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Mendelian Genetics

Mendelian Genetics. Chapter 2. Phenotype and Genotype. Genotype and Phenotype. Genotype – genetic constitution of an organism Phenotype – observable characteristic Genotype and environment Contribution of environment varies between genes Can be controlled by many genes

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Mendelian Genetics

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  1. Mendelian Genetics Chapter 2

  2. Phenotype and Genotype

  3. Genotype and Phenotype • Genotype – genetic constitution of an organism • Phenotype – observable characteristic • Genotype and environment • Contribution of environment varies between genes • Can be controlled by many genes • Random developmental events

  4. Mendel’s Experimental Design

  5. Stamen Carpel Mendelian Genetics • Modern genetics began with Gregor Mendel’s quantitative genetic experiments • Austrian monk • Mathematician • Numerical and observational data • Several generations

  6. 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

  7. Pea Traits

  8. Monohybrid Crosses and Mendel’s Principle of Segregation

  9. 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

  10. 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 self-fertilization • Revealed both phenotypes

  11. GENETIC MAKEUP (ALLELES) P PLANTS PP pp Gametes All P All p F1 PLANTS(hybrids) All Pp Gametes 1/2P 1/2p P P Eggs Sperm PP F2 PLANTS p p Pp Pp Phenotypic ratio3 purple : 1 white pp Genotypic ratio1 PP : 2 Pp : 1 pp 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

  12. Monohybrid Cross • F1 generation had both alleles • Only one expresses • One allele masks • Dominant • Recessive • Identical alleles – homozygous • Different alleles - heterozygous

  13. Monohybrid Cross

  14. Principle of Segregation • Recessive characteristics are masked • Reappear in F2 • Members of a gene pair (alleles) segregated during gamete formation

  15. How cells carry characteristics • Genes on chromosomes • At a specific loci • Homologous pairs carry the same genes at the same locus • Different versions • Separation of homologous chromosomes yields separation of alleles

  16. Branch Diagrams • Punnett squares can become messy with more than one gene • Use branch diagram to figure out genotype and phenotype expected frequency

  17. TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING All black 1 black : 1 chocolate Test Cross • Mendel did several crosses • Followed over several generations • Selfing also very important • Allowed plants to reveal their genotype and not just their phenotye

  18. Test Cross

  19. Recessive Alleles • Wild-type allele – functional allele • Predominates in population • Dominant allele • Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional • Recessive • Function of other in heterozygote is sufficient

  20. Wrinkled Peas • SS type contains more starch and lower sucrose • Also more water • SBEI - starch-branching enzyme • Extra 800 bp piece in mutation

  21. Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment

  22. 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

  23. Branch Diagram of Dihybrid Cross Phenotype

  24. Genotype vs. Phenotype

  25. Test Cross With Dihybrid

  26. Trihybrid Cross

  27. Tribble Traits Activity

  28. Statistical Analysis of Genetic Data: The Chi-Square Test

  29. 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

  30. Chi-Square Test • Goodness of fit test • How much observed number deviates from the expected number

  31. Mendelian Genetics in Humans

  32. Pedigree Analysis • Inheritance patterns are studied using family trees • Pedigree analysis • Phenotypic records • Proband is where gene was discovered

  33. Examples of Human Genetic Traits • Most genetic disorders are recessive • Due to lack of function • Homozygous recessive expression • Dominant usually selected out • Albinism

  34. 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

  35. 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

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