pea Traits Monohybrid Crosses and Mendel’s Principle of Segregation Dominant and Recessive Alleles • Pairs—the factors that control each trait exist in pairs. • Female parent—contributes one factor (♀) • Male parent—contributes one factor (♂) • Together these make a pair • Genes—are the factors that control traits. • Genes consist of pairs of alleles. One that comes from the mother parent and one that comes from the father parent. Dominant and Recessive Alleles • Alleles—the different forms of a gene (such as tall or short, wrinkled or smooth). • Dominant allele—one whose trait always shows up when the allele is present. • Recessive allele—is masked (or covered up) when the dominant allele is present. Recessive alleles only show up if a dominant allele is not present. EXAMPLES recessive dominant Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions. dominant dominant ? dominant recessive dominant ? dominant recessive recessive ? recessive Breeding Crosses • Initial cross is the P generation • Parents • Progeny of parents is first filial generation • F1 generation • Inbreeding of first generation creates second filial generation • F2 generation Monohybrid Crosses • Cross between true-breeding individuals with one different trait • Mendel’s first crosses • Resembled only one of the parents • Planted progeny and allowed selffertilization • Revealed both phenotypes aa AA Aa Aa Aa [S] [W] Smooth seeds (allele A) are completely dominant to wrinkled seeds(allele a) Monohybrid Cross • Mendel determined that • Particulate factors for genes, each contains a set of two • Transmitted by both parents • Alternate forms called alleles • True breeding forms contains identical set GENETIC MAKEUP (ALLELES) P PLANTS Gametes AA aa All A All a F1 PLANTS (hybrids) All Aa Gametes 1/ 2 1/ 2 A Eggs A A a Sperm AA F2 PLANTS Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 AA : 2 Aa : 1 aa a a Aa Aa aa Monohybrid Cross • F1 generation had both alleles • Only one expresses • One allele masks • Dominant • Recessive • Identical alleles – homozygous • Different alleles - heterozygous Monohybrid Cross Principle of Segregation • Recessive characteristics are masked • Reappear in F2 • Members of a gene pair (alleles) segregated during gamete formation How cells carry characteristics ? Aa male Formation of sperm F1 genotypes Aa female Formation of eggs 1 2 1 2 1 2 A A A a A 1 4 F2 genotypes 1 2 a A a 1 4 A a 1 4 a a 1 4 Branch Diagrams • Punnett squares can become messy with more than one gene • Use branch diagram to figure out genotype and phenotype expected frequency Test Cross • Mendel did several crosses • Followed over several generations TESTCROSS: GENOTYPES B_ • Selfing also very important • Allowed plants to reveal their genotype and not just their phenotye bb Two possibilities for the black dog: BB or Bb B B b GAMETES b OFFSPRING Bb All black b Bb bb 1 black : 1 chocolate Test Cross Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment The Principle of Independent Assortment • Factors for different traits assort independently of one another • Genes are inherited independently of each other • Segregate randomly in gametes • Dihybrid Cross Branch Diagram of Dihybrid Cross P h e n o t y p e Genotype vs. Phenotype Test Cross With Dihybrid Trihybrid Cross Tribble Traits Activity Statistical Analysis of Genetic Data: The Chi-Square Test Probability & Genetics Statistical Analysis • Data from genetics is quantitative • Use statistics to show deviation of observed results from predicted results • Chance factors cause deviations • Null-hypothesis – no difference between the predicted and observed • If not accepted then have to come up with a new hypothesis for deviation Genetics & Probability • Mendel’s laws: • segregation • independent assortment reflect same laws of probability that apply to tossing coins or rolling dice Probability & genetics • Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin • probability of tossing heads? 50% • probability making a P gamete… P 50% Pp p PP P 100% P Probability & genetics • Outcome of 1 toss has no impact on the outcome of the next toss • probability of tossing heads each time? 50% • probability making a P gamete each time? 50% P Pp p Calculating probability Pp x Pp male / sperm P p sperm egg offspring P P PP P p 1/2 x 1/2 = female / eggs 1/2 x 1/2 = P PP Pp p Pp pp Pp 1/4 P 1/2 x 1/2 = p 1/4 1/4 1/2 p p 1/2 x 1/2 = pp 1/4 2005-2006 Rule of multiplication • Chance that 2 or more independent events will occur together • probability that 2 coins tossed at the same time will land heads up 1/2 x 1/2 = 1/4 • probability of Pp x Pp pp 1/2 x 1/2 = 1/4 Calculating dihybrid probability • Rule of multiplication also applies to dihybrid crosses • heterozygous parents — YyRr • probability of producing yyrr? • probability of producing y gamete = 1/2 • probability of producing r gamete = 1/2 • probability of producing yr gamete = 1/2 x 1/2 = 1/4 • probability of producing a yyrr offspring = 1/4 x 1/4 = 1/16 Rule of addition • Chance that an event can occur 2 or more different ways • sum of the separate probabilities • probability of Pp x Pp Pp sperm egg offspring P p Pp 1/2 x 1/2 = p P 1/2 x 1/2 = 1/4 Pp 1/4 1/4 + 1/4 1/2 Chi-square test • Test to see if your data supports your hypothesis • Compare “observed” vs. “expected” data • is variance from expected due to “random chance”? • is there another factor influencing data? • null hypothesis • degrees of freedom • statistical significance Chi-Square Test • Goodness of fit test • How much observed number deviates from the expected number Chi - Square Test Mendelian Genetics in Humans Pedigree Analysis • Inheritance patterns are studied using family trees • Pedigree analysis • Phenotypic records • Proband is where gene was discovered Examples of Human Genetic Traits • Most genetic disorders are recessive • • Homozygous recessive expression • • Due to lack of function Dominant usually selected out Albinism Characteristics of Recessive Inheritance Traits • Most have normal heterozygous parents • Heterozygotes have 3:1 ratio • When both parents have the trait then all progeny have the trait • Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs Characteristics of Dominant Inheritance Traits • Gain of function mutations • New property of the mutant gene • No loss of function • Must have one parent with disease • Does not skip generations • Will transmit to half its progeny • Huntingtons disease, Marfan syndrome, achondroplasia